Esopus crassus A. Milne-Edwards, 1875

Figs. 1 – 4

Esopus crassus A. Milne-Edwards, 1875: 91, pl. 17, fig. 1–lc.—A. Milne-Edwards 1880: 2.—A. Milne-Edwards & Bouvier 1923: 389.— Rathbun 1925: 192, pl. 222, figs. 10–12.— Chace 1940: 61, fig. 21A, B.— Ng et al. 2008: 100.— Carmona- Suárez & Poupin 2016: 384.— Poupin & Corbari 2016: fig. 15k.— Poupin 2018: 184, fig. 198.— Guinot 2019: 783, 787, figs. 19A, B, 20.

Type data. Holotype: Female 9.0 × 15.0 mm (MCZ) [Curiously, although “the species is known by the unique type”, Rathbun (1925: 192) provided for the female holotype a significantly higher size, 13.0 × 8.4 mm, than that provided by A. Milne-Edwards (1875: 91), i.e. the above-mentioned measurements.

Type locality. Lesser Antilles of the West Indies, off the Barbados, Sandy Bay, 183 m, “Hassler” Expedition led by L. Agassiz in 1871–1872, from Cambridge to San Francisco, California, and around coasts of South America.

Material examined. One ovigerous female 11.0 × 6.7 mm, Guadeloupe, W Marie-Galante, KARUBENTHOS 2, st. DW4586, 15°59.62’N, 61°22.51’W, 251– 204 m, 21 June 2015, with some large eggs remaining from a recent egg-laying, MNHN-IU-2013-19002 (Figs. 2, 3); one male 9.1 × 5.5 mm, Guadeloupe, W Marie-Galante, KARU- BENTHOS 2, stn DW4592, 15°58’N, 61°22’W, 201–214 m, 22 June 2015, MNHN-IU- 2016-2530; one male 9.1 × 5.4 mm, one female 11.3 × 6.9 mm, Guadeloupe, W Marie-Galante, KARUBENTHOS 2, stn DW4597, 15°56’N, 61°23’W, 208–210 m, 22 June 2015, MNHN-IU- 2016-2350; 2 ovigerous females 10.5 × 6.2 mm, 10.7 × 6,4 mm, Guadeloupe, S Marie-Galante, KARUBENTHOS 2, st. DW4637, 16°52’N, 61°20’W, 217-225 m, 28 June 2015, MNHN-IU- 2016-2562; 3 ovigerous females 10.7 × 6.6 mm, 10.7 × 6.4 mm, 10.4 × 6.3 mm, one female 10.5 × 6.3 mm, Guadeloupe, S Marie-Galante, KARUBENTHOS 2, st. DW4638, 15°50.29’N, 61°19.47’W, 312– 305 m, 28 June 2015, MNHN-IU-2013-19150; one male 10.8 × 6.9 mm (photographed Fig. 4), MNHN-IU- 2019-2552; 2 males 10.5 × 6.4 mm, 10.4 × 6.6 mm, 6 females (3 ovigerous), 12.2 × 7.9 mm, 12.1 × 7.4 mm, 12.0 × 7,4 mm, 11.8 × 7.0 mm, 11.5 × 6,9 mm, 10.9 × 6.6 mm, Guadeloupe, S Marie-Galante, KARUBENTHOS 2, st. CP4624, 15°57’N, 61°32’W, 242–243 m, 26 June 2015, MNHN-IU-2016-8345; one female 10.5 × 6.3 mm, MNHN-IU- 2016-2571; one female 11.1 × 6.6 mm, Guadeloupe, S Marie-Galante, KARUBENTHOS 2, stn DW4645, 15°52’N, 61°20’W, 208–210 m, 29 June 2015, MNHN-IU- 2016-2631; one female 8.9 × 5.4 mm, Guadeloupe, E Desirade, KARUBEN- THOS 2, st. DW4560, 16°25’N, 60°52’W, 185–250 m, 16 June 2015, MNHN-IU- 2016-2061; one ovigerous female 11.1 × 6.8 mm, Guadeloupe, E Desirade, KARUBENTHOS 2, st. CP4569, 16°17.25’N, 61°01’W, 250–359 m, 17 June 2015, MNHN-IU-2013-18951 (Fig. 1, crab photographed on board).

Redescription Carapace narrow, much longer than wide, gibbous (Fig. 1). Cuticle very thick, hard. Lateral margins rounded, without spines. Dorsal surface as blistered, sculpted, with conspicuously inflated regions delineated by several deep, marked grooves, entirely covered by pronounced rounded granules; no spines; only some setae; four main distinct areas: frontal (tripartite, posteriorly limited by deep depression), gastric (elevated, divided in prominent regions: protogastric, mesogastric, metagastric, urogastric), cardiac (swollen), intestinal (flat). Cervical groove widely U-shaped, medially interruped. Cardiac region prominent; intestinal region large, flat; branchial region weakly subdivided. Dorsal surface and most part of ventral surface of body covered by rounded, almost confluent granules. Exposure of latero-external portions of pleurites 5–8 on same level as carapace, calcified and ornamented like dorsal carapace surface, forming wide collar all around posterolateral margins of pleural walls, and including wide and dorsal first (male and female) pleonal somite, similarly ‘integrated’ to carapace (Fig. 2A). Portion in front of eyes delimited by deep transversal depression; interorbital space divided into three nearly equal lobes by two longitudinal depressions. Front broader than long, formed by large, rounded, tuberculiform prominence, with two tiny spines (hardly noticeable) on each side; folded ventrally. Rostrum wide, blunt in dorsal view, strongly deflexed, protruded downward as narrow, granulate beak-shaped plate (?proepistome) joining long and slender anterior process of epistome (Figs. 1, 2B, 3A, 4A). Antennular fossae very narrow; antennules folded almost longitudinally in closed fossae. Antenna: article 1 (urinary) with rounded urinary orifice; basal article (articles 2+3) adjacent to article 1 coalescent with rostrum and epistome, very large, markedly prominent, extending well beyond eyes, without keel, covered with rounded granules; distal and disto-lateral portions visible dorsally; each basal article expanding medially on epistome by two large, rounded tubercles and thus close to the corresponding article of other side; following antennal articles proportionally very small in size; articles 4, 5 free; flagellum very short (Figs. 3A, 4A). Preorbital tooth absent. Ocular peduncle short, stout, folding into a fossa, i.e. incomplete orbit, formed by frontal edge and subhepatic region. Postorbital tooth thick, fused to rounded subhepatic region and cup-shaped to receive eye. Branchiostegite extremely reduced. Constriction of carapace marked ventrally, at level of epistome. Epistome rather small, recessed, forming posteriorly narrow raised wall, with straight margin. Buccal cavity wide. Milne-Edwards openings separated from chelipeds, large, entirely filled by developed mxp3 coxa (Figs. 3A, 4A, B). Mxp3 flat, completely covering buccal frame; merus slightly dilated outward and forward, slightly notched on inner side for insertion of palpus; crista dentata with small teeth and row of setae. Male chelipeds very long, narrow, usually more than twice as long as carapace; large gap between fingers, near base (Chace 1940: fig. 21B). Female chelipeds shorter, less than length of carapace; fingers not gaping (Fig. 1); in females, fingers proportionally much shorter than palm compared to male fingers and palm. Ambulatory legs very slender, rather long, without any spines, with short setae; merus without spine(s) on dorsoproximal margin; dactyli very thin, long, not subchelate, applying along propodi; P2–P5 dactyli with longer setae. Female sterno-pleonal cavity deeply hollowed. Female pleon formed of four somites, somites 5–6 coalescent with pleotelson, swollen, forming large disc limited by high sternal ridge, and without spine or tooth (Figs. 1, 2A, 3B); pleonal somites 2, 3 and portion of 4 visible dorsally. Male pleons composed of five free somites plus pleotelson (somite 6 plus telson), first somites dorsally exposed (Fig. 4A, B). Male thoracic sternum markedly wide, with strong granular ridges and depressions, continuing in smooth condition inside sterno-pleonal cavity; anterior shield inserted between mxp3 as broadly triangular, inflated plate, separated by depression from following sternites: sternites 1–3 fused without visible demarcation in males (Fig. 4A, B) (Chace 1940: fig. 21A), as small, narrow, seemingly undivided piece in front of sternal wall in ovigerous females (Fig. 3B); suture 3/ 4 in depression; sternite 4 not inflated; sutures 4/5–7/8 interrupted, located in depressions, continuing on sides of sterno-pleonal cavity. Deep, small depression at base of sternite 8, instead of median line. Sternum/pterygostome junction complete thanks to curved anterior extensions of sternite 4. Wide sternal extensions joining exposed pleurites (sternum/pleurites connections) between P1/P2, P2/P3, P3/P4, and P4/P5 (Fig. 4B). Female thoracic sternal sutures obliquely directed forward. Vulvae anteriorly displaced, with protruded openings on forward projected sternite 6, anteriorly to suture 4/5 (Fig. 3C). G1 slightly curved, with broadened half proximal portion, ending in distal lobe with large aperture (Fig. 4C); G2 very small. Ovigerous females with few large eggs.

Remarks. Esopus, which is monotypic, shares the synapomorphies that support the family Inachoididae, i.e. the typical inachoidid organisation, with the exposure of pleurites and pleon significantly increasing the surface area of the “carapace” and the edge of the true carapace in a setting gutter (Figs. 1, 2A). But Esopus differs from other inachoidids by a unique combination of characters: body particularly thicker; dorsal surface strongly lobulated and grooved (Fig. 1); disposition of rostrum and proepistome; antennae (Figs. 2C, 3A, 4A) (see also the excellent figure by A. Milne-Edwards 1875: pl. 17 fig. 1c; reproduced by Rathbun 1925: pl. 22, figs. 10–12); short eyestalk included in an incomplete orbit, posteriorly bordered by thick, curved, cup-shaped postorbital tooth fused to subhepatic region (Fig. 2B, C); sternum/pterygostome junction complete, due to curved extension of sternite 4; Milne-Edwards openings separated from chelipeds, large, entirely filled by mxp3 coxa; thoracic sternum with thick granular ridges (Fig. 4A, B) between the sutures; G1 with distal lobe (Fig. 4C). These features are sufficient to introduce for the genus a new subfamily, Esopinae subfam. nov., within the Inachoididae whose characters we will briefly review below.