Austinograea chubacarc, Guinot, 2025

Austinograea chubacarc n. sp.

( Figs 3-7 View FIG View FIG View FIG View FIG View FIG ; 8A, B View FIG )

urn:lsid:zoobank.org:act:4946C58F-0337-4C49-8898-CF60271B3265

Austinograea sp. – Boulart et al. 2022: 7, 8,?fig. 5c, e.

Crabs – Hourdez & Jollivet 2023: 74, tables 10, 11.

TYPE LOCALITY. — Woodlark Basin, La Scala vent Field.

TYPE MATERIAL. — Holotype. SW Pacific • ♂ 22.8 × 35.7 mm, right-handed; Woodlark, La Scala vent Field ; PL738 - ASPI2; 9°47.943’S, 155°3.160’E; 3388 m; 27.V.2019; CHUBACARC 2019 cruise; Hourdez det. Austinograea ; Chu-025 writen on carapace; MNHN-IU-2024-6548 . GoogleMaps

Paratypes. SW Pacific • 1 ♂ 6. 3 × 8.1 mm, right-handed, 1 juvenile ♂ 9.8 × 14.6mm, 1 ♀ 8.5 × 12.4 mm; Woodlark, La Scala vent Field ; PL738 - ASPI3 ; 9°47.943’S, 155°3.160’E; 3374 m; 27.V.2019; CHUBACARC 2019 cruise; Hourdez det. Austinograea hourdezi ; MNHN-IU-2024-6549 GoogleMaps • 1 ♀ 13.5 × 19, 3 mm; same data as for preceding; Chu-026 writen on carapace; MNHN-IU-2024-6550 GoogleMaps • 1 ♀ 24.7 × 39.2 mm, with two chelae gaping; Woodlark, La Scala , PL738 - GBT10 ; 9°47.943’S, 155°3.164’E; 3388 m; 27.V.2019; CHUBACARC 2019 cruise CHU-027; MNHN-IU-2024-6092 GoogleMaps • 1 ♀ 28.6 × 45.7 mm, right-handed, but cutter bigger than crusher; same data as for preceding; CHU-028; MNHN-IU-2018-5228 GoogleMaps • 1 ♀ 18.2 × 28.3 mm, slightly right-handed; same data as for preceding; CHU-029; MNHN-IU-2024-6090 GoogleMaps • 1 ♀ 17.7 × 28.3 mm, with two cutters; same data as for preceding; CHU-030; MNHN-IU-2024-6089 GoogleMaps • 1♂ 15.6 × 25.0 mm, right-handed; same data as for preceding; det. Austinograea sp. , CHU-031; MNHN-IU-2024-6036 GoogleMaps • 5 ♀, 1 ♀ 14.6 × 22.2 mm; with two cutters; same data as for preceding; CHU-032; MNHN-IU-2024-6091 GoogleMaps • 1 ♂ 14.0 × 22.4 mm, right-handed; same data as for preceding; Hourdez det. Austinograea ; CHU-033; MNHN-IU-2024-6032 GoogleMaps • 1 ♂ (right-handed) 14.3 × 22.4 mm; same data as for preceding; det. Austinograea ; CHU-034 writen on carapace; MNHN-IU-2024-6022 GoogleMaps • 1 juvenile ♂ 11.8 × 18.3 mm, right-handed; same data as for preceding; Hourdez det. Austinograea sp. ; MNHN-IU-2024-6034 GoogleMaps • 1 ♂ 10.0 × 15.3 mm (without chelipeds); same data as for preceding; MNHN-IU-2018-5227 GoogleMaps .

OTHER MATERIAL. — SW Pacific • 1 ♂ 14 × 21.6 mm, left-handed (gonopods detached); Manus Basin, Pacmanus, Solwara 8, PL733 - ASPI7 ; 3°43.819’S, 151°40.454’E; c. 1750 m; 18.V.2019; CHUBACARC 2019 cruise; MNHN-IU-2024-6086 GoogleMaps • 1 ♂ 10.3 × 15.9 mm, right-handed; MNHN-IU-2024-6088 • 1 ♀ 22.5 × 34.6 mm; slightly right-handed, unreadable numbers writen on carapace; MNHN-IU-2024-6087.

ETYMOLOGY. — The expedition name CHUBACARC 2019 cruise rhymes with one StarWars character; and the specific name chubacarc is an oblique reference to Chewbacca , the furry hero of the popular American Star Wars saga by George Lucas (Lucas film), alluding to the dense setae on the internal surface of the cheliped palm of the species. The name is used as a noun in apposition.

DESCRIPTION

Carapace

Carapace tranversely elliptical, flat. Dorsal surface with regions indistinct, smooth, except for a few granules on frontal and suborbital regions; may have very sparse, extremely short setae, only discernible at high magnification. Front not protuding, straight or slightly convex, bluntly pointed medially (see Variations), with tiny granules. Anterolateral margin marked by rounded edge, slightly granular proximally. Posterolateral margins convergent. Posterior margin slightly concave. Proepistome sunken, very thin. Margin of epistome formed of two minutely granular, moderately concave lobes, with a median projection. Pterygostomial lobe tuberculous; pterygostomial region smooth. Subhepatic regions covered by patches of setae (see Variations).

Cephalic structures

Eyes, antennules and antennae recessed. Suborbital plate absent. Orbit not delimited, only an orbital area that extends as groove lateral to region containing vestigial eyestalks and antennae; outer side of orbital area tuberculated. Eyestalk not moveable, showing as fixed piece fused to floor of orbital region; no cornea, no pigment. Antennules folded horizontally. Antenna inserted in wide supraorbital notch; urinary article fixed, broad, recessed; basal article (2 + 3) cylindrical, moveable, recessed; article 4 slightly elongated, inclined; flagellum short, curved.

Third maxillipeds

Third maxillipeds closing buccal cavity, except anteriorly where there is a gap between them and pterygostomial lobe. Ischium long, with marked longitudinal internal groove. Merus short: external margin entirely and regularly curved, oblique, without marked angle; distal part distinctly narrower and produced; internal margin with proximal half produced as squarish lobe. Carpus inserted in notch of antero-internal margin of merus; propodus thick, developed; dactylus thick and moderately extending beyond articulaion of ischium/merus; inner margins of propodus and dactylus with some brush-like setae. Mxp3 coxa with only proximal portion visible, its lateral projection hidden by junction of thoracic sternum (sternite 4) with pterygostome. Exopod with relatively long flagellum.

Chelipeds

Chelipeds heteromorphic: distinctive heterochely (also in females, see below) and marked heterodonty, with two types of chelae. Major chela (crusher, see below) stout and short, with blunt tips, gaping in large males; minor chela (cutter, see below) narrower, with elongate, more pointed fingers. Ischio-basis glabrous or with potentially very small setal patch. Merus relatively short, triangular in cross section; anterior border sinuous, with strong teeth; merus glabrous in males; in contrast, in females usually each with patch of setae (see Variations). Carpus glabrous on outer part, with patch of setae only along margin articulating on propodus or more developed; surface pitted. Propodus of both chelae having outer surface of palm near base of dactylus either with two coalescing spots of variable dark colour or with a single elongated spot (actually composed of two more or less coalescing spots) and another one rounded forming a depression (see holotype Fig. 3E, G View FIG ), these spots more or less distinct, tending to fade, then disappearing (see Variations, below). Spots absent in females (see below). Inner surface of both chelae and fixed finger with a depressed surface on half posterior part; on internal median part of palm, thick, wellcircumscribed patch of dense light brown setae, extending thinly along occluding margins.

Crusher. Palm convex, inflated, smooth, pitted, with one or two more or less distinct spots, with depressed posterior portion on inner posterior surface. Fingers thick, not gaping at occluding margins, with blunt tips; two main, proximal and median, teeth on occluding margin of dactylus, becoming blunt in large males as holotype; a marked subproximal tooth and two smaller ones on occluding margin of fixed finger, becoming blunt, and only one remaining in large males as holotype. Median surface of thick dactylus of major male chela hollowed out by deep depression extending distally into fissure, becoming thinner and up to subdistal portion of dactylus (see Variations), this character constituting the most distinctive trait of the species. In females, such a depression absent, but more or less replaced along most part of dactylus by thin, interrupted line. In males, dark colour extending over approximately two-thirds of dactylus and most part of fixed finger.

Cutter. Palm elongate, with subparallel borders; surface smooth, except for two spots of different size and shape (one as a depression, like in holotype) near base of dactylus, more or less similar to those of crusher. Both fingers not gaping at occluding margins; dactylus elongate, hollowed, with occluding margin smooth; fixed finger very thick, depressed on inner surface; occluding margin nearly straight, with two or three small teeth, decreasing in size gradually forward, becoming blunt in large individuals. In males, dark colour extending over approximately most part of both dactylus and fixed finger, but sometimes fading.

In females, both chelae either similar and showing as two elongate and thin cutters or slightly heteromorphic and showing as a crusher and a cutter but without significant differences between them. Palm and fixed finger with posterior half part markedly depressed. Fingers very long, narrow, without depression on superior margin of dactylus, sometimes being replaced by thin interrupted line along most of dactylus of crusher; this line sometimes indistinct, for example in female 22.5 × 34.6 mm, MNHN-IU-2024-6087 ( Fig. 7C, D View FIG ), with dark (reddish) colour very marked, located only in subdistal tips of both dactylus and fixed finger. Inner surface of palm with well circumscribed thick patch of dense light brown setae, weakly extending along occluding margins; patches of setae also present on ischio-basis and proximal part of carpus and merus.

Ambulatory legs

P2-P5 shorter than chelipeds; P3 and P4 longest. Ischio-basis, merus and propodus with thick patches of dense setae on ventral margins (thicker on P2 and P3, weak on P5), mixed with sparse longer setae; superior border of carpus and propodus fringed with setae. Dactyli relatively stout, also with patches of short setae.

Thoracic sternum

Thoracic sternum with incomplete sutures 4/5-7/8, separared by short gap; suture 2/3 complete. Median line only along sternite 8. Junction of sternite 4 with pterygostome only represented by short juxtaposition. At level of antero-external portion of sternite 4 and of episternites 5, 6, 7, some setal patches in males, denser in young adult females, and, in larger females, becoming well circumscribed patches with decreasing size from front to back. Press-button of locking mechanism very acute, close to suture 5/6.

Pleon

Male pleon of six free somites and telson, regularly triangular; pleonal somites 1, 2 dorsally exposed; pleonal somite 3 widest; pleonal somite 6 longest; telson triangular, slightly rounded distally. On posterior angles of somite 6, a deep socket delineated by raised, thick margin.

Penis and gonopods

Penis emerging from anterior margin of coxo-sternal P5 condyle. Both G1 crossing subdistally, joining at tips, the latter not lodged in special depression but normally ending in relatively wide extremity of sterno-pleonal cavity at this level. G1: slender, faintly curved, with only short, small spiniform setae arranged in two rows along mid-part only. G2: short, a little less half length of G1, bent to slightly less three-quarters of length at level of small setiferous area; flagellum curved, flattened, bladelike.

Vulvae

Vulvae big, rounded, occupying most of sternite 5, with thick curved membrane on inner side; large median opening.

VARIATIONS

In males, the depression on the superior margin of the major cheliped dactylus is relatively long and deep, both in the right-handed male holotype 22.8 × 35.7 mm (MNHN-IU-2024-6548) ( Figs 3E View FIG ; 8 View FIG ) and in the right-handed paratype male 14.3 × 22.4 mm (MNHN-IU-2024-6022) ( Figs 4E, F View FIG ; 5C View FIG ) from the Woodlark Basin. Only in a small right-handed paratype male 15.6 × 25.0 mm (MNHN-IU-2024-6036), the depression is shallow and short, as if atrophied ( Fig. 6D View FIG ), whereas in the right-handed juvenile paratype male 11.8 × 18.3 mm (MNHN-IU-2024-6034) the depression of the dactylus is already present and as developed as in the holotype. Such a depression is absent in female chelae ( Fig. 7A, C View FIG ).

The frontal margin is straight or slightly bluntly pointed medially ( Figs 3A View FIG ; 4A, B, E, F View FIG ; 5A View FIG ).

The degree of setation varies: it is more developed in females, particularly on the subhepatic area and in the latero-anterior angles of sternite 4; there are usually patches of setae on the ischio-basis and carpus of the chelipeds of females and, at less extent, in large males such as the holotype ( Fig. 3C View FIG ); but the merus is glabrous in males ( Figs 3C, I View FIG ; 4C View FIG ). The setal patch on the suborbital margin is regularly dense (such as in the holotype, Fig. 3B View FIG ) or denser medially than externally.

The spots on the outer surface of the male palm are variable but usually present in Austinograea chubacarc n. sp. from the Woodlark Basin, whereas spots are absent on the chelae of females ( Fig. 7C, E View FIG ). In the right-handed holotype ♂ 22.8 × 35.7 mm (MNHN-IU-2024-6548) ( Fig. 3E View FIG ), the palm of the major chela bears two dark spots, an elongated one with a depression near the base of the dactylus and a rounded one in a depression at its upper middle, whereas the minor chela has two coalescent spots, the upper one with depression ( Fig. 3G View FIG ). Only a trace, difficult to perceive (and not really visible in the photograph), can be guessed on the minor chelae of the right-handed male 14.3 × 22.4 mm (MNHN-IU-2024-6022) ( Fig. 4C, E, F View FIG ). In the right-handed male paratype 14.3 × 22.4 mm (MNHN-IU-2024-6036) (with an atrophied depression), two spots are visible on the palm of the minor chela, whereas only one is clearly visible and a second one barely discernible on the palm of the major chela ( Fig. 6D View FIG ). In A. chubacarc n. sp. from the Manus Basin, only one spot is clearly visible on each chela of the left-handed male 14 × 21.6 mm (MNHN-IU-2024-6086) and in the right-handed ♂ 10.3 × 15.9 mm (MNHN-IU-2024-6087).

REMARKS

Austinograea chubacarc n. sp. differs from all known species of Austinograea in that the movable finger – the dactylus– of the male major chela has a long, deep depression (it can also be defined as a ‘wide groove’) on its outer superior border ( Figs 3E View FIG ; 4E, F View FIG ); this depression is absent in females. Such a deeply grooved dactylus is an unusual, unique feature that we have never observed in other brachyuran crabs, and its function remains unclear. It is a singular species character admittedly but it is probably not that weird. Many crabs do have the outer surface of the dactylus lined with narrow grooves and, in some cases, there is a dorsal crest that highlights the longitudinal groove. Are these structures homologous?

Interestingly, the depression on the dactylus of A. chubacarc n. sp. can be discerned on the major palm of certain crabs seen in situ from video images ( Fig. 8B View FIG , close-up).

The overall shapes and occlusal geometries of the cheliped dactylus (which probably correspond strongly with diet) and its micro- and ultrastructure have been described in detail, with each cheliped having a range of mechanical advantages at its disposal and a diversity in occlusive design (e.g. Brown et al. 1979: 157; Masunari et al. 2015, 2020). Agnew (2008), who investigated the evolutionary patterns of the dactylus in decapod crustaceans, including the fossils (where the mobile finger is often the only represented remnant in fossil assemblages), and Fujiwara & Kawai (2016), for whom the decapod chela is a suitable structure to analyse the evolutionary process of functional diversifications, do not mention the existence of such a curious feature (namely the wide groove on the dactylus) that cannot alter in any way – apparently – the crushing, chipping, gripping and pinching abilities of the dactylus.

Austinogeaea chubacarc n. sp. has fairly strong chelae in both males ( Figs 3 View FIG A-C, E-H; 4A, C E, F; 5C-F; 6D) and females ( Fig. 7 View FIG A-F), admittedly, without any teeth and complex armouring; however, it seems clear that chelae are used for competition and for foraging. Chelipeds, sexually dimorphic, also play a role in reproduction: although they are much less robust in males than in females, they hold the female firmly during mating (see below, Fig. 8C View FIG ).

In Austinograea chubacarc n. sp., the outer surface of the palm may bear, near the base of the dactylus of the major and minor chelae, more-or-less coloured spots: either very dark and partly in the form of depressions, as in the male right-handed holotype 22.8 × 35.7 mm ( Fig. 3E, G View FIG ) and paratype 15.6 × 25.0 mm ( Fig. 6D View FIG ); or superficial and tending to fade or even disappear, as for example in the right-handed paratype male 14.3 × 22.4 mm ( Fig. 4E, F View FIG and see Variations). There are also two characteristic depressions on the propodus of male chelae, crusher and cutter, of A. hourdezi View in CoL ( Guinot & Segonzac 2018: figs 3A; 4A, B; 5E, G), similarly located in all the individuals examined, variously delineated by a raised margin and appearing fairly deep or shallower, and almost always coloured, appearing as dark spots. As in A. chubacarc n. sp., the spots are absent in the females of A. hourdezi View in CoL ( Guinot & Segonzac 2018: fig. 7E, G).

Austinograea chubacarc n. sp. shares with A. hourdezi View in CoL the following characters: setal patches on the inner surface of the palm of the major and minor chelae in both sexes, on the subhepatic region and on the inner surface of the legs; triangular male pleon. Austinograea chubacarc n. sp. and A. hourdezi View in CoL differ in the following characters: G1 slightly curved in A. chubacarc n. sp., straighter in A. hourdezi View in CoL ; flagellum of G2 proportionally shorter in A. chubacarc n. sp. (see Fig. 6 View FIG A-C); merus of mxp3 shorter in A. chubacarc n. sp. ( Figs 4B, F View FIG ; 5A, C View FIG ); merus of the chelipeds cylindrical and weakly toothed in A. hourdezi View in CoL ( Guinot & Segonzac 2018: fig. 5A), instead of being shorter and with marked teeth in A. chubacarc n. sp. ( Fig. 3A, B View FIG ); thoracic sternum completely glabrous in male A. hourdezi View in CoL ( Guinot & Segonzac 2018: fig. 5C, D), whereas it has tufts of setae on the antero-external portion of sternite 4 and at level of episternites 5, 6, 7 in male A. chubacarc n. sp. ( Figs 3I View FIG ; 4D View FIG ); ischio-basis and carpus of chelipeds may bear small setal patches in males and females of A. chubacarc n. sp. ( Figs 3C View FIG ; 7G View FIG , respectively), but not in A. hourdezi View in CoL .

In Austinograea chubacarc n. sp. the tips of the G1s, crossing subdistally, normally end in the relatively wide extremity of the sterno-pleonal cavity at this level, instead to end in the extremity of the sterno-pleonal cavity that is narrower at this level in A. hourdezi View in CoL ( Guinot & Segonzac 2018: figs 4C, 6C). These two dispositions differ significantly from that of A. alayseae View in CoL , in which Guinot (1990: 884, 891, fig. 2C) described the extremities of the G1 as joining in a small deep, well-delineated depression (d) at the narrow end of the sterno-pleonal cavity. In A. jolliveti , the morphology is quite different:the G1s are directed obliquely, separated even at their tips, and the sterno-pleonal cavity is not excavated distally, without a small special distal depression to receive the distant G1 tips (see Guinot & Segonzac 2018: figs 9B-D; 11C-E). Austinograea chubacarc n. sp. can be distinguished from A. williamsi View in CoL by the G1 ( Fig. 6A, B View FIG ) that is more curved than that, straighter, of A. williamsi View in CoL (see Hessler & Martin 1989: fig. 14a, c, d; Tsuchida & Fujikura 2000: figs 6; 7A; Tsuchida & Hashimoto 2002: fig. 8). In A. rodriguezensis Tsuchida & Hashimoto, 2000 View in CoL , the G1s are nearly straight ( Tsuchida & Hashimoto 2002: fig. 8), but it is not known whether or not they meet at their tips: we can predict that they are distant. Note that in Gandalfus puia View in CoL and G. yunohana View in CoL the tips of the G1s reach the extremity of the sterno-abdominal cavity that is relatively wide at this level.

The G2 of A. chubacarc n. sp. ( Fig. 6B, C View FIG ) is short, a little less half the length of G1, bent at a slightly less three-quarters of the length.

The most diagnostic feature of A. chubacarc n. sp. remains the depression on the cheliped dactylus of males ( Figs 3E View FIG ; 4E, F View FIG ).

The larval morphology may in the future confirm the distinction of Austinograea chubacarc n. sp. It was carried out in certain vent species from the Indian Ocean and the western Pacific: megalopa of Austinograea rodriguezensis from the Indian Ocean ( Demidow et al. 2021); 16 pre-zoeal stages and first zoeal stage of A. rodriguezensis from Kairei Hydrothermal Vent field ( Kihara et al. 2024), with a welldeveloped spinature on carapace and plenoal somites, not only attributed to defence from predators but considered an effective adaptation that helps in the dispersal of the zoeal stages to maintain the resident population; first zoeal stage of Gandalfus yunohana ( Nakajima et al. 2010) ; and three zoeal stages and megalopa of the same species ( Hamasaki et al. 2010), with larval data supporting the recognition of Austinograea and Gandalfus as two distinct clades comprising A. rodriguezensis and G. yunohana , respectively ( Kihara et al. 2024).

A phylogenetic analysis based on the CHUBACARC material was published by Chabert (2021: 26, 27, 32, table 2) for her master thesis. We support the Chabert’s result ( Chabert 2021: 11, 26, 32, table 2) that the specimens from the Woodlark Basin identified as ‘Gen. sp. B’ would be a new species: they could well belong to A. chubacarc n. sp.