Abyssal vent field habitats along plate margins in the Central Indian Ocean yield new species in the genus Anatoma (Vetigastropoda: Anatomidae)

. New species in Anatoma Woodward, 1859 (Anatomidae, Vetigastropoda) are found in abyssal hydrothermal vent field habitats on the oceanic plate margins in the Indian Ocean. Six species are identified using molecular sequence analyses of which four species are described as new based on their morphological characters: Anatoma discapex sp. nov., Anatoma declivis sp. nov., Anatoma laevapex sp. nov. and Anatoma paucisculpta sp. nov. Inadequate material was available for a morphological description of the other two species with genetic identification, but it is likely that all six species are new to science and endemic to the Indian Ocean. The northern Central Indian Ridge localities are dominated by Anatoma declivis sp. nov.; its closest relative is Anatoma discapex sp. nov. which occurs in the central area near the Rodriguez Triple Junction. Anatoma laevapex sp. nov. and Anatoma paucisculpta sp. nov. as well as a fifth undescribed species are mainly found on the Southeast Indian Ridge.


Introduction
As a part of the research executed by the German Federal Institute for Geosciences and Natural Resources (BGR) to identify inactive polymetallic sulphide deposits along the Central Indian Ridge (CIR) and the Southeast Indian Ridge (SEIR), the INDEX project conducted cruises to the oceanic plate margins in the Indian Ocean in 2015, 2018 and 2019 with the aim to investigate geological, geophysical and biological aspects of hydrothermal vent developments. These vents include morphological structures from outflow of hydrothermal water and gases that are commonly found at diverging oceanic plate margins. The structures are often referred to as black or white smokers. Significant variations of chemical compositions in seawater and on the seafloor, as well as wide hydrothermal variations, yield a large number of microhabitats for marine life.
Knowledge of the hydrothermal vent fauna is an important tool to detect vulnerable taxa and can be used to designate marine protected areas. The investigation of biodiversity at these sites is an objective of activity related to the INDEX project, with recent studies mapping the vent fauna and describing benthic assemblages along the German claim showing that Mollusca account for a considerable number of benthic taxa (Gerdes et al. 2019a(Gerdes et al. , 2019b(Gerdes et al. , 2021. Similar observations were reported from other vents of the Central Indian Ridge (CIR) Van Dover et al. 2001;Van Dover 2002;Nakamura et al. 2012;Watanabe & Beedessee 2015;Kim et al. 2020) and Southwest Indian Ridge (SWIR) (Zhou et al. 2018;Sun et al. 2020). Table 1. Anatomidae McLean, 1989 from the Indian Ocean as given by Geiger (2012). The last four species are described as new in this study.

Sampling methods
The material used for the study of Anatoma was collected during the INDEX2015, INDEX2018 and INDEX2019 cruises on the research vessels Pelagia and Sonne using the Canadian ROV (Remotely Operated Vehicle) ROPOS (Remotely Operated Platform for Ocean Sciences). All samples have been taken by ROV-operated manipulators (grabs and suction device) and sampling information is listed in Table S1 (Supp. file 1). Live species have often been sampled as associated fauna with sediment and rock samples. Biological specimens have been collected by hand pickings and they have been sorted at species level and stored in ethanol (96%). Part of the samples has been kept under cooled or frozen condition to enable reliable genetic analyses at DZMB in Wilhelmshaven.
From the material obtained during three cruises from six different vent field areas, each specimen received a unique reference number (voucher specimen code). Detailed specimen data (taxonomy, collection sites, voucher specimen codes and museum number) is available in Barcode of Life Data Systems (BOLD; www.boldsystems.org) (Ratnasingham & Hebert 2007), project "Indian Ocean Hydrothermal Vent Fauna -INDEX" -subproject "Indian Ocean Hydrothermal Vent Macrofauna" in the public dataset "DS-INMAC03".

Imaging
To enable specific determination, each specimen used for former molecular analyses has been photographed with a Leica MZ 95 binocular microscope and a Lumix DMC -TZ 5 camera. This image set has been used to complete a species classification. Specimens within each species have subsequently been selected for morphological imaging of the soft parts and for the shells. Imaging of the soft parts has been carried out after removal of the shell by organic acid; the imaging has been done using a confocal laser scanning microscope (CLSM) at DZMB. Imaging of the shells has been done using a VEGA3-Tescan scanning electron microscope (SEM). Species descriptions are based on the photo sets for each species.

Molecular methods
After the morphological identification, several specimens of Anatoma were processed molecularly (Supp. file 1: Table S1). For 169 specimens genomic DNA (Supp. file 1: Table S1) was extracted by removing the soft part of the gastropod from the shell by microwaving for some seconds in a glass vial filled with water (Galindo et al. 2014). Further, 30 μl of CHELEX (BIO-RAD Insta Gene Matrix) was added and heated for 20 minutes at 56°C and 10 minutes at 99°C. For some samples the extraction was performed using columns to receive higher concentrated DNA (E.Z.N.A.® Mollusc DNA Kit, NucleoSpin® Tissue) following the manufacturer's protocol.
For the PCR, the primer pair jgHCO2198 / jgLCO1490 (Geller et al. 2013) with M13 tails was used to define nucleotide sequences for sequencing. For PCR, AccuStart II PCR SuperMix with a total volume of 12.5 µl (6.25 µl of AccuStart, 4.75 µl of H20, 0.25 µl of each primer and 1 µl of DNA) was added. Specimen that failed with the long fragment of COI were redone with the primer pair Lobo F1 and Lobo R1 (Lobo Arteaga et al. 2013) in combination with the abovementioned named primer pair and the same protocol. These primers resulted in shorter fragments and are appropriate for defragmented DNA. All amplified products were purified using Exo-SAP-IT ®. Finally, the PCR-products were sent to the Macrogen Europe Laboratory in Amsterdam for sequencing.
The obtained sequences of Anatoma were cleaned using Geneious Prime® 2020.1.2 (Kearse et al. 2012) to check for ambiguities and errors. All edited sequences were aligned and trimmed with MAFFT (Katoh et al. 2002) alignment in Geneious. Short sequences and sequences with bad quality were not included in the analysis. Afterwards, similarity analyses were done with BLASTn (Nucleotide BLAST) (Altschul et al. 1990) search against GenBank and BOLD (Ratnasingham & Hebert 2007).

Species delimitation and phylogenetic analyses
Combining our dataset of 95 sequences of COI with the four available sequences for Anatoma from GenBank (Table 2) we try to infer the number of sampled species and their classification or relation within the genus Anatoma. Therefore, the genetic distances of Molecular Taxonomical Units (MOTUs) were analysed. These MOTUs were ascertained with five methods using the COI dataset consisting of 95 sequences.
First of all, intra-and interspecific distances for our dataset were calculated by the Barcode Gap Analysis provided by BOLD. As a result, the threshold value that is mandatory for some delimitation methods was set at 0.983 or 98.3% (mean intraspecific divergence is 1.7%) (Supp. file 1: Fig. S1, Table S2).
The used delimitation methods can be grouped into three distance-based and two tree-based methods. Automatic Barcode Gap Discovery (ABGD) is calculating the "barcoding gap" (threshold distance) between inter-and intraspecific variation (Puillandre et al. 2012). Another distance-based method is CD-HIT (Li & Godzik 2006), a heuristic clustering process that requires defined sequence similarity thresholds. The third method of delimitation that uses calculated distances is the Barcode Index Numbers (BINs) system implemented in BOLD (Ratnasingham & Hebert 2013). This system registers each cluster of sequences and assigns a unique and specific code (BIN).
In contrast, the following two methods build groups of species based on a tree. General Mixed Yule Coalescent (GMYC) is a method that determines the point of transition from speciation to coalescent branching patterns (Pons et al. 2006;Monaghan et al. 2009). Another method is the Bayesian Poisson Tree Process (BPTP) for larger datasets (Zhang et al. 2013).
To present the different MOTUs and morphospecies in a graphic way, Bayesian tree analyses were performed. Therefore, the best fitting model was determined by the modeltest using MEGA X (Kumar et al. 2018) using both the Akaike Information Criterion (AIC) and the Bayesian Information Criterion (BIC). The best model for our dataset was the Tamura-Nei (TN93+G) model. For the construction of the tree, BEAST ver. 1.8.3 package and Yule-coalescent model as implemented in the R package 'splits' (Suchard et al. 2018) were used. The tree was produced and annotated with Bayesian posterior probabilities (PP) using TreeAnnotator in the BEAST ver. 1.8.3 package.

Molecular analyses
A total of 643 anatomid specimens were collected from the sediment samples during this study. A subset of 95 specimens could be identified as 6 different MOTUs ( Fig. 2 and Table 3). Two MOTUs are only represented by a single specimen. In this study, 95 high-quality sequences of COI were obtained (Table 3 and Supp. file 1: Table S1). Fragment lengths ranged from 513 to 658 bp; no indels (genetic insertions or deletions in genomes) or stop codons were found. In addition, a subset of 17 sequences of shorter fragments of COI was analysed (306-339 bp).
The molecular species delimitation methods revealed 5 to 6 MOTUs (Fig. 2). Plausible consensus clusters were maintained by combining all the methods and creating consensus clusters for conforming results of at least three methods. As a result, six MOTUs with four clusters and two singletons ( Fig. 2) were identified. The four clusters will be described in the following part. The two singletons remain undescribed (Anatoma sp. 1 DZMB_2021_0095, Anatoma sp. 2 DZMB_2021_0096).
The maximum intra-specific distances in the COI sequence are less than 1.5% (mutations in ca 10 bp in the investigated COI sequence) for all species whereas the inter-species distances are more than 6% (mutations in ca 40 bp) between all investigated species. The mean intra-specific distance is less than 0.6% for all species. The comparison with reference libraries (GenBank and BOLD) shows the highest identity to Anatoma sp. Lau (Kano 2008) (82-98% identity; GenBank accession number: AB365210). According to species delimitation analysis, all Anatoma records from literature (Table 2) are clearly separated from our species, except the species Anatoma sp. Lau (Fig. 2). Nevertheless, the consensus indicated this species to be separate from all other anatomid species in this study.
All sequences were deposited in GenBank with the accession numbers: OM951021-OM951133. Relevant voucher information, pictures, taxonomic classifications, and sequences are deposited in the dataset "DS-INMAC03" in the Barcode of Life Data System (BOLD) (www.boldsystems.org) (Ratnasingham & Hebert 2007) (GenBank accession numbers in Supp. file 1: Table S1).

Etymology
The species name 'discapex' refers to the flattened apex. Table 3. The list of taxa identified in the study presented for the sampled location: Gauss vent field (Gau), Vent field (VF). Unidentified or juvenile specimen are not named (see Supp. file 1: Table S1). Selected sequences for COI, morphological delimitation (M), MOTUs for ABGD (Ai, initial and Ar, recursive partition), CD-Hit (C), GMYC (G) and bPTP (B).

Shell description
Protoconch (Fig. 3K of paratype 4). One flat whorl, coarse pitted sculpture; raised flexuous lip with rim at transition to teleoconch, transition clear by change in sculpture; diameter 0.20 mm. teleoconch ( Fig. 3A-B). Flattened upper spire and disk-shaped outline with strongly protruding margins of selenizone; flexuous axial ribs; pear-shaped aperture with slit; suture deep. Height 1.3 mm, width 2.1 mm, height of aperture 0.85 mm (60 % of total height); colour opaque grayish white. 2¼ regular whorls with flattened, slightly convex shoulder area and rounded base, selenizone slightly above periphery; suture well below selenizone of penultimate whorl in last half of body whorl. Teleoconch I from protoconch to start of selenizone: ¾-1 convex whorl, in plane with protoconch; about 15 strong axial ribs, flexuous; no spiral cord. Teleoconch II with selenizone: initial whorl about 30 axial ribs, last ¼ whorl of body whorl about 22 axial ribs; bifurcating and closely spaced near lip. Margins of selenizone sharply protruding upwards or sideways near the lip. Axial ribs on external margins of selenizone; inside margins smooth. Frequency of axial ribs below selenizone similar to that above; rough growth lines between ribs, typically 3-6 major stages. Umbilicus open, tortuous and deep; steeply spiralling keel at union of columellar callus with parietal area (Fig. 3A of holotype and 3C, E, G of paratypes). APerture (Fig. 3A). Rounded base, funnel-shaped towards selenizone, pointed at union with penultimate whorl. Columellar lip sharp, protruding, rounded at base, flexuous towards parietal area; parietal callus thin and reclining. Semi-circular lower lip; slit slightly above periphery with concave edge of lip on either side, slit ¼ whorl deep with flattened sharp margins above and below. Callus thin; inside aperture smooth.
VAriAtion. Little morphological variation of shell. Variable length of selenizone I and number of axial ribs. Adult height range 1.1-1.4 mm; width range 2.1-2.2 mm.

Anatomical description
Snout smooth, broad, flattened with one symmetrical front lobe with pointed front margins on left and right sides (Fig. 4D-E). No eyes observed. Broad cephalic tentacles, tapered, with eight rounded lobes, deeply folded between lobes, gradually decreasing in diameter towards bluntly rounded tip, nonpapillate. Foot small when contracted, with irregular surface. Four epipodial appendages on right side; anteriorly two smaller, papillate epipodial tentacles and two large ones posteriorly; the most posterior probably epipodial sensory organ (ESO; Sasaki et al. 2010;Haszprunar et al. 2017). Three epipodial appendages on left side; small papillate tentacle anteriorly; second smaller epipodial tentacle or small ESO; most posteriorly a large ESO. Visceral mass with little more than one whorl. Mantle smooth with rounded margin, non-papillate. oPerculum (Fig. 4). Yellowish transparent, thin, corneous, multispiral (about 10 whorls) with central nucleus.
rAdulAr teeth. Configuration 30-4-1-4-30; width 0.10 mm. Central tooth broad cusp, directed anteriorly. Four lateral teeth on either side. Inner marginal teeth with multiple large curved hooks on thick stems changing towards external margin into long and thin stems with fine cusps at external margin.

Habitat
Living on rocky surface with bacterial mats near active and inactive vents. Shells frequently covered by red bacterial deposit.

Etymology
The species name 'declivis' refers to the sloping shoulders.

Shell description
Protoconch (Fig. 5C of paratype 1). One flat whorl, pointed nucleus, coarsely pitted sculpture; raised sharp lip with smooth flexuous margin at the transition to teleoconch: transition clear by change in sculpture; diameter 0.20 mm. teleoconch ( Fig. 5F-G). Flattened apex, biconical outline with sloping convex shoulders, protruding margins of selenizone; flexuous axial ribs; round aperture with slit; suture deep. Height 1.19 mm, width 1.90 mm, height of aperture 0.81 mm (67 % of total height); colour opaque grayish white. 2½ regular whorls with convex shoulder area and rounded base, selenizone at periphery; distance between base selenizone and suture minute. Teleoconch I from protoconch to start of selenizone: 0.8-0.9 convex whorls, in plane with protoconch; about 12-25 strong axial ribs, flexuous; no spiral cord. Teleoconch II with selenizone: 20-35 flexuous axial ribs on first whorl, 40-65 ribs on body whorl; ribs at regular spacing, decreasing spacing towards lip. Smooth margins of selenizone sharply protruding upwards, axial ribs weaker and more numerous on external margins; inside margins smooth. Frequency of axial ribs below selenizone slightly higher to that above; rough growth lines between ribs. Base of body whorl shows few spiral cordlets. Umbilicus open, narrow, tortuous and deep; steep spiral keel to centre of columellar callus.
APerture. Rounded at base, columella, parietal area and shoulder, funnel-shaped at selenizone, pointed at union with penultimate whorl. Lip sharp, flexuous following external ribs, protruding on shoulder area, protruding at columella, thin reclining callus in parietal area. Disk-shaped lower lip; slit at periphery, slit ¼ whorl deep with flattened sharp margins above and below; union of lip aligned with penultimate whorl. Callus thin; inside aperture smooth.
VAriAtion. Little morphological variation of shell. Teleoconch I up to 25 axial ribs, teleoconch II up to 65 axial ribs on body whorl. Base of body whorl occasionally with few spiral cordlets, or with one stronger spiral cord. Adult height range 1.2-1.7 mm; width range 1.9-2.0 mm; H/W ratio range 0.6-0.8. Juvenile shells lower H/W ratio.

Habitat
Living on rocky surface with bacterial mats near active and inactive vents. Shells are frequently covered by an amorph red bacterial deposit.

Remarks
Shells of Anatoma declivis sp. nov. are similar to those of Anatoma finlayi (Powell, 1937), an Indo-Pacific species living on the shelf and in bathyal depths, including off Madagascar (Geiger 2012; Table 1). However, the latter species has fine regular spiral cordlets on the teleoconch and its outline is more elevated (H/W 0.8-1.0). All other known species in the Indian Ocean have a stepped outline with a spiral sculpture.

Shell description
Protoconch (Fig. 8C). One slightly elevated whorl, pointed nucleus, coarsely pitted sculpture; sharp lip with flexuous margin at the transition to teleoconch: transition clear by change in sculpture; diameter 0.19 mm. teleoconch ( Fig. 8A-B). Flattened spire, disk-shaped outline with sloping weakly convex shoulders, smooth apex, protruding margins of selenizone; flexuous axial ribs; pear-shaped aperture with slit; suture deep. Height 1.0 mm, width 1.5 mm, height of aperture 0.67 mm (67% of total height); colour opaque grayish white. 2¼ regular whorls with convex shoulder area and rounded base, selenizone above periphery; distance between base of selenizone and suture small but increasing regularly. Teleoconch I from protoconch to start of selenizone: one convex whorl, decending below protoconch; smooth, flexuous growth lines; no spiral cord. Teleoconch II with selenizone: 44 regular weak axial ribs on first whorl, last ¼ whorl of body whorl with 17 axial ribs; ribs at regular spacing. Smooth margins of selenizone sharply protruding outwards, axial ribs absent on margins; inside margins smooth. Frequency of axial ribs below selenizone similar to that above; rough growth lines between ribs. Umbilicus open, tortuous and deep; steep spiral keel to base of columellar callus (Fig. 8A of holotype and 8D, F-G of paratypes). APerture (Fig. 8B). Rounded base, columella and parietal area; flattened shoulder, funnel-shaped selenizone, pointed union with penultimate whorl. Lip sharp, strongly reclining at union with penultimate whorl, protruding at upper shoulder and columella, thin reclining callus at parietal area; disk-shaped lower lip; slit above periphery, slit ¼ whorl deep with flattened sharp margins above and below. Callus thin; inside aperture smooth.

Habitat
Living on rocky surface with bacterial mats near active and inactive vents. Shells frequently covered by red bacterial deposit.

Remarks
Shells, anterior soft parts and radulae of Anatoma laevapex sp. nov. are similar to those from Anatoma declivis sp. nov. However, the shell of the latter species has fewer and coarser axial ribs including on teleoconch I; its protoconch is sunken. The shell differences with the similar Anatoma finlayi (Powell, 1937) and other known species in the Indian Ocean are mentioned under the remarks of Anatoma declivis sp. nov.

Shell description
Protoconch (Fig. 11F). One slightly elevated whorl, evenly rounded, sculpture small, irregularly oriented, straight or curved, short ridges; few spiral striae; smooth straight rim near margin; sharp flexuous lip, protrusion at periphery; transition to teleoconch clear by change in sculpture; diameter 0.21 mm. teleoconch ( Fig. 11D-E). With elevated spire, swollen whorls with protruding margins of selenizone; with spiral cordlets; rounded aperture with slit; suture deep. Height 1.0 mm, width 1.2 mm, height of aperture 0.7 mm (70 % of total height); colour translucent grayish white. 2¼ regular whorls with convex shoulder area and rounded base, selenizone slightly above periphery. Teleoconch I from protoconch to start of selenizone: slightly more than one well-rounded whorl, descending below protoconch; smooth, with up to six spiral cordlets, numerous prosocline growth lines. Teleoconch II with selenizone: slightly more than one whorl; shoulder area with seven spiral cordlets at the lip margin; new weaker cordlets emerging between older ones. Margins of selenizone sharply protruding outwards; growth lines on external sides of selenizone; inside margins smooth; surface between margins smooth with curved growth lines (Fig. 11B, E, H). Base body whorl smooth background with about 20 irregularly spaced spiral cordlets and about 60 regular axial riblets; spiral and axial sculpture stronger in umbilical area (Fig. 11G).
umbilicus. Open, tortuous and deep; steep smoothly curved spiral keel ending at base of columellar callus (Fig. 11D, G). APerture (Fig. 11D). With semi-circular rounded base, funnel-shaped towards selenizone, convex shoulder, pointed at union with penultimate whorl, well rounded on parietal and columellar side. Lip sharp; protruding, oblique on columella with minor rib joining in v-shape with columellar keel (Fig. 11G); rounded towards base and parietal area with thin callus. Slit ¼ whorl deep with flattened sharp margins above and below, lip prosocline, union perpendicular to penultimate whorl (Fig. 11E). Callus thin; inside aperture smooth.

Anatomical description
Animal unknown; the soft parts of the live-collected specimen have been used for molecular analyses or retained in ethanol for future studies.

Habitat
Living on rocky surface with bacterial mats near active and inactive vents.

Remarks
The lack of distinct and regular spiral and or axial sculpture is uncommon. The outline is similar to that of Anatoma africanae (Herbert, 1963) from the Gallieni Fracture Zone, south of Madagascar at a depth of 400 m, but this species has a distinct and fine axial and spiral sculpture.

Discussion
To date, thirteen species in Anatomidae were known from the Indian Ocean, most with a shelf to bathyal range (Geiger 2012; Table 1). Only two species were found alive at depths of 2500-3000 m (Geiger 2012): Anatoma munieri (Fischer, 1862) and Anatoma porcellana Geiger 2012, both found at a considerable distance from the investigated areas in this study and morphologically different from the new species presented. The identification of six species from the Central Indian Ridge and SE Indian Ridge gives testimony to the unique variability in habitats and related malaco-diversity in this relatively small area of the Indian Ocean. The four newly described species are considered endemic to the vent areas in the central Indian Ocean.
Three newly described species are distributed over an 800 km long trajectory along the central Indian plate margins: Anatoma declivis sp. nov., Anatoma laevapex sp. nov. and Anatoma paucisculpta sp. nov. Anatoma discapex sp. nov. has only been found on the Central Indian Ridge near the Rodriguez Triple Junction. The simple protoconchs indicate a direct larval development or a short lecitotrophic phase. It is yet unclear how they are capable of crossing abyssal gaps without food supply; this paradox is shared with many other Vetigastropods (Hoffman et al. 2021 and references therein).
The radulae in three investigated new species have simple cusped central teeth and lateral teeth with few denticles, inner marginal teeth with large, curved hooks with up to four denticles and outer marginal teeth with stilted cusps with many denticles. The number of marginal teeth varies among these three species between 21 and 30. Anatoma discapex sp. nov. had the highest number of marginal teeth (30) in a relatively narrow teeth-row (0.10 mm) when compared to Anatoma declivis sp. nov. (26 marginal teeth; row width 0.15 mm) or Anatoma laevapex sp. nov. (21 marginal teeth; row width 0.15 mm). Similar radulae were observed in Anatoma agulhasensis (Thiele, 1925), a species from the upper shelf of Africa (Geiger 2012: 749, fig. 602). Anatoma biconica Geiger, 2012 from bathyal depths in the southern Indo-Pacific (including off S Madagascar) has more denticles on the central and inner marginal teeth (Geiger 2012: 815, fig. 658). Anatoma flexidentata Geiger & Sasaki, 2008 is an abyssal Indo-Pacific species described from nearby Réunion (3300-3240 m) with completely different radular morphology with serrated pointed central teeth and curved denticles as outer marginal teeth (Geiger 2012: 892, fig. 719). The Indo-Pacific Anatoma japonica (Adams, 1862) is also found at upper bathyal depths off Madagascar and Réunion; it has similar central teeth but more serrated inner marginal teeth (Geiger 2012: 935, fig. 759). Anatoma porcellana Geiger, 2012 is an Indo-Pacific species, which has been found in bathyal depths off S Madagascar; its central and inner marginal teeth are similar but more serrated than those of our new species (Geiger 2012(Geiger : 1023 fig. 892). Anatoma janetae Geiger, 2006 from 2567-2606 m on the East Pacific Rise also has a similar rachidian (central) tooth, lateral teeth and inner marginal teeth but it has different outer marginal teeth (Geiger 2006 : fig. 3).
The morphologies of the anterior soft parts of three new species are similar. All cephalic tentacles have a tapered outline, non-papillate with multple lobes. Geiger (2012) imaged cephalic tentacles of some hitherto known Anatoma; they show an elongated shape with many tiny papillae; for example, the Antarctic Anatoma euglyptae (Pelseneer, 1903) and A. weddelliana (Zelaya & Geiger, 2007), the Japanese A. fujikurai Sasaki, Geiger & Okutani, 2010, the W American A. kelseyi (Dall, 1905 and A. janetae Geiger, 2006, the Peruvian A. peruviana (Geiger & McLean, 2010) and the NE Atlantic A. tenuisculpta (Seguenza, 1877). Haszprunar et al. (2017) studied the epipodial and cephalic appendages in basal gastropods. They confirmed that ESO pairs are present in species in Cocculiniformia, Neomphalida and Vetigastropoda. Species in Scissurellidae (Vetigastropoda) showed a pair of papillate cephalic tentacles, one or two pairs of papillate epipodial tentacles and one ESO pair. Species in Scissurellidae are considered closely related to Anatomidae. Geiger (2006: fig. 2) indicates several pairs of epipodial tentacles on Anatoma janetae; his image does not allow confirmation of one or more ESO pairs. Our observations indicate a posterior non-papillate pair of ESOs in three new vent species. Up to four pairs of epipodial tentacles and/or additional ESO pairs have been observed. Some epipodial tentacles seem papillate. The cephalic tentacles of three new species are elongate, tapered, strongly lobed and nonpapillate.
It is likely that anatomical traits of anatomids were adapted to food type available in the abyssal vent habitats. We presume an oligotrophic environment for all habitats; high oxygen concentration but little falling detritus. Nevertheless, the mineral-rich habitat is ideal for a bloom in Bacteria and Archaea (Han et al. 2018), which likely provide food for small vetigastropods. Many specimens are covered by a red deposit, probably containing bacteria. Different mineral environments will yield different Bacteria and Archaea, which might enhance the introduction of different species.
Switching on and off of food supplies by changes in mineral output across the oceanic plate margins in recent geological times will enhance distribution as well as isolation of new species. The genetic divergence of various species is probably associated with a morphological divergence: Anatoma laevapex sp. nov. and A. declivis sp. nov. are morphologically very similar, followed by A. discapex sp. nov. In contrast, the COI sequences of Anatoma laevapex sp. nov. and A. discapex sp. nov. are similar, with A. declivis sp. nov. being more distant. Anatoma paucisculpta sp. nov. shows greater differences in shell morphology and COI sequence. A relatively large COI distinctness is also observed in the two additional MOTUs.
Genetic and morphological divergence is expected to increase with increasing linear distance along the plate margins. Future sampling could be extended North, SE and SW to establish the limits of the species ranges. Moreover, a possible relationship between different types of bacterial mats and anatomid species could indicate different or preferred food sources for species.
A systematic analysis on the occurrence of anatomids in other plate margin areas has not been done to date; most studies were done on the Mid Atlantic Ridge (MAR) and the East Pacific Rise (EPR). Anatoma tenuis (Jeffreys, 1877) was originally described near the MAR at a depth of 2521 m but the species has also been found on the North Atlantic abyssal plane at depths of 5500-5987 m as was the synonymous A. josephinae Odhner, 1960(Geiger 2012. Anatoma umbilicata (Jeffreys, 1883) has been found on the MAR around the Azores but this species is widely distributed in the NE Atlantic Ocean and into the Mediterranean Sea (Geiger 2012). Anatoma aspera (Philippi, 1844) and A. tenuisculpta (Seguenza, 1877) have been found on the northern MAR south-west of Iceland at depths of 242-321 m (unpublished records from the reference collection at SaM). These latter species have a wide NE Atlantic and Mediterranean distribution range (Geiger 2012). Therefore, the anatomids known from the MAR cannot be regarded as typical vent fauna. Anatoma janetae Geiger, 2006 is the only anatomid orginally reported on the EPR from 2567-2606 m depth (Geiger 2006: 108). More recent records (Geiger 2012: 922) also report A. janetae in 850-900 m depth off the northwestern USA. In summary, the new anatomids on the plate margins in the Indian Ocean are solely known as endemic vent fauna. To date, anatomids known from MAR and EPR are not restricted to plate margin habitats and they have shown more extensive distribution areas.