A new genus and three new species of mangrove slugs from the Indo-West Pacific (Mollusca: Gastropoda: Euthyneura: Onchidiidae)

Mangroves of the Indo-West Pacific have remained poorly explored, so even the diversity of the onchidiid slugs, which are some of the most abundant animals in mangroves of the Indo-West Pacific, is not well known. Thanks to several years spent exploring mangroves in the Indo-West Pacific (more than 260 stations), especially in South-East Asia, the diversity of mangrove gastropods can now be addressed through revisions following an integrative taxonomy approach (nomenclature, field observations, comparative anatomy and DNA sequences). A new genus of onchidiid slugs is described, Paromoionchis Dayrat & Goulding gen. nov., which includes five species, three of which are new: Paromoionchis boholensis Dayrat & Goulding gen. et sp. nov., P. daemelii (Semper, 1880) com. nov., European Journal of Taxonomy 500: 1–77 (2019) 2 P. goslineri Dayrat & Goulding gen. et sp. nov., P. penangensis Dayrat & Goulding gen. et sp. nov. and P. tumidus (Semper, 1880) comb. nov. Paromoionchis gen. nov. is distributed from western India to the subtropical waters of Japan (33° N) and southeastern Australia (33° S). The creation of new taxon names is supported by rigorous nomenclature: the types of all existing species names in the family were examined, the original descriptions carefully studied and nomenclatural issues addressed. The diversity and biogeography of this new genus is discussed in a broader context.


Introduction
Onchidiid slugs are closely related to land snails and slugs, but they are truly marine because their larvae develop in seawater and because their adult life takes place in the intertidal (Dayrat et al. 2011a). Exceptionally, a few species are terrestrial and live in high-elevation rainforests (Dayrat 2010). Onchidiids are distributed worldwide, except at the poles. One subclade -traditionally referred to as Onchidium Buchannan, 1800 -has diversifi ed in the Indo-West Pacifi c, especially South-East Asia. Another subclade -traditionally referred to as Onchidella J.E. Gray, 1850 -has diversifi ed outside the Indo-West Pacifi c, especially in temperate waters. These subclades overlap geographically at the borders between the subtropical Indo-West Pacifi c and temperate waters (i.e., southeastern Australia, South Africa and Japan).
Even though onchidiids are widespread and common, their biodiversity has remained very poorly understood (Dayrat 2009). Some of the obvious reasons explaining this situation are that 1) onchidiid taxonomy has not been revised for more than 80 years, 2) new species names were often being created with little to no consideration for existing names, 3) internal characters (which are key at both specifi c and generic levels) were often ignored and 4) species were described based on preserved specimens without any fi eld observations on live animals. As a result, 150 species names exist in the literature, but, until now, no onchidiid species could be properly identifi ed, which is especially true in the Indo-West Pacifi c, where onchidiids are common and diverse (with dozens of species). Species relationships at the generic level also need to be completely re-evaluated. For instance, it was recently demonstrated that the genus name Onchidium actually applies to a small clade including only three species (Dayrat et al. 2016).
The Dayrat lab is in the process of revising the taxonomy of the entire family, using an integrative approach (Dayrat 2005). Our efforts have focused on collecting fresh material, getting new morphological and molecular data, addressing the nomenclatural status of every single existing species-and genusgroup name by re-examining the totality of the types available (many museum collections were visited specifi cally for that purpose) and critically going through the entire primary and secondary literature. Over the past few years, we have spent considerable time collecting fresh material, most especially in the mangroves of South-East Asia and Australia, where onchidiids are often the most abundant animals. We have collected thousands of specimens from more than 300 stations worldwide (as of August 2018), including many original type localities. This exploration has allowed us to gather invaluable information on the natural history and color variation of live onchidiids. Those new collections were used to build a large integrative data set which now includes approximately 80 species and 10 clades of generic level. Each taxon is strongly supported by both morphological and molecular data.
In order to fi nally establish some order in the taxonomy of the Onchidiidae, we are comprehensively revising every genus, focusing on one genus at a time. Several genera and many species in our data set are new to science. Also, species with existing names are often known only from the original description and need to be fully re-described. Six revisions have already been published on Onchidium (Dayrat et al. 2016), Onchidina Semper, 1882 , Melayonchis , Alionchis Goulding & Dayrat, 2018(Goulding et al. 2018a, Peronina Plate, 1893 (Goulding et al. 2018b) and Wallaconchis Goulding & Dayrat, 2018(Goulding et al. 2018c. The purpose of the present contribution is to describe a new genus, Paromoionchis Dayrat & Goulding gen. nov., which includes fi ve species. Two of those species were known only from the original description and are re-described here for the fi rst time with many new geographical records. The three other species are new to science. The nomenclatural status of several other existing species names is addressed. Three species names are shown to be junior synonyms of a Paromoionchis gen. nov. species name. Five species names are regarded as names of doubtful application (nomina dubia) for a variety of reasons.

Field collecting and sampling
Only fi ve specimens (out of 156) used in this study were not collected by us: two Queensland specimens of P. daemelii (Semper, 1880) and one Queensland specimen of P. tumidus (Semper, 1880) were found in the collections of the Australian Museum, Sydney; two specimens of P. tumidus collected during an expedition led by Philippe Bouchet to Madang, Papua New Guinea, were obtained from the Muséum national d'Histoire naturelle. All other 151 specimens examined here were collected by us in the context of an exploration of mangrove snails and slugs across the Indo-West Pacifi c, which provided fresh material for DNA sequencing and invaluable natural history observations. Collecting parties were led by Benoît Dayrat in the Andaman Islands (India), Brunei Darussalam, Malaysia, New South Wales and Northern Territory (Australia) and the Philippines, by Tricia Goulding in eastern and western India, Queensland (Australia) and Vietnam, by Rebecca Cumming in Japan and by Munawar Khalil in Indonesia. We were often accompanied by local guides (villagers or fi shermen). Sites were accessed by car or by boat. Each site was explored for an average of two hours, but the exact time spent at each site also depended on the time of the low tide, the weather, etc. At each site, photographs were taken to document the kind of mangrove being visited as well as the diverse microhabitats where specimens were collected.
In the fi eld, specimens were individually numbered and photographed in their habitat (it is very important to take photographs before animals are touched because they retract when disturbed and do not relax again for a long time). Importantly, a piece of tissue was cut from all specimens individually numbered (for DNA extraction) and the rest of each specimen was relaxed and fi xed for comparative anatomy.
Prior to Maximum Likelihood and Bayesian analyses, the best-fi tting evolutionary model was selected independently for each marker using the Model Selection option of Topali ver. 2.5 (Milne et al. 2004).
A GTR + G model was independently selected for each mitochondrial marker and a HKY + G model was independently selected for each nuclear marker. Other (unpublished) analyses were performed using different models, which all yielded identical results. Maximum Likelihood analyses were performed using PhyML (Guindon & Gascuel 2003) as implemented in Topali. Node support was evaluated using bootstrapping with 100 replicates. Bayesian analyses were performed using MrBayes ver. 3.1.2 (Ronquist & Huelsenbeck 2003) as implemented in Topali, with fi ve simultaneous runs of 1.5 × 10 6 generations each, sample frequency of 100 and burn-in of 25% (posterior probabilities were also calculated). Topali did not detect any issue with respect to convergence. Maximum Parsimony analyses were conducted in PAUP ver. 4.0 (Swofford 2002), with gaps coded as a 5 th character state and 100 bootstrap replicates conducted using a full heuristic search. All analyses were run several times and yielded the same result. In addition, pairwise genetic distances between COI sequences were calculated in MEGA 6. COI sequences were also translated into amino acid sequences in MEGA using the invertebrate mitochondrial genetic code to check for the presence of stop codons (no stop codon was found).

Molecular phylogenetic analyses
DNA sequences were used to test species limits within Paromoionchis gen. nov. The monophyly of this genus is strongly supported in all analyses . In analyses based on mitochondrial COI, 16S and 12S concatenated sequences, three clades (clades A, B and C in Fig. 1) are strongly supported, with bootstrap support and posterior probabilities of 88 and 1, 100 and 1, and 100 and 1, respectively. The relationships between clades A, B and C are not well supported. Each of these three clades includes three strongly-supported, least-inclusive units that are reciprocally monophyletic (Fig. 1). The monophyly of each unit is strongly supported by a bootstrap support of 100 and a posterior probability of 1; the bootstrap support for P. goslineri gen. et sp. nov. unit #1 is 98 (Fig. 1).
The phylogenetic analyses with just COI yielded similar results, even though, as expected, the deeper nodes are not as strongly supported (Fig. 2). The monophyly of Paromoionchis gen. nov. is recovered but not well supported.  (Dayrat et al. 2011a(Dayrat et al. , 2016. Information on specimens can be found in the lists of material examined and in Table 1. The letters A, B and C correspond to three clades which are referred to in the text. The color used for each (mitochondrial) unit is the same as that used in Figs 2-6.  (Semper, 1880) comb. nov. from China found in GenBank (in which they were misidentifi ed as Paraoncidium reevesii (J.E. Gray, 1850)). Information on specimens can be found in the lists of material examined and in Table 1. The letters A, B and C correspond to three clades referred to in the text. The color used for each (mitochondrial) unit is the same as that used in Figs 1 and 3-6.

Species
There also is a strong gap between intra-and inter-unit distances for P. tumidus unit #1, but it is slightly shifted: intra-unit distances are below 3.4% and inter-unit distances vary from 5.0 to 8.4%. Genetic distances do not mean anything in absolute terms and one should not expect the gap between intra-and inter-unit distances to always be the same between genera and even within genera. It all depends on the context. In clade A, the gap is between 0.8% and 3.4% and inter-unit divergences do not exceed 8.9%. In clade B, data display a similar pattern but numbers are slightly shifted, with a gap between 1.3% and 5% and inter-unit divergences going up to 9.8%. Finally, in clade C, the gap between intra-and inter-unit distances seems smaller if all three units are considered together (between 3.2% and 4.2%). However, in clade C, distances need to be analyzed with P. tumidus unit #1 being taken separately because of its higher intra-unit distances (up to 3.2%). There is still an obvious gap in genetic distances between P. tumidus unit #1 and unit #3 (inter-unit distances above 5.0%) and between P. tumidus unit #1 and unit #2 (inter-unit distances above 5.7%). Finally, the gap is obvious between P. tumidus unit #2 and unit #3 (intra-unit distances below 0.6% and inter-unit distances above 4.2%).
So, overall, there is always a gap between intra-unit and inter-unit distances, but the actual values vary depending on the phylogenetic context and the units being considered (Fig. 5). In other words, one should not focus on where the gap is situated (between 2% and 5%, between 3% and 6%, etc.) but on whether there is an actual gap separating units, especially those that are most closely related.

Comparative anatomy
In the fi eld, specimens of Paromoionchis gen. nov. were often correctly identifi ed at the generic level, i.e., we most often recognized that they were in the same genus. Because these slugs bear a large dorsal,

Fig. 5.
Diagram that helps to visualize the data on pairwise genetic distances between COI sequences within and between mitochondrial units in Paromoionchis gen. nov. (see Table 3). Ranges of minimum to maximum distances are indicated (in percentages). For instance, within P. tumidus (Semper, 1880) unit #1, individual sequences are between 0 and 3.2% divergent; individual sequences between P. tumidus unit #1 and the other units are minimally 5% and maximally 8.4% divergent; overall, the distance gap between P. tumidus unit #1 and the eight other units is between 3.2 and 5%. The colors used for each unit are the same as those used in Figs 1-4 and 6.
central, retractable papilla that looks like a peduncle, we called them the ʻpeduncle' slugs in the fi eld. This peduncle, however, is not fully reliable, because a similar structure is found in other genera; for instance, a similar peduncle is found in Wallaconchis buetschlii (Stantschinsky, 1907) (see Goulding et al. 2018c), and it often cannot be seen because it is fully retractable inside the notum. In the fi eld, ʻpeduncle' slugs were numbered individually without any a priori species designation, because they all live in a similar habitat and are not distinct externally (their color patterns are similar and individual variation is high). Even though slugs in Paromoionchis gen. nov. are not distinct externally, they differ internally for characters from the male copulatory apparatus (Table 3).  Within clade C, the three mitochondrial units of P. tumidus are not anatomically distinct from each other, but they differ from all the slugs in clades B and C (Figs 1-4, Table 3): P. tumidus is characterized by an accessory penial gland, which is lacking in clade A (P. penangensis gen. et sp. nov. and P. goslineri gen. et sp. nov.); also, the penis of P. tumidus bears hooks which are lacking in clade B (P. daemelii and P. boholensis gen. et sp. nov.).

Species delineation
According to mitochondrial DNA sequences, there are nine least-inclusive, reciprocally monophyletic units (

Diagnosis
Body not fl attened. No dorsal gills. Dorsal eyes present on notum. Retractable, central papilla (usually with four dorsal eyes) present, often raised above dorsal surface. Eyes at tip of short ocular tentacles. Male opening below right ocular tentacle and to its left. Foot wide. Pneumostome median, on ventral hyponotum. Intestinal loops of type II. Rectal gland absent. Accessory penial gland present or absent. When present, accessory penial gland with muscular sac. Penis with or without hooks.

Differential diagnosis
No external diagnostic feature unambiguously distinguishes Paromoionchis gen. nov. from all other genera (which is not surprising because many onchidiid species from different genera are very similar externally). However, Paromoionchis gen. nov. is characterized by a unique combination of internal and external characters: no dorsal gills, male opening below and to the left of the right eye tentacle, no rectal gland and intestinal loops of type II (see Labbé 1934a: 177, fi g. 3, for a comparison of digestive types). According to our data, any onchidiid slug with this combination of characters must belong to a species of Paromoionchis gen. nov.

Etymology
The name Paromoionchis is a combination of parómoios (παρóμoιoς), which means ʻsimilarʼ in Greek (because members look very similar externally) and onchis, a word derived from the Greek onchos (ὁ ὂγκος) and one of the early names used to refer to onchidiid slugs.

Gender
Masculine, the gender of onchis (ICZN Art. 30.1.1), a word derived from the masculine Greek word ὁ ὂγκος (onchos), which means ʻmassʼ or ʻtumor.ʼ As a result (ICZN Art. 31.2), the ending of the specifi c name tumidum (a Latin adjective) must be changed from neuter (because Onchidium is a name of neuter gender) to masculine (tumidus).

Distribution
The new genus described here is distributed from the western coast of India in the west, all the way to the subtropical waters of Japan (~33° N), Papua New Guinea and the subtropical waters of southeastern Australia (~32° S) in the east (Fig. 6). We did not fi nd Paromoionchis gen. nov. in South Africa, Madagascar or Mauritius, but it is possible that it is present in areas east of Papua New Guinea, such as Fiji and New Caledonia, where we did not collect.

Habitat
The fi ve known species of Paromoionchis gen. nov. primarily live on mud, in or next to mangroves, which explains why three species have just been discovered now, because the mangroves of South-East Asia have been very poorly explored. Occasionally, these slugs can also be found in or on muddy logs, coral rubble, sandy mud or even sand with very little mud in it. Paromoionchis tumidus, which is widespread and very common, can be found in nearly all these habitats, even though the mud surface remains where it is most commonly found, like all other species of the genus. Because members of Paromoionchis gen. nov. prefer the mud surface, live animals are often covered with mud.

Remarks
A new generic name is needed because no existing name applies to the clade described here. Our remarks are based on the examination of all the type specimens available, especially those of the type species of all genera, the careful analyses of all the original descriptions (especially when no type specimens were available), and our ongoing taxonomic revision of each genus of the family. Three existing generic names are junior synonyms of Onchidella J.E. Gray, 1850, which is not found in the tropical Indo-West Pacifi c and is characterized by a completely different anatomy (Dayrat 2009;Dayrat et al. 2011b). Seven generic names apply to the clade including all the onchidiid slugs with dorsal gills, i.e., Peronia Fleming, 1822 (Dayrat 2009). Labella Starobogatov, 1976 is a junior synonym of Onchidium Buchannan, 1800, which applies to a distinct clade including three species (Dayrat et al. 2016). Paraoncidium Labbé, 1934 is a junior synonym of Onchidina Semper, 1882, which applies to a distinct monotypic genus from southeastern Australia . Peronina Plate, 1893 applies to a clade including slugs characterized by a pneumostome located at the margin between the dorsal notum and the ventral hyponotum. Platevindex Baker, 1938 applies to a clade including species with a distinctly fl attened body and a narrow foot. Semperoncis Starobogatov, 1976 applies to species characterized by a very different anatomy and which are adapted to terrestrial life in the Philippines (Dayrat 2010). And, fi nally, Melayonchis  applies to a distinct clade including slugs with a different anatomy ).

Fig. 6.
Geographic distribution of the fi ve species of Paromoionchis gen. nov. Distinct colors are used for each mitochondrial unit found in P. tumidus (Semper, 1880) comb. nov., P. boholensis gen. et sp. nov. and P. goslineri gen. et sp. nov., and the colors used for each unit are the same as those used in Figs 1-5. Colored areas correspond to hypothetical ranges proposed based on known records (all of which are new here except for the type localities of P. tumidus and P. daemelii (Semper, 1880) comb. nov.). (Semper, 1880)

Notes on type material
Onchidium tumidum. Lectotype, 28/22 mm, designated here (ZMB 39019a). All other syntypes become paralectotypes (the 15 paralectotypes from the same lot are now ZMB 39019b). According to the original description, the type material included 42 specimens from Singapore and an unknown number of specimens from Port Mackay, Queensland, Australia. A total of 21 syntypes were located in museum collections: 19 specimens from Singapore (16 specimens, ZMB 39019; 2 specimens, NHMD 300305; 1 specimen, SMF 333603/1) and 2 specimens from Mackay (ZMB 39020). There also are two possible syntypes from Australia (ZMH 27480/2). Two similar species of Paromoionchis gen. nov. are found at Port Mackay, P. tumidus and P. daemelii, which anatomically can only be distinguished based on the insertion of the retractor muscle of the penis. In the lectotype designated here from Singapore, the retractor muscle inserts near the heart, exactly as in the species described here. However, in one of the two paralectotypes of P. tumidus from Mackay (ZMB 39020), the retractor muscle is vestigial, as in P. daemelii (in the other paralectotype from Mackay, the male apparatus was destroyed prior to the present investigation and could not be examined). Hence, it was necessary to designate a lectotype from Singapore in order to clarify the application of P. tumidus. Note that the type material was fi xed in formalin more than 130 years ago and no DNA sequencing could be attempted.
Onchidium samarense. Lectotype, 22/17 mm, designated here (ZMB 39025a). The two other syntypes become paralectotypes (ZMB 39025b). According to the original description, the type material included only two specimens from the same locality in Samar, Philippines. However, the jar with the type material currently contains three similar-looking specimens (syntypes), all of which were dissected prior to the present study. It is not excluded that the original description was based on only two of those three specimens but it is also possible that Semper himself identifi ed all three specimens as O. samerense [sic] (with a minor typo in the original description). The lectotype still contains all its internal organs, including the male copulatory parts. One paralectotype (24/20 mm) is mostly destroyed, with no internal organs left except the digestive gland (a few destroyed pieces of organs are in a vial). The other paralectotype (20/15 mm) still contains internal organs, but the male parts are missing. Our observations and comments are mostly based on the only specimen with male parts; hence its designation as a lectotype. Note that the type material was fi xed in formalin more than 130 years ago and no DNA sequencing could be attempted. Note also that if, in the future, Onchidium samarense were to be regarded as a valid species name in Paromoionchis gen. nov., the specifi c name samarense (neuter) would need to become samarensis (masculine).
Onchidium mertoni. Lectotype, 15/9 mm, designated here (ZMB 121591a). The four other syntypes become paralectotypes (ZMB 121591b). Simroth mentioned in the original description that all fi ve specimens were very hard. Indeed, it seems that they dried out at some point and they are very poorly preserved. The lectotype designated here is the only specimen that is complete. It was partially dissected for the present study (the penial hooks, identical to those of O. tumidum, are illustrated here). Two paralectotypes (14/8 and 14/10 mm) were dissected prior to the present study and are completely empty. Two other paralectotypes (15/14 and 14/10 mm) are in very poor condition (the body is extremely hard and the digestive system is partly outside the body through the foot). A lectotype is designated here to clarify the application of the name O. mertoni because several species of Paromoionchis gen. nov. are potentially sympatric in the Aru Islands and so it cannot be excluded that the fi ve original syntypes belong to different species. Note that the type material was fi xed in formalin more than 100 years ago and no DNA sequencing could be attempted.
Onchidium hongkongense. Holotype, 17/13 mm, by original designation (NHM 1982290) and 15 paratypes (NHM 1982291, NHM 1982292). The holotype is largely destroyed due to prior dissection, likely by Britton. Large parts of the notum and of the reproductive organs are missing. Even though it is mostly destroyed, the digestive system is confi rmed to be of type II. A few paratypes were checked for the present study and their anatomy matches that of the holotype. Note that the type material was fi xed in formalin more than 40 years ago and no DNA sequencing could be attempted. Note also that the specifi c name hongkongensis (masculine or feminine gender) originally used by Britton is corrected to hongkongense (neuter) for gender agreement with Onchidium. Should Onchidium hongkongense ever become a valid species name in Paromoionchis gen. nov., hongkongense would then need to be changed back to hongkongensis.  (Figs 7-10) Live animals of units #1 and #2 are often abundantly covered with mud, in which case their dorsal color can hardly be seen. The background of the dorsal notum is brown, light to dark. That background can be homogenous or clearly mottled with darker or lighter areas and, occasionally, also with red areas.  (Semper, 1880)  In addition, in some animals the tip of the dorsal papillae (with and without dorsal eyes) can be bright yellow. The foot varies from gray (light or dark) to yellow or orange. The hyponotum is almost always yellow, from pale yellow to bright yellow and even orange. This variable yellow component can cover the entire hyponotum or just an outer ring (the inner ring being light to dark gray). The color of the foot and of the hyponotum of an individual can change rapidly, especially when disturbed. The ocular tentacles are brown (variable from light to dark) and may or may not be speckled with tiny white dots, exactly like the head. The ocular tentacles are short (just a few millimeters long).

Color and morphology of live animals
No live pictures were available for unit #3, so the following description is based on preserved specimens (Fig. 10). It is possible that bright colors (yellow, orange) were lost during preservation on both the ventral and dorsal sides. The background of the dorsal notum is brown, mottled with darker or lighter areas. The foot is light gray. The hyponotum is gray-brown, with a reddish hue on the margin (which could possibly be orange in live animals). The color of the ocular tentacles (retracted, likely short) cannot be determined.
Generally speaking, the dorsal notum of any given live animal can rapidly change from almost perfectly smooth to covered by many papillae. However, when animals are not disturbed, the dorsum is usually covered by papillae of various sizes. In some animals, larger papillae may be arranged in two longitudinal and lateral ridges (on either side of the median line), but those ridges can appear and disappear rapidly. Some papillae bear from two to four black dorsal eyes at their tip (most papillae bear three eyes). The number of papillae with dorsal eyes is variable (between 10 and 15, on average) and they mostly are on the central part of the notum. Their tip is usually yellow, but not always. A central, much larger papilla, which bears four dorsal eyes (sometimes three), is entirely retractable within the notum. In addition to all these large papillae, the notum is covered by smaller, rounded papillae, which can make it look granular.
External morphology (Fig. 11A-B) Preserved specimens no longer display the color of live animals. The body is not fl attened (although, exceptionally, animals on mud with a thin layer of water can look fl attened). The notum is oval. Dorsal gills are absent. The large, central, retractable papilla at the center of the notum can only be seen in live animals. In preserved specimens, it is retracted inside the notum. The hyponotum is horizontal. The width of the hyponotum relative to the total width of the ventral surface (pedal sole and hyponotum) varies among individuals, from approximately one third to half. In the anterior region, the left and right ocular tentacles are superior to the mouth. Eyes are located at the tip of the two ocular tentacles. Inferior to the ocular tentacles, superior to the mouth, the head bears a pair of oral lobes. The latter are smooth, with no transversal protuberance. The male opening (of the copulatory complex) is below and to the left of the right ocular tentacle (i.e., between the two ocular tentacles). The anus is posterior, median, close to the edge of the pedal sole. On the right side (to the left in ventral view), a peripodial groove is present at the junction between the foot and the hyponotum, running longitudinally from the buccal area to the posterior end, a few millimeters from the anus and the pneumostome. The pneumostome is median. Its position on the hyponotum relative to the notum margin and the edge of the pedal sole varies among individuals but averages in the middle. The position of the female pore (at the posterior end of the peripodial groove) does not vary much among individuals.

Visceral cavity and pallial complex
The anterior pedal gland is oval and fl attened, lying free on the fl oor of the visceral cavity below the buccal mass. The heart, enclosed in the pericardium, is on the right side of the visceral cavity, slightly posterior to the middle. From the anterior ventricle an anterior vessel exits that supports several anterior organs such as the buccal mass, the nervous system and the copulatory complex. The auricle is posterior. The kidney is more or less symmetrical, the right and left parts being equally developed. The kidney is intricately attached to the respiratory complex. The lung is in two more or less symmetrical parts, left and right.
Digestive system  There are no jaws. The left and right salivary glands, heavily branched, join the buccal mass dorsally, on either side of the esophagus. The radula is in between two large postero-lateral muscular masses. Radulae measure up to 5.2 mm in length (unit #1), 4.1 mm (unit #2) and 2.8 mm (unit #3). Each radular row contains a rachidian tooth and two half rows of lateral teeth of similar size and shape. Examples of radular formulae are presented in Table 4. The rachidian teeth are unicuspid: the median cusp is always present; there are no conspicuous cusps on the lateral sides of the base of the rachidian tooth. The length of the rachidian teeth (approximately 25 μm) tend to be approximately half the size of the lateral teeth (approximately 50 μm). The lateral aspect of the base of the rachidian teeth is straight, occasionally slightly convex. The half rows of lateral teeth form an angle of 45° with the rachidian axis. With the Fig. 11. Paromoionchis tumidus (Semper, 1880) comb. nov., external morphology and nervous system. A. Anterior, ventral view, lectotype of Onchidium mertoni Simroth, 1918 (ZMB 121591a). B. Unit #1, posterior, ventral view, Indonesia, Sulawesi [2240] (UMIZ 00125). C. Nervous system, dorsal view, same as B. Abbreviations: a = anus; f = foot; fo = female opening; h = hyponotum; lcg = left cerebral ganglion; lplg = left pleural ganglion; mo = male opening; ol = oral lobe; ot = ocular tentacle; pn = pneumostome; ppg = peripodial groove; rcg = right cerebral ganglion; rplg = right pleural ganglion; vg = visceral ganglion. Scales: A-B = 2 mm; C = 0.5 mm.  Britton, 1984 (NHM 1982290  exception of the few innermost and few outermost lateral teeth, the size and shape of the lateral teeth do not vary along the half row, nor do they vary among half rows. The lateral teeth seem to be unicuspid with a fl attened and curved hook (approximately 50 μm long) with a rounded tip, but there is also a pointed spine on the outer lateral expansion of the base (basal lateral spine). In most cases, the basal lateral spine cannot be observed because it is hidden below the hook of the next, outer lateral tooth. It can only be observed when the teeth are not too close (such as in the innermost and outermost regions) or when teeth are placed in an unusual position. The inner and outer lateral aspects of the hook of the lateral teeth are straight (i.e., not wavy and not with any protuberance).
The esophagus is narrow and straight, with thin internal folds. The esophagus enters the stomach anteriorly. Only a portion of the posterior aspect of the stomach can be seen in dorsal view because it is partly covered by the lobes of the digestive gland. The dorsal lobe is mainly on the right. The left, lateral lobe is mainly ventral. The posterior lobe covers the posterior aspect of the stomach. The stomach is a U-shaped sac divided into four chambers. The fi rst chamber, which receives the esophagus, is delimited by thin tissue and receives the ducts of the dorsal and lateral lobes of the digestive gland. The second, posterior chamber, delimited by thick muscular tissue, receives the duct of the posterior lobe of the digestive gland. The third, funnel-shaped chamber is delimited by thin tissue with high ridges internally. The fourth chamber is continuous and externally similar to the third, but it bears only low, thin ridges internally. The intestine is long and narrow and the intestinal loops are of type II. There is no rectal gland.
Nervous system (Fig. 11C) The circum-esophageal nerve ring is post-pharyngeal and pre-esophageal. The paired cerebral ganglia are close and the cerebral commissure is short (but its length does vary among individuals). Paired pleural and pedal ganglia are also all distinct. The visceral commissure is short but distinctly present and   the visceral ganglion is more or less median. Cerebro-pleural and pleuro-pedal connectives are short and pleural and cerebral ganglia touch each other on either side. Nerves from the cerebral ganglia innervate the buccal area and the ocular tentacles and, on the right side, the penial complex. Nerves from the pedal ganglia innervate the foot. Nerves from the pleural ganglia innervate the lateral and dorsal regions of the mantle. Nerves from the visceral ganglia innervate the visceral organs.

Reproductive system (Figs 16-23)
Sexual maturity is correlated with animal length. Mature individuals have large female organs (with a large female gland mass) and fully-developed male copulatory parts. Immature individuals (< 15 mm long) may have inconspicuous (or no) female organs and rudimentary anterior male parts.  The female organs are located at the posterior end of the visceral cavity, mixed with some male parts (Figs 16A, 17A-B). The hermaphroditic gland is a single mass, joining the spermoviduct through the hermaphroditic duct (which conveys the eggs and the autosperm). There is a narrow and bent receptaculum seminalis (caecum) along the hermaphroditic duct. The female gland mass contains various glands (mucus and albumen) which can hardly be separated by dissection and of which the exact connections remain uncertain. The hermaphroditic duct becomes the spermoviduct (which conveys eggs, exosperm and autosperm). Proximally, the spermoviduct is not divided (at least externally) and is embedded within the female gland mass. Distally, the spermoviduct branches into the deferent duct (which conveys the autosperm up to the anterior region, running through the body wall) and the oviduct. The free oviduct conveys the eggs up to the female opening and the exosperm from the female opening up to the fertilization chamber. The large, ovate-spherical spermatheca connects to the oviduct through a narrow and short duct. The oviduct is narrow and straight. There is no vaginal gland.    The male anterior organs consist of the penial complex (penis, penial sheath, vestibule, deferent duct, retractor muscle) and the accessory penial gland (Figs 16B-C, 17C-D). The penial complex and the accessory penial gland share the same vestibule and the same anterior male opening. The penial gland is a long, tube-like fl agellum with a proximal dead end. The length of the fl agellum of the penial gland varies among individuals but it is always heavily coiled. Near its distal end (just before the hollow spine), the fl agellum is enlarged into a thick muscular sac. Distally, the fl agellum ends in a hard, hollow spine protected by a sheath which opens into the vestibule. The hollow spine is narrow, elongated and slightly curved (Figs 18-20). Its base is conical. Its diameter is between 60 and 100 μm.  (Figs 18-20). There is no disc separating the spine of the penial gland and the vestibule.
The penial sheath is narrow and elongated (Figs 16B-C, 17C-D). The penial sheath protects the penis for its entire length. The beginning of the retractor muscle marks the separation between the penial sheath (and the penis inside) and the deferent duct. The retractor muscle is shorter than the penial sheath and inserts on the wall of the body cavity, near the heart. The deferent duct is also highly convoluted, with many loops. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube of approximately 200 μm in diameter. Inside the tube-like penis, six longitudinal ridges bear sparse, tiny, conical (but not pointed) hooks which are less than 20 μm long in unit #1, less than 22 μm long in unit #2 and less than 28 μm in unit #3 (Figs 21-23). When the penis is retracted inside the penial sheath, the hooks are inside the tube-like penis; during copulation, the penis is evaginated like a glove and the hooks are outside.

Distinctive diagnostic features
Externally, Paromoionchis tumidus (Semper, 1880) cannot be distinguished from other species of Paromoionchis gen. nov. Internally, the presence of penial hooks distinguishes it from other species of the genus (Table 3).

Habitat (Figs 24-25)
Paromoionchis tumidus unit #1 is predominantly found on mud, hard or soft, inside or near mangroves, or on mudfl ats (Fig. 24). It is also found on old, muddy logs, inside or near mangroves. It occasionally is found on muddy sand, or even rocks and coral rubble, usually in the proximity of some mangrove trees. It is not found on rocky shores. Paromoionchis tumidus unit #2 is found in mangroves, mostly on mud and occasionally on sand (Fig. 25). Paromoionchis tumidus unit #3 is found in Nypa palm swamps and seems rare (only two specimens are known).
Paromoionchis tumidus is very common across its entire distribution range. It is by far the most abundant species of Paromoionchis gen. nov. and arguably the most abundant onchidiid species in the Indo-West Pacifi c. Most individuals of P. tumidus are part of unit #1, because P. tumidus unit #2 is rare across its entire distribution (it is only known from a total of nine specimens collected at nine stations) and P. tumidus unit #3 is restricted to two individuals from Papua New Guinea.

Remarks
The publication dates of the various sections of the volume on Landmollusken by Carl Semper in the Reisen im Archipel der Philippinen series were clarifi ed by Johnson (1969). The species name Onchidium tumidum was published by Semper with a complete description (text and fi gures) in 1880.
The anatomy of the species described here is fully compatible with Semper's original description of Onchidium tumidum as well as our own observation of the lectotype (and paralectotypes) from Singapore ( Table 3). The most important characters are the lack of a rectal gland, a digestive system of type II, an accessory penial gland, a retractor muscle of the penis inserting near the heart, a male opening between the two eye tentacles (not just below the right eye tentacle), and a penis with hooks (Figs 12H, 21D). According to Semper (1880: 263, our translation), the male opening is "almost exactly midway between the two [eye tentacles]," but it actually is closer to the right tentacle. Also, Semper (1880: 263, our translation) described a penis with an "anterior tooth-bearing portion [which] is reduced, namely at most 2 mm long." It is confi rmed here with SEM (Fig. 21D) that the penis of the lectotype bears tiny hooks (< 20 μm) and is fully compatible with the species described here.
The lectotype of O. mertoni is anatomically identical to the species described here, P. tumidus. Simroth did not describe the internal anatomy of O. mertoni, but a description of its lectotype is provided here.
Simroth mentioned a male aperture below the right ocular tentacle, but it clearly is to the left of the right tentacle (Fig. 11A); the intestinal loops are of type II; the male apparatus includes an accessory penial gland; the penial retractor muscle inserts to the body wall near the heart; the penis bears tiny hooks which are identical to the hooks of the species described here (Fig. 21C).
The type material of O. hongkongense is anatomically identical to the species described here and it is characterized by the exact same combination of characters: no rectal gland, intestinal loops of type II (Fig. 12D), an accessory penial gland, a retractor muscle inserting near the heart, a male opening between the two eye tentacles and a penis with tiny hooks (which Britton illustrated and measured as < 20 μm).
Strictly speaking, Onchidium samarense probably should be regarded as a nomen dubium because 1) the jar with the type material includes three specimens while Semper only mentioned two specimens in the original description, 2) the male organs of two of the syntypes are gone (possibly dissected by Semper) and cannot be checked, and 3) important features (e.g., the insertion of the retractor muscle) are not mentioned in the original description. However, our observations are compatible with the original description and it is possible that all three specimens were actually identifi ed as O. samarense by Semper himself. According to Semper (1882: 269, our translation), the penis of O. samarense is "very similar to that of O. tumidum." Indeed, the male apparatus of the lectotype is very similar to that of the species described here. In particular, the retractor muscle of the penis (not described by Semper) is thin but reaches the heart, following some nerves, as in the species described here. Semper described no ʻcartilaginous teeth.' However, that can be easily explained by the fact that the hooks inside the penis of O. tumidum are soft and tiny (< 20 μm) and, unlike the large and solid hooks found in some other onchidiids, can hardly be seen under a light microscope (in fact, because there are not many hooks, they are hard to fi nd even with SEM). Our collections currently do not include any specimen from Samar, the type locality of O. samarense, but we did collect many species of onchidiids in Luzon, just next to Samar, as well as in Bohol, a bit further south in the Philippines. Given the anatomy of O. samarense (no rectal gland, digestive system of type II, male opening clearly on the left of the right eye tentacle, accessory penial gland and retractor muscle inserting near the heart), it is most likely that O. samarense applies to the same species as O. tumidum. Because its nomenclatural status still remains problematic (it could be regarded as a nomen dubium) and because its written description was published in 1882 and not in 1880, O. samarense is regarded as a junior synonym of O. tumidum.
Plate (1893) identifi ed fi ve specimens from Ponape (now Pohnpei, Micronesia) and one specimen from Singapore as Onchidium tumidum. Given that their intestinal loops were of type I, the specimens from Ponape were misidentifi ed. It is impossible to determine whether the specimen from Singapore (with intestinal loops of type II) actually belongs to P. tumidus. Bretnall (1919) Boettger (1923) from the Aru and Kei Islands, was a misidentifi cation for O. mertoni (Boettger provided a record but did not describe any specimens). However, Hoffmann's claim cannot be checked because the application of Onchidella tabularis Tapparone-Canefri, 1883 (as Oncidiella) is very unclear (the type material could not be located and the original description is not informative).
Three onchidiid sequences from mainland China were obtained from GenBank, the only ones that are not new in our data set (Table 1). These sequences were misidentifi ed as Paraoncidium reevesii (J.E. Gray, 1850). Paraoncidium Labbé, 1934 is not a valid name: it is a junior synonym of Onchidina Semper, 1882. Also, Onchidium reevesi (J.E. Gray, 1850) is actually one of the three valid species of Onchidium (Dayrat et al. 2016).

Notes on type material
The lectotype, 17/14 mm, is designated here (ZMB 31640a). All other syntypes become paralectotypes. According to the original description, the type material included only three specimens. However, fi ve possible syntypes could be located in museum collections, all from Sydney, Australia: 2 specimens, one of which, dissected with male parts remaining inside (17/14 mm), is designated as lectotype (ZMB 31640a) and the other one, still entire (17/17 mm), is a paralectotype (ZMB 31640b); 1 specimen destroyed, in pieces and completely dried (ZMB 39035); and 2 specimens (ZMH 27476/2), both entire. It is unclear exactly which specimens Semper used for the description, but it is safe to assume that the anatomical details he provided are based on the only two dissected specimens. Two species of Paromoionchis gen. nov. are present in Sydney, P. tumidus and the species described here, which are cryptic externally but distinct internally. Thus, the specimens that were not dissected by Semper could belong either to P. tumidus or to the species treated here. Hence the necessity of designating a lectotype in order to clarify the application of the name Onchidium daemelii.  .468917.001). B. Digestive system, lectotype of Onchidium daemelii (ZMB 31640a). C. Posterior hermaphroditic (female) reproductive system, same as A. D. Male copulatory organs, same as A. Abbreviations: ag = accessory penial gland; dd = deferent duct; ddg = dorsal lobe of digestive gland; fgm = female gland mass; hd = hermaphroditic duct; hg = hermaphroditic gland; i = intestine; ms = muscular sac (of accessory penial gland); ov = oviduct; pdg = posterior lobe of digestive gland; ps = penial sheath; r = rectum; rm = retractor muscle; rs = receptaculum seminis; sp = spermatheca; st = stomach; v = vestibule. Scales: A = 4 mm; B-D = 3 mm. Color and morphology of live animals (Fig. 26) Live animals are often covered with mud, in which case their dorsal color can hardly be seen. The background of the dorsal notum is brown, occasionally mottled with darker or lighter areas. In addition, in some animals, the tip of dorsal papillae (with and without dorsal eyes) can be bright yellow. The foot is gray. The hyponotum is gray (same color as the foot), yellow, or both (yellow outer ring and gray inner ring). The color of the foot and of the hyponotum of an individual can change rapidly, especially when disturbed. The ocular tentacles are gray or brown, and may or may not be speckled with white dots, like the head. The ocular tentacles are short (just a few millimeters long).

Digestive system (Figs 27A-B, 28)
Radulae measure up to 5.4 mm in length. Examples of radular formulae are presented in Table 4.  (Figs 27C-D, 29) The male anterior organs consist of the penial complex (penis, penial sheath, vestibule, deferent duct, retractor muscle) and the accessory penial gland (fl agellum and hollow spine). The hollow spine of the accessory penial gland is narrow, elongated, slightly curved. Its base is conical. Its diameter is approximately 80 μm for most of its length, except at its base (200 μm) and tip (60 μm). Its length ranges from 2.5 mm ([1519] AM C.468917.001) to 2.7 mm ([1521] AM C.468919.001), and its shape does vary between individuals (Fig. 29). The penial sheath is narrow and elongated. The retractor muscle is short (shorter than the penial sheath) or even vestigial (its distal end being free in the visceral cavity, with no clear insertion). The deferent duct is highly convoluted, with many loops. Inside the penial sheath, the penis is a narrow, elongated, soft, hollow tube of approximately 100 μm in diameter; internally, the penis bears a few smooth (no hooks) longitudinal ridges.

Distinctive diagnostic features
Externally, Paromoionchis daemelii cannot be distinguished from other species of Paromoionchis gen. nov. (Table 3). Also, its internal anatomy (accessory penial gland, vestigial penial retractor muscle, smooth penis) is very similar to that of P. boholensis gen. et sp. nov. The distribution range of P. daemelii overlaps with that of only one species of Paromoionchis gen. nov., P. tumidus (Fig. 6). Both species live in similar habitats and can even be found at the same station. They can only be distinguished internally thanks to a few anatomical details: in P. daemelii, the penis is smooth and the penial retractor muscle is very short or even vestigial, while in P. tumidus the penis bears some tiny hooks and the penial retractor muscle inserts near the heart (Table 3). Distribution (Fig. 6) Australia: New South Wales (type locality, present study), Queensland (present study).
Habitat (Fig. 30) Paromoionchis daemelii is found on mud or muddy logs, inside or near mangroves, or on muddy sand. It is not common in central Queensland or New South Wales, but its abundance in southern Queensland is unknown.

Remarks
The publication dates of the various sections of the volume on Landmollusken by Carl Semper in the Reisen im Archipel der Philippinen series were clarifi ed by Johnson (1969). The species name Onchidium daemelii was fi rst published by Semper in 1880 with one fi gure (pl. 20, fi g. 2) but no written description. Because Onchidium daemelii was published before 1931, ICZN Article 12.2.7 applies and the name is available (Semper's fi gures are regarded as an indication accompanying the name Onchidium daemelii). Also, the specifi c name was originally spelled dämelii. However, according to ICZN Article 32.5.2.1., the correct spelling is daemelii. Both daemelii (e.g., Labbé 1934a) and damelii (e.g., Kenny & Smith 1987, 1988 are spelling mistakes. According to our current data, there are only two species of Paromoionchis gen. nov. in New South Wales (Fig. 6): P. tumidus and P. daemelii. They cannot be distinguished externally but they differ anatomically (Table 3). Both species are characterized by the lack of a rectal gland, a digestive system of type II, a male opening clearly to the left of the right eye tentacle (Semper described a male opening under the right eye tentacle, but it is distinctly below and left of it) and an accessory penial gland. The retractor muscle of the penis of O. daemelii, described as "very thin" by Semper, is vestigial in the lectotype, whereas the retractor muscle of P. tumidus is not vestigial and inserts near the heart. No "teeth" (the term he used to refer to penial hooks) are mentioned by Semper in the original description of O. daemelii, while the penis of P. tumidus bears some hooks. Therefore, the combination of characters found in the lectotype of O. daemelii and in Semper's original description (retractor muscle vestigial and soft penis with no hooks) is only compatible with the species described here, not with P. tumidus, which justifi es the present application of P. daemelii. (Semper, 1880) comb. nov., habitats. A. Australia, New South Wales, muddy sand next to small patch of mangrove and rocks on sandy beach (station 39). B. Australia, Queensland, Rhizophora and Avicennia mangrove (station 119).

Fig. 30. Paromoionchis daemelii
Onchidium daemelii was recorded from New South Wales (Lendenfeld 1886; Tenison-Woods 1888) and even New Guinea (Tapparone-Canefri 1883) but it is not possible to determine whether it was identifi ed properly without re-examining the material which these authors examined (which may or may not have been deposited). Bretnall (1919), Hoffmann (1928) and Labbé (1934a) mentioned Onchidium daemelii without adding any new material. Finally, Kenny & Smith (1987) published an ecological study on a species they identifi ed as Onchidium damelii in a mangrove on Magnetic Island, Queensland. However, given that P. daemelii is rare in northern and central Queensland and that its identifi cation requires detailed study of the internal anatomy, Kenny & Smith likely studied P. tumidus rather than P. daemelii (or a mix of both species). Dayrat & Goulding gen. et sp. nov. urn:lsid:zoobank (Figs 31-32) Live animals are often covered with mud, in which case their dorsal color can hardly be seen. In unit #1, the background of the dorsal notum is brown, occasionally mottled with darker or lighter areas, while in unit #2 it ranges from very light brown (almost white) to dark brown, mottled or not. In some animals, there is a reddish hue on the margin of the dorsal notum (unit #1). In addition, in most animals the tip of the dorsal papillae (with and without dorsal eyes) can be bright yellow. The foot is orange (unit #1) or varies from gray to yellow and orange (unit #2). The hyponotum is also orange, often with a darker ring on the margin which may be bright red (unit #1) or homogenously gray, yellow, or orange, but can also display a mix of two or three of those colors (unit #2). The color of both the foot and the hyponotum of an individual can change very rapidly, especially when disturbed. The ocular tentacles are reddish brown and may or may not be speckled with white dots, like the head. The ocular tentacles are short (just a few millimeters long).

Color and morphology of live animals
Digestive system (Figs 33A,34A,(35)(36) Radulae measure up to 4.5 mm (unit #1) and 4 mm (unit #2) in length. Examples of radular formulae are presented in Table 4.     Reproductive system (37)(38) The male anterior organs consist of the penial complex (penis, penial sheath, vestibule, deferent duct, retractor muscle) and the accessory penial gland (fl agellum and hollow spine). The hollow spine of the accessory penial gland is narrow, elongated, slightly curved. Its base is conical. Its diameter is approximately 50 to 70 μm for most of its length and 100-130 μm at its base (unit #1) and approximately 70 to 80 μm for most of its length and 150-200 μm at its base (unit #2). Its length ranges from 1 mm ( in unit #2, and its shape does vary between individuals (Fig. 38). The penial sheath is narrow and elongated. The retractor muscle is vestigial, i.e., with its distal end being free in the visceral cavity, with no clear insertion (unit #1), or absent or vestigial (unit #2). The deferent duct is highly convoluted, with many loops. Inside the penial sheath, the penis is a narrow, elongated, soft, smooth (no hooks) and hollow tube of approximately 200 μm in diameter.

Distinctive diagnostic features
Externally, the color of the foot and hyponotum can help one to identify Paromoionchis boholensis gen. et sp. nov., but unfortunately it is not fully reliable. Specimens with a bright orange foot and hyponotum are only found in P. boholensis gen. et sp. nov., especially in unit #1 but also in unit #2; the  ventral side of P. tumidus, which is sympatric with P. boholensis (Fig. 6), can be orange but not bright orange. However, specimens with a more yellowish or greyish foot and hyponotum cannot be identifi ed externally. The internal anatomy of P. boholensis gen. et sp. nov. (accessory penial gland, vestigial penial retractor muscle, penis with no hooks) is similar to that of P. daemelii. However, P. boholensis gen. et sp. nov. and P. daemelii do not overlap geographically, at least based on the present data. Thus, within its distribution range P. boholensis gen. et sp. nov. is the only species with this combination of internal characters. Indeed, the internal characters of the two species of Paromoionchis gen. nov. with which P. boholensis gen. et sp. nov. is sympatric (P. goslineri gen. et sp. nov. and P. tumidus) are different (Table 3). It must be noted that the known distribution of species of Paromoionchis gen. nov. may change as new records are found in the future and so the use of geographic data should only be used with caution for identifi cation.
Habitat  Unit #1 of Paromoionchis boholensis gen. et sp. nov. is found on sandy mud or sand with very little mud, in mangroves or near mangrove trees and is rare (it was found at only four stations). Unit #2 is found in open or dense mangroves, on soft or hard mud, as well as on muddy sand and is common (but not as common as P. tumidus unit #1). Dayrat & Goulding gen. et sp. nov. urn:lsid:zoobank Color and morphology of live animals (Fig. 41) Live animals are most often covered with mud, in which case their dorsal color can hardly be seen. The background of the dorsal notum is brown, occasionally mottled with darker or lighter areas. In addition, in some animals, the tip of dorsal papillae (with and without dorsal eyes) can be yellow. The foot is gray, occasionally with a light yellow hue. The hyponotum is uniform gray or gray (inner ring) and yellow (outer ring). The color of both the foot and the hyponotum of an individual can change rapidly, especially when disturbed. The ocular tentacles are brown and may or may not be speckled with white dots, like the head. The ocular tentacles are short (just a few mm long).

Paromoionchis penangensis
Digestive system (Figs 42A,43) Radulae measure up to 3.2 mm in length. Examples of radular formulae are presented in Table 4. Reproductive system  In the posterior (female) organs, the distal portion of the oviduct and of the duct to the spermatheca is wider than in other species, which makes sense given the wide penis. The male anterior organs consist of the penial complex (penis, penial sheath, vestibule, deferent duct, retractor muscle). An accessory penial gland is absent. The penial sheath is large (at least ten times as large as the deferent duct). The retractor muscle is strong, long and inserts near the heart. The deferent duct is convoluted, with many loops. Inside the penial sheath, the penis is a large (wider than long), smooth (no hooks), muscular mass.

Distinctive diagnostic features
Externally, Paromoionchis penangensis gen. et sp. nov. cannot be reliably distinguished from other species of Paromoionchis gen. nov. Its distribution only overlaps with that of P. tumidus. Our data suggest that the tips of the dorsal papillae of P. penangensis gen. et sp. nov. tend to be paler yellow, while they tend to be brighter yellow in P. tumidus. However, the internal anatomy of P. penangensis gen. et sp. nov., especially the large penis inside the large penial sheath, is very distinct from that of all other species and reliably distinguishes it from P. tumidus. Habitat (Fig. 44) Paromoionchis penangensis gen. et sp. nov. is found on soft and hard mud, in mangroves or in open areas near mangroves. This species was only found at three stations in the Strait of Malacca, three stations in the Andaman Islands (Bay of Bengal), and three stations in Maharashtra (W coast of India). However, at each station it was found to be quite abundant.   Live animals are most often covered with mud, in which case their dorsal color can hardly be seen. The background of the dorsal notum is gray-brown, mottled with darker and lighter areas. In addition, in some animals, the tip of dorsal papillae (with and without dorsal eyes) can be lighter (pale yellow or white). The foot and the hyponotum are dark or light gray. The color of both the foot and the hyponotum of an individual can change rapidly, especially when disturbed. The ocular tentacles are gray-brown and may or may not be speckled with white dots, like the head. The ocular tentacles are short (just a few millimeters long). The tip of dorsal papillae is usually white or pale yellow, but not always (in any case generally covered with mud).

Reproductive system (Figs 47B-C, 48B-C)
The male anterior organs consist of the penial complex (penis, penial sheath, vestibule, deferent duct, retractor muscle). An accessory penial gland is absent. The penial sheath is narrow and elongated. In unit #1, the retractor muscle is very short (much shorter than the penial sheath), inserting on the body wall near the nervous system, or vestigial (its distal end being free in the visceral cavity, with no clear insertion). In unit #2, the retractor muscle is long (as long as the penial sheath), inserting near the heart. The deferent duct is also highly convoluted, with many loops. Inside the penial sheath, the penis is a narrow, elongated, soft, smooth (no hooks) and hollow tube of approximately 200 μm in diameter.

Distinctive diagnostic features
Externally, Paromoionchis goslineri gen. et sp. nov. cannot be distinguished from other species of Paromoionchis gen. nov. The ventral side (foot and hyponotum) is gray, i.e., never yellow or orange. Unfortunately, a gray ventral side can occasionally be found in all other species of the genus, so the use of that color trait is not fully reliable for identifi cation. However, the internal anatomy of P. goslineri  anatomically because its penis is very large. Units #1 and #2 of P. goslineri gen. et sp. nov. differ slightly with respect to the penial retractor: it is short and thin, inserting near the nervous system, or even vestigial in unit #1, while it is as long as the penial sheath, inserting near the heart in unit #2. However, given that only four specimens could be dissected in unit #1, it is very possible that intermediates may be found in the future, especially considering that units #1 and #2 are widely distant geographically. Distribution (Fig. 6) Philippines (unit #1): Luzon (type locality). Indonesia (unit #2): Ambon, Bali, Halmahera, Sulawesi and Timor.
Habitat (Fig. 51) Paromoionchis goslineri gen. et sp. nov. unit #1 is found on mud, in Avicennia forests near the shore and is rare (only four specimens are known from two stations). Unit #2 is found on soft and hard mud, in mangroves or in open areas near mangroves and is rare (except in Bali, where several specimens were found at a few stations).

Nomenclature
Five species names are regarded as names of doubtful application (nomina dubia) for a variety of reasons (the type locality is too vague, the original description is not informative enough, the type material is destroyed or lost): Onchidium griseum Plate, 1893, O. lixii Labbé, 1934, O. palaense Semper, 1880, O. papuanum Semper, 1880and O. straelenii Labbé, 1934 Onchidium palaense Semper, 1880 could belong to Paromoionchis gen. nov., but its application is doubtful and it is regarded as a nomen dubium. The publication date for O. palaense is 1880 because it is the year in which Semper's plate 23 was published. ICZN Article 12.2.7 applies and Semper's fi g. 8 on plate 23 is an indication accompanying the name. The written description of O. palaense was published two years later (Semper 1882: 275-276, pl. 21, fi g. 8). Its type locality is Aibukit, Palau Islands, in the western Pacifi c. However, the type material (two syntypes) could not be located, which makes it impossible to determine some key characters not described by Semper. Semper indicated the absence of a rectal gland and of an accessory penial gland, but O. palaense cannot be reliably assigned to a genus without information on its digestive system type (type I or type II), because slugs without a rectal gland or an accessory penial gland are found in more than one clade. The position of the male aperture (between the two eye tentacles) seems to suggest that O. palaense could belong to Paromoionchis gen. nov. However, the position of the male aperture cannot be verifi ed here and Semper did not always describe it accurately (see our remarks on P. daemelii and P. tumidus above). That the original description of Onchidium palaense by Semper does not provide enough information to decide on a generic placement is demonstrated by the fact that Hoffmann (1928: 82) thought that O. palaense and O. gracile Stantschinsky, 1907 were synonyms because of a "striking agreement" in their anatomy. However, O. gracile belongs to a different genus characterized by a digestive system of type I and is now known as Wallaconchis gracilis (see Goulding et al. 2018c). When the type material is lost and the original description is incomplete, a name can apply to basically anything! Therefore, it is more reasonable to regard O. palaense as a nomen dubium. Even if a distinct species of Paromoionchis gen. nov. were to be found one day in Palau, there is no guarantee that it would actually belong to Semper's species. Finally, Plate (1893: 180) reported O. palaense from Ambon based on a single specimen (with intestinal loops of type I). However, Plate acknowledged that the identifi cation of that specimen was uncertain.
Onchidium papuanum Semper, 1880 could belong to Paromoionchis gen. nov., but is regarded as a nomen dubium because the type locality is too vague and because the type material is lost. Since Semper's plate 23 was published in 1880, ICZN Article 12.2.7 applies and Semper's fi g. 9 on plate 23 is an indication accompanying the name. The written description of O. papuanum was published two years later (Semper 1882: 276-277, pl. 21, fi g. 17). The type locality ("New Guinea") is too vague because it could be anywhere on the shore of the entire island of New Guinea (i.e., Indonesia and Papua New Guinea). Also, the type material could not be located and is likely lost. As a result, important characters cannot be checked, such as the type of the digestive system, which Semper does not mention. Slugs with no rectal gland and no accessory penial gland are found in more than one clade, so O. papuanum may or may not belong to Paromoionchis gen. nov. Even if O. papuanum was assumed to belong to Paromoionchis gen. nov., its male anatomy (penis with a large spherical vestibule) does not match that of any of the species described here. Previous authors have struggled with this species. Tapparone-Canefri (1883: 215) transferred O. papuanum to Peronia, as Peronia papuana, with no justifi cation. Bretnall (1919: 317) commented on the anatomy of O. papuanum, which he regarded as valid, but without examining any new material. Based on two non-type specimens from New Guinea, Labbé (1934a: 230) transferred O. papuanum to his genus Paraoncidium (which is actually a junior synonym of Onchidina Semper, 1882), but acknowledged that the identifi cation of those two specimens as Paraoncidium papuanum was only probable.
Onchidium griseum Plate, 1893 belongs to Paromoionchis gen. nov., but is regarded as a nomen dubium because the type locality is uncertain. In the original description (Plate 1893: 179), the specimens are said to be "of unknown origin, probably from one of the Polynesian islands." Also, no locality is indicated on the labels of the four syntypes (ZMB 45657). However, given its anatomy (digestive system of type II, no rectal gland, presence of an accessory penial gland and a male opening left of the right eye tentacle), we know that O. griseum belongs to Paromoionchis gen. nov.
Onchidium straelenii Labbé, 1934 may or may not belong to Paromoionchis gen. nov., but is regarded as a nomen dubium. The two syntypes used by Labbé were located (RBINS) and an examination of them revealed that his description is seriously erroneous regarding several important characters. For instance, Labbé (1934b: 76, our translation) described "numerous, small and very ramifi ed gills" on the dorsal notum, which explains why Labbé (1934a) later transferred that species to Scaphis, a genus he created for onchidiids with dorsal gills. However, there are no gills at all on the notum of the two  Philippines, Luzon, impacted mangrove next to a village, narrow band of Avicennia by the shore (station 185, type locality). B. Indonesia, Sulawesi, mostly Rhizophora, with sand, small rocks and pieces of wood outside narrow coastal mangrove (station 89). C. Indonesia, Bali, short mangrove shrubs, high intertidal, muddy area with pools (station 155). D. Indonesia, Bali, short mangrove shrubs high in intertidal, muddy area with pools (station 156). E. Indonesia, Halmahera, Rhizophora trees in open mangrove with sandy and muddy areas (station 219). syntypes of O. straelenii. Furthermore, Labbé described a digestive system of type I, but it is clearly of type II. Sadly, these kinds of mistakes are not unusual in Labbé's work. Consequently, these mistakes make it impossible to trust any of the other features he described for the anatomy of the male apparatus (accessory penial gland present and penis with hooks), which cannot be checked because the male parts are missing in both syntypes. The characters of the syntypes (digestive system of type II, male aperture clearly on the left of the right eye tentacle, no rectal gland) suggest that O. straelenii belongs to Paromoionchis gen. nov. However, its application remains doubtful, although it is clear that O. straelenii does not belong to Peronia, the clade including all onchidiids with dorsal gills. Labbé, 1934 likely belongs to Paromoionchis gen. nov. but it is regarded as a nomen dubium because the original description is confusing and the type locality ("New Guinea") is too vague. The four syntypes used by Labbé were located (MNHN Malacologie 22955). An examination of them revealed that the original description is based on specimens that belong to two species. Three syntypes were fully dissected by Labbé. In two of these syntypes (both 27/22 mm long), the male parts are completely missing, so we cannot verify the presence of an accessory penial gland (present according to Labbé's original description). The digestive system of one of these syntype is of type II (as described by Labbé); the digestive type of the second syntype could not be checked because it was destroyed by Labbé. In the third syntype dissected by Labbé (25/20 mm), the male parts are still inside the specimen, there is no accessory penial gland, and it does not seem that this gland was removed by Labbé. Thus, the presence of an accessory penial gland cannot be confi rmed in that syntype. Its digestive system is of type II. Finally, a fourth syntype was left almost intact by Labbé, who only cut a small square of the dorsal notum but did not open it (and so none of its internal organs were touched by Labbé). Some characters of that fourth syntype are consistent with Labbé's original description: dorsum with no gills, digestive system of type II, and no rectal gland. However, an accessory penial gland is clearly absent. All four syntypes must belong to some species of Paromoionchis gen. nov., because they share a unique combination of two traits which characterizes this genus (a digestive system of type II and no rectal gland). However, those four syntypes may not all belong to the same species. If Labbé did really see an accessory penial gland in two (or three) syntypes, then they belong to a different species than the fourth syntype (in which the accessory penial gland is absent). If Labbé made a mistake and described an accessory penial gland that did not exist, then all those syntypes could belong to the same species. The fact that Labbé actually illustrated an accessory penial gland and its spine (Labbé 1934a: fi g. 67) is not a guarantee that he actually saw it. We know for a fact that he illustrated various structures that he could not have seen. For instance, Labbé (1934a: 206) erroneously described dorsal gills and an accessory penial gland in the syntypes of Onchidium ater. Because of the uncertainty regarding the accessory penial gland, the application of the name O. lixii remains doubtful. One could designate the syntype which Labbé did not open as a lectotype. However, this would not help with the fact that the type locality ("New Guinea") is too vague; the syntypes could have been collected anywhere in West Papua (Indonesia), Papua (Indonesia) or mainland Papua New Guinea. So, in conclusion, Onchidium lixii is regarded as a nomen dubium.

Diversity
The analysis of three distinct data sets (comparative anatomy, mitochondrial DNA sequences and nuclear DNA sequences) provides invaluable insight on species boundaries. The mitochondrial units within P. tumidus, P. boholensis gen. et sp. nov. and P. goslineri gen. et sp. nov. are found to be reciprocally monophyletic and are separated by a wide gap in genetic divergences using mitochondrial DNA sequences (Figs 1-2, 5). However, they are not regarded as species for two main reasons: 1) nuclear markers suggest that there still is gene fl ow between mitochondrial units (because units are not recovered as reciprocally monophyletic in analyses based on nuclear markers) and 2) mitochondrial units are not distinct anatomically, especially for the male copulatory apparatus, which is essential for maintaining or preventing interbreeding. Naturally, one cannot completely exclude the hypothesis that nuclear markers are not variable enough to distinguish species and that the least-inclusive units based on mitochondrial DNA sequences (especially COI) should be regarded as species. However, it seems easier to interpret those mitochondrial units as divergent haplotypes that have been maintained due to maternal inheritance. Studies on land snails and slugs (Stylommatophora), to which onchidiids are closely related, have shown that very old (20 million years) distinct haplotypes can co-occur within a single population (e.g., Thomaz et al. 1996;Pinceel et al. 2005). Because of the multiple changes in sea levels in South-East Asia, especially during the glacial-interglacial cycles in the Quaternary (e.g., Bowen et al. 2016), there were many opportunities for small populations to be isolated in refuges at regular periods. Furthermore, the results from nuclear sequences suggest that isolation has not always led to speciation, because gene fl ow is still happening between individuals representing old, distinct mitochondrial haplotypes. The present study shows the critical importance of using nuclear DNA sequences in addition to mitochondrial DNA sequences because, had we used mitochondrial DNA sequences alone, we could have erroneously postulated the existence of nine species of Paromoionchis gen. nov., including several cryptic species.
Prior to the present study, only two species of Paromoionchis gen. nov. were known: P. tumidus, for which there are three junior synonyms, and P. daemelii, described once. These two species were originally classifi ed in Onchidium, traditionally used by default for most Indo-West Pacifi c onchidiids. However, Onchidium refers to a small clade of only three species (Dayrat et al. 2016) from which species of Paromoionchis gen. nov. highly differ anatomically (for example, the latter slugs lack a rectal gland). Also, molecular phylogenetics strongly supports the monophyly of Paromoionchis gen. nov., with respect to all other onchidiids. Both P. tumidus and P. daemelii were known exclusively from the types, showing that it has remained very diffi cult for authors to re-identify them. Here, dozens of new records are provided for P. tumidus, from the Andaman Islands (Bay of Bengal) all the way to the subtropical waters of Japan (33° N) and southeastern Australia (33° S). It is not surprising that P. tumidus was described four different times, because it is a very common species across its geographic distribution. New records are provided for P. daemelii (in Queensland) and it is shown to be endemic to eastern Australia.
The three other species of Paromoionchis gen. nov. are new to science. One might wonder how those species of large slugs have remained unnoticed for so long. However, it is not so surprising considering that the mangroves of South-East Asia have been very poorly explored and that these new species tend to be rare and found at only a few of the numerous stations that were visited (more than 260 stations across the Indo-West Pacifi c).
Species of Paromoionchis gen. nov. all live on mud, in or near mangroves. They are mostly found directly on the mud surface, which is their preferred habitat, but they can also be found on old logs covered with mud. Most species live on both soft mud (saturated in water) and hard mud (not saturated in water). Occasionally, some species (P. tumidus, P. daemelii, P. boholensis gen. et sp. nov.) can also be found on muddy sand (sand that is slightly muddy, usually with a few Avicennia trees). It is interesting to notice that some onchidiid genera seem to be more or less specialized to a particular habitat: for instance, Peronina is specialized to very soft mud, Melayonchis to tree roots and trunks, Platevindex to tree trunks and old logs covered with mud and Peronia to the rocky intertidal. We have never found Paromoionchis gen. nov. in the rocky intertidal and only very occasionally have we found it on old logs (mostly P. tumidus).
Each onchidiid genus is characterized by a distinct combination of anatomical characters. Thus, all onchidiids can easily be identifi ed at the generic level as long as some key anatomical characters are known (presence or absence of dorsal gills, type of intestinal loops, position of the male opening, absence or presence of rectal gland, absence or presence of an accessory penial gland). Members of Paromoionchis gen. nov. are characterized by lacking dorsal gills and a rectal gland, and by having a male opening below and to the left of the right eye tentacle and intestinal loops of type II. Furthermore, the monophyly of each genus is strongly supported by molecular data. Within each onchidiid genus, however, it is common for species of to be indistinguishable externally, although in most cases species differ internally, especially for the male copulatory apparatus. Species of Paromoionchis gen. nov. are cryptic externally due to their similar color patterns and to high individual variation, but they are distinct internally (Table 3).

Identifi cation key
A key is provided here to help identify the fi ve species of Paromoionchis gen. nov. Because species cannot be distinguished externally, the key is based on internal characters of reproductively mature specimens.
of Environment and Conservation. This work was supported by the Eberly College of Science at the Pennsylvania State University and by a REVSYS (Revisionary Syntheses in Systematics) award from the US National Science Foundation (DEB 1419394).