Eutrichopagurus crassoculus, Osawa & Fujita, 2022

Osawa, Masayuki & Fujita, Yoshihisa, 2022, A new species of the genus Eutrichopagurus Komai, 2015 (Crustacea: Decapoda Anomura: Paguridae) from a submarine cave in the Ryukyu Islands, southwestern Japan, Zootaxa 5214 (3), pp. 440-450 : 441-447

publication ID	https://doi.org/ 10.11646/zootaxa.5214.3.6
publication LSID	lsid:zoobank.org:pub:3FE60779-E207-433F-B827-4C4D68C17432
DOI	https://doi.org/10.5281/zenodo.7400374
persistent identifier	https://treatment.plazi.org/id/03AB8793-FF92-AB67-FF3B-FED0FE88F8B0
treatment provided by	Plazi
scientific name	Eutrichopagurus crassoculus
status	sp. nov.

Treatment

Eutrichopagurus crassoculus View in CoL n. sp.

[New Japanese name: Kurayami-yadokari]

( Figs. 1–4 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 )

Type material. Holotype: RUMF-ZC 6115 (HD 2020-56), male (sl 1.2 mm), Hedo Dome (= Ginama) cave, Kunigami, Okinawa Island , Ryukyu Islands , zone 2, 3–5 m depth, 24 September 2020, SCUBA, collected by Y. Fujita & T. Fujii . Paratype: RUMF-ZC 6116 (HD 2020-54), 1 male (sl 1.2 mm), same data as in holotype .

Description. Eleven pairs of deeply quadriserial gills.

Shield ( Fig. 1A, B View FIGURE 1 ) subtriangular in general outline, 1.1–1.2 times as long as wide; anterior margin between rostrum and lateral projections concave; anterolateral margins sloping; posterior margin roundly truncate, slightly concave on median part; dorsal surface slightly convex transversely, with 1 median, 1 submedian, and 1 lateral pairs of few short setae on anterior half. Rostrum triangular, terminating in acute tip, distinctly overreaching lateral projections. Lateral projections obtusely triangular, each with short marginal spine. Posterior carapace ( Fig. 1A View FIGURE 1 ) with lateral lobes very narrow. Cardiac sulci not discernible. Sulci cardiobranchiales extending to posterior carapace margin, converging posteriorly. Posteromedian and posterolateral plates poorly calcified; branchiostegites membranous, almost glabrous.

Ocular peduncle (including cornea) ( Fig. 1B, C View FIGURE 1 ) stout, about half-length of shield, widest at about midlength, with sparse tufts of setae distally on each dorsomesial surface; cornea greatly reduced, rounded on distal margin, much narrower than peduncle; lateral surface strongly convex on proximal half and concave and somewhat decalcified on distal part including cornea (in preserved condition, showing strongly concave appearance, but in fresh condition, only weakly concave; Fig. 4A View FIGURE 4 ); mesial surface flattish, with bluntly delimited dorsomesial margin. Ocular acicle ovate, with small submarginal spine reaching proximal 0.2 of ocular peduncle; dorsal surface flattish. Interocular lobe separated from ocular acicles, overhung by rostrum, visible in dorsal view; anterior surface slightly convex.

Antennular peduncle ( Fig. 1B, D View FIGURE 1 ) long, when fully extended, overreaching distal corneal margin by full length of article 3 and distal one-third of article 2. Article 3 3.2 times as long as high, approximately 2.1 times as long as article 2, somewhat widened distally, with prominent tuft of long setae (length subequal to that of dorsal flagellum) on dorsodistal margin. Article 1 with distolateral margin not markedly produced; statocyst lobe slightly inflated, with small spine on lateral face. Dorsal flagellum 1.4 times longer than peduncular article 3, consisting of short aesthetasc-bearing proximal portion (about 10 articles) and long distal portion (about 10 articles, about 2.8 length of aesthetasc-bearing portion); ventral flagellum about half length of dorsal flagellum, consisting of about 10 articles.

Antennal peduncle ( Fig. 1B, E View FIGURE 1 ) overreaching distal corneal margin by nearly full length of article 5. Article 5 slightly flattened, with few setae marginally. Article 4 with few short setae ventromesially. Article 3 unarmed but with oblong capsule-like structure and few short setae on ventromesial distal margin. Article 2 dorsolateral distal angle produced, barely reaching midlength of article 4, terminating in simple spine, bearing few distal setae; dorsomesial distal angle with small spine; ventromesial margin with few short setae. Article 1 with minute distal spine on lateral surface; ventromesial distal margin somewhat produced anteriorly, but without armature. Antennal acicle overreaching distal corneal margin by distal one third length and reaching midlength of peduncular article 5, terminating in small spine; dorsomesial margin with row of sparse setae. Antennal flagellum ( Fig.1F View FIGURE 1 ) 3.1–3.3 times shield length, overreaching extended right cheliped; each article with 0–3 short setae on distal margin.

Mouthparts not dissected. Maxilliped 3 ( Fig. 1G, H View FIGURE 1 ) moderately stout; carpus and merus unarmed; ischium with crista dentata composed of about 10 small closely set corneous teeth and 1 accessory tooth, dorsal margin with minute distal spine; basis-ischium fusion incomplete; basis with few minute denticles on ventromesial margin; exopod reaching slightly short of distal margin of carpus.

Chelipeds ( Fig. 2A, B, E, F View FIGURE 2 ) subequal in length, dissimilar with right much more robust than left. Right cheliped ( Fig. 2A–D View FIGURE 2 ) moderately stout, moderately large compared to body. Chela elongate subovate in dorsal view, 1.8 times as long as wide (greatest width at midlength of palm), 1.7 times longer than carpus; dorsomesial margin somewhat narrowing distally; dorsolateral margin gently arched. Dactylus 0.7 length of palm, weakly curved in dorsal and mesial views, with sparse tufts of setae on surfaces, terminating in small corneous claw; dorsal and mesial surfaces proximally with sparse granules, dorsomesial margin not delimited; occlusal margin with 3 subacute calcareous teeth on submedian part and row of small corneous teeth on distal concaved part. Palm with sparse short to moderately long setae and covering of coarse granules on surfaces, granules on fixed finger sparse; dorsomesial margin delimited by row of short granular ridges; dorsolateral margin weakly delimited; dorsal and ventral surfaces gently convex. Fixed finger terminating in small corneous claw; occlusal margin with low rounded calcareous tooth proximally and prominent roundly triangular calcareous tooth subdistally, concaved distal part with row of small corneous teeth. Carpus slightly widened distally in dorsal view, 1.4–1.5 times longer than distal width, 0.9 length of merus, bearing sparse bristle-like setae on dorsomesial and dorsomesial margins and short transverse ridges and granules dorsally on mesial and lateral surfaces; few short setae on ventral surface; all surfaces with coarse granules, fewer on dorsal surface; dorsomesial margin with 3 or 4 distal spines followed by some short granular ridges; dorsolateral margin with blunt distal projection followed by some short granular ridges; mesial surface dorsally with 2 distal spines and few short granular ridges, distomesial margin granular; lateral surface with few short granular ridges dorsally and distal small spine ventrally, distolateral margin with row of coarse granules; ventral surface convex. Merus with very low, short transverse ridges and some bristle-like and simple setae on dorsal surface, dorsodistal margin unarmed; mesial surface with coarse granules and few distal bristle-like setae ventrally, ventromesial margin with 2–4 distinct distal spines followed by some short ridges or minute spines; lateral surface with few short setae and short granular ridges and single granules ventrally; ventrolateral margin 3 spines on distal half followed by some short ridges; ventral surface slightly convex on proximal half, with coarse granules. Ischium with sparse setae on dorsodistal margin and ventral surface; ventromesial margin unarmed. Coxa laterally with minute distal spine.

Left cheliped ( Fig. 2E–H View FIGURE 2 ) slender. Chela 2.7–2.9 times longer than wide (greatest width at midlength of palm), 1.3–1.5 times longer than carpus; dorsolateral margin weakly undulate on distal two-thirds Dactylus 1.5 times longer than palm, gently curved distally in dorsal view, terminating in small corneous claw; surfaces with sparse setae, dorsal surface proximally with coarse granules, otherwise nearly smooth; mesial surface proximally with arc of granules, dorsomesial margin not delimited; occlusal margin with row of minute corneous teeth (teeth on proximal half widely to narrowly spaced, those on distal half contiguous). Palm with sparse setae on dorsal and ventral surfaces; dorsal surface convex, elevated with some proximal subacute or spinulose tubercles on midline, covered with minute granules, dorsomesial and dorsolateral margins each delimited by row of small tubercles; lateral surface covered with granules; ventral surface gently convex, nearly smooth. Fixed finger distinctly curved distally in dorsal view, with scattered setae on dorsal and ventral surfaces, terminating in small corneous claw; occlusal margin bordered with thin corneous plate. Carpus slightly widened distally in dorsal view, 2.8–3.2 times longer than distal width, 0.9 length of merus; dorsolateral and dorsomesial margins with 4 (1 distal and 3 median) and 2 distal spines followed by few short ridges, respectively, spines and short ridges bearing bristle-like and simple setae; mesial surface smooth, with few bristle-like setae dorsally and distally; lateral surface with some scaley ridges medially, distolateral margin with row of small tubercles and few short bristle-like setae; ventrodistal margin with 2 small distal spines; ventral surface convex, with few very short setae. Merus with row of short ridges bearing few bristle-like and simple setae on distal half of dorsal surface, dorsodistal margin unarmed; mesial surface smooth, with few very short setae ventrally, ventromesial margin with row of 4 or 5 small spines and terminal bristle-like setae on distal half and with some short somewhat elevated ridges and bristle-like setae on proximal half; lateral surface with sparse short ridges bearing short setae, ventrolateral margin with 2 or 3 small spines on distal one third followed by some short somewhat elevated ridges; ventral surface flattish, bearing few short setae. Ischium unarmed, with few setae. Coxa laterally with minute distal spine.

Ambulatory legs ( Fig. 3A–D View FIGURE 3 ) moderately long and slender; surfaces mostly smooth. Dactyli 1.0–1.1 times as long as propodi, in dorsal view slightly curved mesially, in lateral view slightly curved ventrally; dorsal margins each with irregularly arranged row of setae, setae on distal one-third more closely set than on those on remaining part; lateral and mesial faces almost glabrous; ventral margins each with row of 3 or 4 (second) and 5 or 6 (third) slender corneous spines. Propodi distinctly longer than carpi, slightly narrowing distally, 5.0–5.1 times as long as maximum high; dorsal surfaces each with sparse setae; lateral and mesial faces glabrous; ventral margins each with 1 or 2 slender corneous spines at distal end and 1 or 2 corneous spines on median one third. Carpi unarmed, with sparse setae on dorsal surfaces; ventrodistal margin unarmed. Meri 3.6–3.8 (second) and 3.0–3.9 (third) times as long as maximum high, with sparse setae on dorsal surfaces; lateral surfaces almost glabrous; ventral margins with 1–3 small spines on posterior half (second) and unarmed (third). Ischia unarmed, with sparse tufts of setae distally on dorsal and ventral margins.

Pereopod 4 ( Fig. 1I View FIGURE 1 ) semichelate. Dactylus slightly curved. Propodus with rasp consisting of single row of corneous scales. Carpus unarmed on dosrsal surface. Dactylus to merus with sparse tufts of setae on dorsal surfaces, dorsodistal tuft of setae on merus prominent.

Pereopods 5 chelate. Coxae ( Fig. 1J–L View FIGURE 1 ) asymmetrical; right coxa with long, filamentous sexual tube (about 4.7 times of coxal length) directed posterolaterally and recurved toward anterior between dorsal surface of ischium and base of pleon, with hooked part on about distal one third, prominent tuft of setae at base of sexual tube; left coxa with very short sexual tube, directed mesially and tapering distally.

Thoracic sternite 6 anterior lobe ( Fig. 1M View FIGURE 1 ) transversely ovate, with pair of short setae anterolaterally. Thoracic sternite 8 ( Fig. 1J View FIGURE 1 ) consisting of 2 contiguous lobes.

Pleon dextrally twisted. Male with unpaired, unequally biramous left pleopods 3–5.

Uropods markedly asymmetrical, protopods unarmed.

Telson ( Fig. 1N View FIGURE 1 ) with lateral indentations at posterior one-third; posterior lobes subtriangular, terminal margins oblique, each with 3–5 minute acute or blunt spines, posterolateral margins each with fringe of long setae.

Coloration. In fresh condition, body and appendage are entirely translucent white with pale brown tint ( Fig. 4 View FIGURE 4 ).

Distribution. So far only known from the type locality, Okinawa Island (Ryukyu Islands, southwestern Japan).

Habitat. The two type specimens were obtained from the area between “main tube” zone and “innermost part” zones in Hedo Dome cave (cf. Osawa & Fujita 2019), having an anchialine environment with salinity cline. They each carried a shell of gastropod referred to an unidentified species of the genus Collonista Iredale, 1918 ( Fig. 4B; H View FIGURE 4 . Kubo, personal communication). Numerous empty and living shells of that species were found in the same environment of the cave.

Prior to this study, hermit crabs found in Hedo Dome cave included 12 species ( Osawa & Fujita 2019): Aniculus retipes Lewinsohn, 1982 ; Paguristes jalur Morgan, 1992 ; Pseudopaguristes shidarai Asakura, 2004 ( Diogenidae , three species); Catapaguroides foresti McLaughlin, 2002 ; C. longior Komai, Yamada & Shirakawa, 2010 ; Pagurixus longipes Osawa, Fujita & Okuno, 2006 ; P. nomurai Komai & Asakura, 1995 ; Pylopaguropsis bellula Osawa & Okuno, 2007 ; P. fimbriata McLaughlin & Haig, 1989 ; P. granulata Asakura, 2000 ; P. keijii McLaughlin & Haig, 1989 ; and P. speciosa McLaughlin & Haig, 1989 ( Paguridae , nine species). The present new species represents the 13th hermit crab species found in the cave. Among these species, only Catapaguroides foresti , C. longior , and Pylopaguropsis granulata occurred in “silty room” zone leading to “innermost part” zone of Hedo Dome cave. The other species were found in “main tube” zone and the present new species is obtained from the area adjacent to “innermost part” zone.

Etymology. The specific name is derived from the combination of the Latin, crassus (= stout) and oculus (= eye), in reference to the inflated proximal portion of the ocular peduncle of the new species. Used as a noun in apposition.

Remarks. In the preserved condition, the ocular peduncles of both the holotype and a paratype available for study have a strongly concave appearance on each distolateral margin including the cornea. Nevertheless, this condition is likely due to shrinkage by ethanol preservation because the ocular peduncles of the fresh specimens are seemingly only weakly concave on that margin ( Fig. 4A View FIGURE 4 ). Meanwhile, the two type specimens have an oblong capsule-like structure on the ventromesial distal margin of each of the left and right antennal article 3 ( Fig. 1E View FIGURE 1 ). Similar capsulelike structures have been reported in Trichopagurus asper Komai & Poupin, 2012 and T. tenuidactylus Komai, 2013 ; such several structures are present on the ventral surfaces of both cheliped meri and only a single structure is present on the ventromesial margin of the right cheliped merus in the two species, respectively. Komai & Poupin (2012) considered they possibly represent egg sacs of other invertebrates or parasites, but it is unknown if the capsule-like structures are identical among Komai & Poupin’s (2012), Komai’s (2013), and the present specimens, and if they might be actually external ornamentation belonging to the hermit crabs.

In their description of Eutrichopagurus granulimanus Komai, Rahayu & Fujita, 2021 from Thunderdome Cave, a submarine cave of Christmas Island in the southeastern Indian Ocean, Komai et al. (2021) discussed the supposed morphological distinctions between the genera Eutrichopagurus and Cestopagurus Bouvier, 1897 . As in Komai et al.’s (2021) species, the assignment of the present new species to the genus Eutrichopagurus is considered provisional, because only the male specimens were available for study, and the original diagnosis of the genus was based only on female characters. The present new species has diagnostic characters of Eutrichopagurus including eleven pairs of deeply quadriserial phyllobranchiate gills, a prominent tuft of setae on the dorsodistal margin of the article 3 of the antennular peduncle, and the posterolateral margins of the telson each bearing a tuft of long setae (cf. Komai 2015; Komai et al. 2021). The right sexual tube of the new species is directed posterolaterally and filamentous in shape as described in E. granulimanus , whereas that of E. shirakawai Komai, 2015 , the type species of the genus and described from shallow subtidal waters around Okinawa Island, Ryukyu Islands, is unknown because no male of the species has been recorded.

Eutrichopagurus is also substantially similar to Trichopagurus de Saint Laurent, 1968 in having the strongly produced triangular rostrum, a distinct tuft of setae on each posterolateral margin of the telson and the male right sexual tube being directed posterolaterally, but the deeply quadriserial, instead of biserial, gills distinguish the former from the latter (cf. Komai & Osawa 2005; Komai & Poupin 2012; Komai 2013, 2015).

Among two known species of the genus Eutrichopagurus , E. crassoculus n. sp. may be closer to E. shirakawai Komai, 2015 in having reduced corneas basally bearing sparse setae and no small corneous spines or bristle-like setae on the dorsal surface of the meri of the ambulatory legs, but is distinguishable by the following numerous characters. The proximolateral margin of each ocular peduncle is strongly convex in E. crassoculus n. sp., whereas it is gently convex in E. shirakawai . The ocular acicle is short and ovate in the new species, instead of elongate and narrowly triangular in E. shirakawai . The antennular peduncle overreaches the distal corneal margin by the full length of the article 3 and the distal one third of the article 2, rather only approximately the full length of the article 3 in E. shirakawai . The antennal flagellum has at most three short setae on the distal margin of each article, whereas three to five, short and long setae are present on the distal margin, which make the flagellum somewhat setose, in E. shirakawai . The merus of maxilliped 3 is unarmed on both the dorsal and ventral margins, but it has distinct dorsodistal and ventromedian spines in E. shirakawai (cf. Komai 2015: fig. 1B, E, F). The right cheliped is proportionally much shorter in E. crassoculus n. sp. than in E. shirakawai ; the chela and carpus are 1.8 versus 2.0 and 1.4–1.5 versus 2.1 times longer than the distal widths, respectively. The dactylus of the left cheliped is clearly longer than the palm in E. crassoculus n. sp., than in E. shirakawai ; it is 1.5 versus 0.9 length of the palm. The merus of both chelipeds of E. crassoculus n. sp. has sparse bristle-like setae on the dorsodistal margin and adjacent to the ventromesial distal margin, but such setae are absent in E. shirakawai . The ambulatory legs are mostly smooth on all the surfaces, instead of the entire surfaces of the dactylus to carpus and the ventral parts of the lateral surfaces of the merus with numerous fine granules (cf. Komai 2015: figs. 2–4).

Besides the strongly inflated proximolateral margin of the ocular peduncle, the reduced corneas, and the absence of small corneous spines or bristle-like setae on the dorsal surface of the ambulatory meri, E. crassoculus n. sp. differs from E. granulimanus in the following particulars: the comparatively longer shield (1.1–1.2 times versus approximately as long as wide); the proportionally longer article 3 of the antennular peduncle (3.2 versus 2.8 times as long as high); the merus of maxilliped 3 being unarmed on the dorsal and ventral margins (a dorsodistal and a ventromedian spines are present in E. granulimanus ); the proportionally longer dactylus of the right cheliped (0.7 versus 0.4 length of the palm); the proportionally shorter carpus of the right cheliped (1.4–1.5 versus 1.7 times longer than distal widths); the palm of the left cheliped with numerous minute granules, a distinct median elevation on the dorsal surface, and clearly delimited dorsomesial and dorsolateral margins (the dorsal surface being smooth and gently convex, without the delineation of dorsolateral and dorsomesial margins in E. granulimanus ); and the dactyli of the ambulatory legs with less number of ventral spines (three to six versus eight or nine) (cf. Komai et al. 2021: figs. 2, 3).