New giant genus of Parabathynellidae (Crustacea: Bathynellacea): first record of Bathynellacea in an Australian cave

. A new genus and species of Parabathynellidae (Crustacea: Bathynellacea), Megabathynella totemensis Camacho & Abrams gen. et sp. nov., is described from the Northern Territory, Australia. This species is the first to be described from an Australian cave. It is a new giant species (4 to 6 mm). The new species displays several unique morphological character states within Parabathynellidae and is the only known species with: more than 12 articles on antennules, with a short, curved barbed seta on each article from the fifth; eight setae on the last article of antennae; more than three setae on the mandibular palp; up to 17 articles on the exopod of the thoracopods, without ctenidia but with a strong spine on each article at the base of the external seta; strong row of pair of spines on latero-external side of second article of endopod in all thoracopods; the male thoracopod VIII is different from all those known; more than 50 spines on the sympod of the uropod and more than 35 spines on the furcal rami. Specimens of the new species are morphologically different from all known species, but more closely resemble some giant species of the genera Kampucheabathynella (Asia), and Billibathynella and Brevisomabathynella (Australia).


Introduction
The crustacean Malacostraca Latreille, 1802 family Parabathynellidae Noodt, 1965 has a worldwide distribution with 45 genera and 220 species recognized thus far (Camacho & Leclerc 2022).According to Abrams et al. (2013), 47 species were known from Australia (including two in Tasmania), distributed in 10 genera across mainland Australia.Since then, one more genus and species have been described, Lockyerenella danschmidti Camacho & Little, 2017 from Queensland, two new species of Hexabathynella Schminke, 1972 from Rottnest Island (Western Australia) (Perina et al. 2023a) and two new species of Atopobathynella Schminke, 1973 from the Pilbara region (Perina et al. 2023b).In addition, a recent paper by Matthews et al. (2020), using a combined approach of molecular species delimitation methods, identifi ed between eight to 24 putative new species from remote subterranean habitats in the Pilbara region of Western Australia (WA).In the last 10 years, the delineation of parabathynellid species using molecular methods has focused on Queensland (QLD) (Cook et al. 2012;Little et al. 2016;Little & Camacho 2017), South Australia (SA) (Abrams et al. 2012(Abrams et al. , 2013)), New South Wales (NSW) (Asmyhr & Cooper 2012), and the Yilgarn region, WA (Guzik et al. 2008;Abrams et al. 2012Abrams et al. , 2013)).Therefore, the diversity of species is much greater than the 52 species formally described and assigned to 11 genera.
As previously noted by Cho & Humphreys (2010), each new fi nding not only increases the known diversity in the area, but also the morphological novelties.The description of this new genus does not disappoint in this sense since it presents several morphological characters that have not been previously observed in the group and that expand the known morphospace of the Parabathynellidae family.It is a new giant genus and species, more than 4 mm long.Other genera that contain giant species include the Australian genera Brevisomabathynella Cho, Park &Ranga Reddy, 2006, andBillibathynella Cho, 2005, and the Cambodian genus Kampucheabathynella Cho, Kry & Chhenh, 2015.Here, we consider species larger than 4 mm to be 'giant' and those of 2.5-4 mm to be 'large'.
Unfortunately, despite multiple DNA extraction attempts on fi ve specimens, we were unable to obtain sequences for this new genus, therefore, we were not able to place them in current global parabathynellid molecular phylogenetic frameworks (Little et al. 2016;Matthews et al. 2020;Camacho et al. 2021;Perina et al. 2023aPerina et al. , 2023b)).

Study area
Totem Pole Cave is located in the Pungalina Karst area (-16°48′0.9972″S, 137°27′1.0002″E) on the eastern margin of the Northern Territory near the border with Queensland in the Gulf of Carpentaria.The karst is a Precambrian dolomite and stromatolite fossils are common throughout the karst area.
Specimens were collected from a shallow pool of clear water (15 cm deep and 1 m 2 ) in a cave passage approximately 150 m from the entrance and approximately 10 m above the phreatic zone.Numerous pools of water were present in the cave above the phreatic zone due to the recent cyclone and associated rainfall in the area.Specimens were collected by hand, using a forceps and placed into 70% ethanol.The small pool was densely populated with an estimated 80-100 individuals.Several adjacent pools of similar size contained individuals but in lower abundance.No other specimens were observed in the remainder of the cave.

Morphological study
Ten specimens used for the study are listed in Table 1.Two entire specimens (mounted completely) and eight completely dissected (all body appendages separately) were preserved as permanent slides (special metal slides, glycerine-gelatine stained with methylene blue and paraffi n as the mounting medium (see Perina & Camacho 2016).Anatomical examinations were performed using an oil immersion lens Table 1 (  We used the terminology proposed by Serban (1972Serban ( , 1980Serban ( , 1985) ) to describe the mandible and male thoracopod VIII.

Diagnosis
AI multisegmented with more than 12 articles with terminal aesthetascs present on fi fth to last segments.AII 7-segmented.Labrum fl at, free margin dentate.Md pars molaris (molar process) protruding; proximal tooth present; mandibular palp bi-segmented with several setae (more than six).Proximal endite of MxI with four claws; ten to 11 claws present on distal endite.Exopod of ThI to ThVII multisegmented (more than 11 articles); epipod present from ThI to ThVII; fi rst and second article of endopod of ThI to ThVII each with a plumose dorsal seta.Male ThVIII unusually large, twice as long as wide; basal region of penial complex supports three independent lobes: inner lobe, outer lobe and dentate lobe; dentate lobe as inner lobe, rounded and as long as the outer lobe; very long curved outer lobe, as a fi nger; large basipod with a seta and a pronounced crest-like on internal face; dentate small exopod; endopod large with two setae and with the distal end rounded with four teeth and two setae of different sizes, the longest barbed.Female ThVIII one-segmented, almost triangular with three long barbed terminal setae.Pleopods absent.Inhomonomous sympod of uropod, with more than 50 spines, distalmost 25% longer than the rest; endopod with spines, the two terminal spines stronger than the rest, and setae, two barbed apical setae and one subterminal and two basal plumose setae; exopod with more than 16 setae.Furca very enlarged with more than 35 spines.Giant species, more than 4 mm long.

Differential diagnosis
Megabathynella Camacho & Abrams gen.nov.bears some resemblance to the giant genera Billibathynella, Brevisomabathynella and Kampucheabathynella (see Table 2).The new genus has more than 12 segments in AI, while the other genera have seven or 10; however, the new genus shows few teeth on the labrum, 12 to 14 with few lateral denticles, less than the other giant genera (between 12 and 63 teeth); the new genus presents a two-segmented mandibular palp with more than six setae, the general condition is one or up to three setae found in some species of Billibathynella and Brevisomabathynella; the new genus has up to 18 articles in the exopod of the Ths and always more than 10, while the largest number found never exceeded 13 articles.The maximum number of spines found in the furca of giant species is 23, while the new genus always has more than 35.Similarly, the sympod of the uropod of the new genus has more than 50 spines, in comparison, the maximum counted to date is 28 spines in species of Billibathynella.The male ThVIII of the new genus also shows several signifi cant differences: it is very large, with a very special endopod, the rounded and protruded distal end with small teeth and setae is totally novel.

Etymology
This prefi x 'Mega-' comes from the Greek 'μέγας', which means 'big'.The name Megabathynella refers to the unusually large size of the new genus.

Distribution
Australia, Northern Territory (present study).ThVII; fi rst and second article of endopod of ThI to ThVII each with a plumose dorsal seta.Male ThVIII unusually large, twice as long as wide; basal region of penial complex supports three independent lobes: inner lobe, outer lobe and dentate lobe; dentate lobe as inner lobe, rounded and as long as the outer lobe; very long curved outer lobe, as a fi nger; large basipod with a seta and a pronounced crest on internal face; dentate small exopod; endopod large with two basal setae and with the distal end rounded with four teeth and two setae of different length, the longest barbed.Female ThVIII one-segmented, almost triangular with three long barbed terminal setae.Pleopods absent.Inhomonomous sympod of uropod, with many long spines (56-85), distalmost 25% longer than the rest; endopod with four to six spines, the two terminal spines stronger than the rest, and setae, two barbed apical ones, one subterminal and two basal plumose setae; exopod with 15 to 18 setae.Furca very enlarged with 37 to 46 spines, the two distal ones twice as long as the rest.Anal operculum not pronounced.Giant species, more than 4 mm long.

Etymology
The specifi c name 'totemensis' is dedicated to the Totem Pole Cave where the new species was found.

Type locality
Ten specimens, six females and four males, were collected at the type locality by T.A. Moulds on 24 Jul.2006.

Description
MEASUREMENTS AND APPEARANCE.Body total length of male holotype 5.9 mm, allotype 6.2 mm.Body elongated (Figs 1-2), segments widening towards posterior end ~8 × as long as wide.Head slightly longer than broad.All drawings are of the holotype, except ThVIII (Fig. 5C), labrum (Fig. 3D), mandibular palp (Fig. 3F) and ThI (Fig. 4B) which are of the allotype.ANTENNULES (Figs 3A, 6A) (AI).Almost 35% longer than AII.13-segmented; fi rst three articles as long as the next six and slightly longer than the last four articles combined; fi rst article longest, similar to third, second slightly shorter, but just as thick; the fourth article is the shortest; the fi fth to eighth articles are similar in length, short and thick, and the last fi ve are longer and narrower than all the previous ones.First article with three smooth dorsal setae and three plumose ones (Fig. 3A, I).Second article with four plumose setae and eight smooth setae on inner margin.Inner fl agellum on third article, small and almost square with three smooth setae.Third article (Fig. 3J) with two smooth and one plumose outer lateral setae and eight smooth setae of different sizes on inner margin.Fourth article with two plumose setae on outer distal apophysis, one more dorsal plumose seta and one small, plumose stub on dorsal margin.Fifth to ninth articles (Fig. 3A, K) with three smooth setae, one strong and short curved seta with long setules and two aesthetascs, of similar size, on inner margin, and one smooth dorsal seta.Articles 10 to 12 with similar setation as previous ones but with three terminal aesthetascs instead of two.Last article with three subterminal aesthetascs and four terminal smooth setae.

MAXILLULES (Figs 3G, 6G-H) (MXI).
Proximal endite with four unequal strong claws with strong setation; distal endite with ten claws (as spines), along inner edge, one apical smooth and very large and strong, remaining claws denticulate with strong row of denticles and setae, basal claw smallest, half length of others; three smooth setae subdistally on outer margin of endite as fi gured.

MAXILLAE (Figs 3H, 6I
). Four-segmented, setal formula: 8, 16, 31, 8. 4A-B, 7A-D) (THI TO VII).Well developed; length gradually increasing from ThI to ThIV (Figs 1-2), ThV to VII similar in length; small epipod on ThI (Figs 4A, 7A) to VII, each about ⅓ of length of corresponding basipod.Basipod of all Ths with several (three to fi ve) distolateral, barbed setae.Exopods multi-segmented, 14 to 17 articles; number of exopodal articles of thoracopods I to VII: 14-16-16-17-17-17-16; basal article very long and wide with several barbed setae plumose at base on each side; following eight articles almost square and last ones elongated, all with barbed seta (plumose at base) on each side and one strong spine at base of inner seta.Endopod 4-segmented; fi rst article short, second and third long and similar in length and fourth article reduced with two smooth strong spinulose claws and two simple setae; fi rst article with distal plumose inner seta as second article; second article with inner and outer barbed setae, and cluster of pairs of strong spinules along inner margin from base to fi rst seta; third article with barbed setae on inner margin and one barbed distoventral seta.All thoracopods similar to ThI, but varying in number of articles of exopod, number of setae in basal article, number of setae on basipod and on fi rst three articles of endopod.Size ratios between articles, of exopod and endopod, similar to those of ThI (Fig. 4A).B, 8A-J, 9F).Unusually large, twice as long as wide; basal region of penial complex supports three independent lobes: inner lobe, outer lobe and dentate lobe; inclined dentate lobe, as long as outer lobe, with rounded distal end and big teeth (Fig. 5A-B); inner lobe rounded and longer than outer lobe; very long (6 × as long as wide) and curved outer lobe (as a fi nger) (Fig. 5A, 8B) that covers end of dentate lobe and not extending beyond basipod; large basipod with seta and pronounced crest-like protuberance (Fig. 5A-B, 8C) on internal face, and small almost triangular exopod with several denticles; endopod large with two basal setae and with distal end rounded, like skullcap or "helmet" (Figs 5A-B, 8F-G, J) with four teeth and two setae of different length, longest one barbed.

THORACOPODS (Figs
PLEOTELSON.Small ventral plumose seta.Anal operculum not pronounced, almost fl at. FIRST PLEOPODS.Absent.5D-E, 7F-G).Sympod 6.5 times as long as wide, 15% longer than exopod and almost three times as long as endopod, with 59 barbed spines subequal, except distalmost spine, slightly longer than rest.Endopod 2.5 times as long as exopod, with three spines along distal half of inner margin and two stronger and enlarged distal ones; distolateral angle of ramous with one subterminal plumose and two terminal barbed setae; basal part of ramous with two plumose setae of different length.Exopod with 18 barbed setae, 14 lateral and four terminal.

UROPODS (Figs
FURCAL RAMI (Figs 5F,7F).Each ramus almost triangular, very enlarged, with 37 barbed spines, six distal ones of different sizes, four slightly longer than fi rst 31 and two terminal ones two times as long as others; two plumose setae of different lengths oriented dorsally, shorter one without reaching tip of terminal spines.11.
An analysis of the oligomerization of appendages in relation to the size of the species, as a whole, would be required to determine whether it can be explained by simple allometry; however, this is outside the scope of this study The new species is most similar overall to species in the genera Billibathynella, Brevisomabathynella and Kampucheabathynella and shares characters with some species of the other Australian and Asian genera as shown in Table 2.
The new species has the highest number of articles known to date in AI, viz.15.Kampucheabathynella and Sinobathynella have the next highest number of articles (10).The new species is unique in having a very strong, recurved, short, thick and plumose seta from the fi fth to the penultimate article (Fig. 3A, K).
The new genus has numerous claws in the distal endite of the MxI, up to 11, and only some species of Billibathynella come close, with 10 claws, but the most common state is seven claws.
Brevisomabathynella uramurdahensis has a great profusion of setae in MxII but not as many as the new species.
The number of articles of the Th exopods of Megabathynella totemensis gen.et sp.nov. is unusually high (18 in some Th) in comparison with other genera, for example some species of Billibathynella, Brevisomabathynella and Kampucheabathynella have 10 or 12 articles.Sinobathynella decamera Camacho, Trontelj & Zagmajster 2006 also has a relatively high number of articles of the exopod of some Ths (9-10).The new species has an epipod on all Ths, as all species of the genera Billibathynella, Brevisomabathynella, Arkaroolabathynella and Lockyerenella, and some species of Allobathynella and Notobathynella, but the absence of the fi rst and second epipods is common in Asian and Australian genera as in Kampucheabathynella, and even in the third epipod as occurs in some species of Paraeobathynella and Allobathynella.Megabathynella totemensis has several setae on the basipod of all Ths, which is also found in Kampucheabathynella.However, the new species has a large number of setae on the fi rst article (Table 1) of the exopod of all Ths, which makes it diffi cult to see where the second article begins.In other genera, there are few setae, 1 or 2 on each side only.Another exclusive character is the lack of the ubiquitous ctenidia at the base of the setae (which are plumose at the base and barbed elsewhere) of all articles of the exopod that all other species of the family Parabathynellidae show; instead, it presents a strong spine at the base of the outer seta of all articles, at the inner margin.The new species shows a cluster of pairs of strong spinules along the inner margin from the base to the fi rst seta on the second article of the endopod of the Ths; both the second and third articles can have barbed setae on the inner face in a variable number (Tables 1-2); in Brevisomabathynella uramurdahensis, B. magna and some other species of this genus and in Notobathynella octocamura Camacho & Hancock, 2011 there are also barbed setae on the inner face of the second article and some species of Allobathynella have a small seta, as a spinula, but never on the third article as in the new species.
The female ThVIII of the new species is very large with three distal setae as only occurs in Kampucheabathynella khaeiptouka Cho, Kry & Chhenh, 2015.The male ThVIII (Fig. 10C) is distinctive in comparison with all other genera, as it is much larger and more elongated than that of any known species 10).Some examples of male ThVIII in Australian and Asian species are shown in Figs 9 and 10 for comparison.Perhaps the greatest resemblance of this appendage is to the genus Lockyerenella (Fig. 9C) due to the similarities of the long, curved, fi nger-shaped outer lobe and the crestlike protuberance on the internal face of the basipod, smaller than those of Megabathynella totemensis gen.et sp.nov.Due to its elongated appearance, male thoracopod VIII resembles Australian genera (Fig. 9) more than Asian genera (Fig. 10), in which the general appearance is more square.However, the gigantic size and unique morphology of the endopod of the male ThVIII of the new species makes it completely different from all other male ThVIIIs of other species.
The absence of the fi rst pair of pleopods in the new species is quite common in most species of Parabathynellidae.
No species known to date shows such a large number of spines on the sympod of the uropod, nor on the furca as the new species.However, some species of Billibathynella outnumber the new species in the number of setae on the uropod exopod (Table 2).
The unique combination of characters along with the exclusive characters we have listed justifi es the erection of a new genus with similarities to Australian and Asian genera.Elucidating whether the oligomerization of AI and the Th exopods is correlated with size is beyond the scope of this study but will be an interesting question for future comprehensive analyses of the family Parabathynellidae.

Distribution of bathynellaceans of northern Australia
The diversity and distributions of bathynellaceans of the Northern Territory are poorly known, with only a single described species, Atopobathynella readi Cho, Humphreys & Lee, 2005, recorded in the Ngalia Basin, approximately 941 km south-west of the type locality of Megabathynella totemensis gen.et sp.nov.(Fig. 11).However, there are records of a likely new species of possibly Brevisomabathynella from the Cambrian Limestone Aquifer in the Beetaloo Sub-basin (Oberprieler et al. 2021) (Fig. 11).The region has not been extensively sampled for stygofauna and it is likely that new taxa will be collected with further survey.Far west of Totem Pole Cave, near the border with Western Australia, a rich parabathynellid assemblage has been recorded from the eastern Kimberley, with fi ve species of Kimberleybathynella occurring in alluvial and regolith substrata in the Ord River catchment (Cho   et al. 2005), and multiple undescribed species have been collected from environmental impact surveys (Humphreys 1999;Bennelongia 2012).
As new species continue to be discovered and described in Australia and around the world, it is interesting to observe that large and giant species occur on every continent (Table 3) (Fig. 11).Although almost half of them (15 species) have been recorded from Australia, fi ve occur in Europe (Portugal, Spain and France) and fi ve occur in South Korea.One large species has been collected each from Thailand, China, South Africa, USA, Madagascar, Morocco, Cambodia and Japan to date.It is likely that numerous new species will be discovered in these and other countries with further survey of prospective habitats, as is the case in Australia.Future studies could explore the factors that lead to this unusually large size and proliferation of articles and setae observed.It would also be interesting to investigate the paleobiogeographic aspects that have led to this worldwide distribution of genera with large species on all continents when more comprehensive and robust phylogenies based on both molecular and morphological data will have become available.

Rect
Table 2(continued).Character variability in different genera of the 'big' and 'giant' (2.5-4 mm and > 4.0 mm in length respectively) Australian and Asian Parabathynellidae and genera with more than four-segmented exopod of some thoracopods in the world.
MANDIBLES (Figs 3E, 6D-F) (MD).Pars distalis with six well-developed teeth and six small and one triangular strong proximal tooth as in fi gures 3E and 6D; pars molaris (molar process) very big, with row of 18 claws, all strong and denticulate, with two more distal setulose ones, joined basally (Figs 3E, 6E); exceptional two-segmented mandibular palp with seven setae (Figs 3E, 6D, F) not exceeding distal part of Md.

Table 1
Zeiss interference microscope equipped with a drawing tube.Material is deposited in the Museum and Art Gallery of the Northern Territory (MAGNT) and Western Australian Museum (WAM) collections and two specimens of the type series are deposited in the Arthropod Collection of the Museo Nacional de Ciencias Naturales de Madrid (Spain) (MNCN).

Australia Results Systematic account
Camacho & Abrams gen.et sp.nov.