Pagurixus depressus, Komai & Rahayu & Fujita, 2021

Komai, Tomoyuki, Rahayu, Dwi Listyo & Fujita, Yoshihisa, 2021, Hermit crabs of the family Paguridae (Crustacea: Decapoda: Anomura) from Christmas and Cocos (Keeling) Islands, Indian Ocean Territory of Australia, with descriptions of three new species and reappraisal of the genus Kumepagurus Komai & Osawa, 2012, Raffles Bulletin of Zoology 69, pp. 378-402 : 392-397

Pagurixus depressus , new species

( Figs. 9–11 )

Material examined. Holotype: QM W29566, 1 female (sl 1.4 mm), CI-D16-2011, Flying Fish Cove , Christmas Island, 10°25.815′S, 105°40.180′E, 29 March 2011.

Paratype: CHRISTMAS ISLAND: QM W29567, CI-D15- 2011, 1 male (sl 1.2 mm), Boat Cave , 29 March 2011 ; ZRC 2021.0637 , 1 female (sl 1.0 mm), CI-D17-2011, Thunderdome Cave , 20°27.906′S, 105°36.465′E, 3–16.6 m, 30 March 2011 .

Description. Gills biserial, 11 pairs.

Cephalothorax somewhat depressed dorsoventrally. Shield ( Fig. 9A ) approximately as long as wide; anterior margin between rostrum and lateral projections concave; anterolateral margins sloping; dorsal surface glabrous. Rostrum triangular, reaching level of midlength of ocular acicles, terminating acutely. Lateral projections slightly produced, with submarginal spinule.

Ocular peduncle ( Fig. 9A ) moderately long and stout, 0.6–0.7 length of shield, slightly widened proximally, with row of tufts of setae on dorsal surface mesially; cornea artificially shrunk in all specimens examined. Ocular acicle subtriangular, with small submarginal spine.

Antennular peduncle ( Fig. 9A ) overreaching base of cornea by full length of ultimate article. Ultimate article with tufts of long setae at dorsolateral angle; ventral surface with 2 setal rows, each consisting of tufts of few short setae or individual seta ( Fig. 9B, C ). Basal article with small lateral spine on statocyst lobe. Ventral flagellum ( Fig. 9B ) with numerous long setae on lateral and mesial margins.

Antennal peduncle ( Fig. 9A ) overreaching basal margin of cornea by full length of article 5. Articles 5 and 4 with few setae. Article 3 with small spine at ventromesial distal angle. Article 2 with small spine at dorsomesial distal angle; distolateral projection short, not reaching midlength of article 4, terminating in bifid spine. Article 1 with small laterodistal spine; ventromesial distal margin produced, unarmed. Antennal acicle arcuate, overreaching distal margin of article 4; mesial margin with row of tufts of stiff setae. Flagellum moderately long, exceeding 4.0 length of shield.

Maxilliped 3 normal for genus; ischium with well-developed crista dentata consisting of row of closely spaced small corneous teeth and 1 accessory tooth. Merus and carpus unarmed.

Right cheliped of female holotype ( Fig. 10A–C ) moderately long, stout. Chela subovate in dorsal view, 1.8 times as long as wide. Dactylus 0.7 length of palm; dorsomesial margin not delimited, with tufts of short setae on surfaces, arranged in longitudinal rows; dorsomesial surface granular; occlusal margin with 2 widely separated, blunt calcareous teeth and row of minute corneous teeth in distal 0.2, terminating in small corneous claw. Palm shorter than carpus; gently convex dorsal surface with scattered, coarse granules, dorsolateral and dorsomesial margins not delimited; lateral, mesial, and ventral surfaces with only few granules and few short setae. Occlusal margin of fixed finger with row of blunt calcareous teeth, terminating in calcareous claw. Carpus subequal in length to merus, slightly flattened dorsoventrally; dorsolateral and dorsomesial margins not delimited, with few small spines dorsomesially; dorsal surface with sparse granules and short setae; mesial surface with scattered long setae and granules, ventromesial margin unarmed; lateral surface with scattered granules and few setae, without longitudinal ridge, ventrolateral margin with few minute granules distally; ventral surface gently convex, glabrous. Meral-carpal articulation lacking any pronounced clockwise rotation; dorsal surface of merus almost smooth, dorsodistal margin unarmed; lateral face with scattered, very short vertical ridges, ventrolateral margin with 1 subdistal spine; mesial face almost glabrous, ventromesial margin faintly denticulate distally; ventral surface slightly convex, with sparse short to long setae. Ischium unarmed.

Right cheliped of male paratype ( Fig. 11A–C ) not particularly elongate, moderately stout. Chela subovate in dorsal view, 2.1 times as long as wide. Dactylus 0.8 length of palm; dorsomesial margin not delimited, with tufts of short setae on surfaces, arranged in longitudinal rows; dorsomesial surface granular; occlusal margin with 2 widely separated, blunt calcareous teeth and row of minute corneous teeth in distal 0.2, terminating in small corneous claw. Palm shorter than carpus; gently convex dorsal surface with scattered, coarse granules and sparse short setae, dorsolateral and dorsomesial margins not delimited; lateral, mesial, and ventral surfaces also with scattered granules and few short setae. Occlusal margin of fixed finger with row of blunt calcareous teeth in proximal half, no tooth on distal half, terminating in calcareous claw. Carpus 0.8 length of chela, subequal in length to merus, somewhat flattened dorsoventrally; dorsolateral and dorsomesial margins not delimited, with some small spines dorsomesially; dorsal surface with sparse granules and short setae; mesial surface with sparse long setae and granules, ventromesial margin unarmed; lateral surface with scattered granules, without longitudinal ridge, ventrolateral margin with few minute granules distally; ventral surface only slightly convex, glabrous. Meral-carpal articulation lacking any pronounced clockwise rotation; dorsal surface of merus almost smooth, dorsodistal margin unarmed; lateral face with scattered granules, ventrolateral margin with 1 distal spine; mesial face almost glabrous, ventromesial margin unarmed; ventral surface nearly flat, almost glabrous. Ischium unarmed.

Left cheliped ( Fig. 10D–F ) moderately slender. Chela 2.2 times longer than wide. Dactylus subequal in length to palm, with scattered tufts of short setae on surfaces; dorsal surface smooth, dorsomesial margin not delimited; occlusal margin with row of small corneous teeth in distal 0.3, terminating in small corneous claw. Palm half-length of carpus; dorsal surface with sparse short setae, slightly elevated in midline and bearing longitudinal row of small tubercles, with granules mesial to midline, dorsolateral and dorsomesial margins not delimited; lateral face with short oblique ridges; mesial face smooth; ventral surface gently convex, with scattered long setae. Occlusal margin of fixed finger with row of small calcareous teeth in distal half, terminating in small corneous claw. Carpus subequal in length to chela and merus; length about 2.4 of distal width and 2.7 of greatest height; dorsal surface with longitudinal row of tiny spines dorsomesially, rounded dorsolaterally; lateral surface having scattered row tubercles, lacking median ridge; ventrolateral distal angle unarmed; mesial surface smooth, with several long stiff setae, particularly along dorsal margin; ventral surface with long setae distally. Merus glabrous on dorsal surface, dorsodistal margin unarmed; lateral surface glabrous, ventrolateral margin with 2 subdistal spines; mesial surface also glabrous, ventromesial margin with 1 minute distal spine and faintly denticulate thereafter; ventral surface with few short setae. Ischium unarmed.

Ambulatory legs ( Fig. 11D–G ) moderately long and slender, similar from right to left. Dactyli subequal in length to propodi, about 6.1 times longer than high, terminating in large corneous claws; dorsal surfaces each with row of tufts of moderately spaced setae; lateral surfaces with row of shorter setae adjacent to dorsal margins; mesial surfaces with only few short setae, unarmed; ventral margins each with 6 or 7 corneous spines notably increasing in size distally. Propodi not tapering distally, about 4 times longer than high; dorsal surfaces each with tufts of long setae; lateral surfaces with sparse setae; ventral margins each with 2 corneous spinules, ventrodistal margins each with paired corneous spines. Carpi each without dorsodistal spine; dorsal surfaces with row of tufts of setae; lateral faces with few short setae. Meri with row of tufts or individual setae (setae shorter than in third); lateral surfaces glabrous; ventral margins glabrous, ventrolateral parts each with small subdistal spine (second) or unarmed (third), faintly granulated (second) or smooth (third). Unpaired gonopore on left coxa of pereopod 3 in females ( Fig. 9D ).

Pereopods 4 ( Fig. 9E–G ) markedly dissimilar, left larger and more setose on dactylus to carpus. Dactyli nearly straight with arcuate dorsal margins, terminating in small corneous claws; left dactylus much broader than right. Propodi each with single row of corneous scales in distal 0.8, without conspicuous tuft of setae on mesial face on either side. Right carpus only with few short setae on dorsal margin distally.

Male with coxae of pereopods 5 markedly unequal ( Fig. 9H ). Right coxa without protrusion, but with rounded demarcation outside of gonopore; prominent setal tuft consisting of long stiff setae arising from demarcated area and passing to left coxa. Left coxa lacking gonopore.

Anterior lobe of thoracic sternite 6 ( Fig. 9D ) broadly subtrapezoidal, with anterolateral angles rounded, anterior margin with row of short setae. Thoracic sternite 8 ( Fig. 9H ) composed of two subequal, clearly separated, rounded lobes.

Pleon dextrally twisted. Female (holotype) with 4 unpaired pleopods 2–5; male (paratype) with 3 unpaired pleopods 3–5.

Telson ( Fig. 9I ) terminal margins moderately oblique, each bearing 4 small spines, no spinules interspersing those spines.

Colouration in life. Not known.

Distribution. Presently known only from Christmas Island; at depths of 3–16.6 m.

Remarks. Pagurixus depressus , new species, is referable to the subgroup B of the P. boninensis species group because of the combination of the presence of ventral setal rows on the antennular peduncle ultimate article, and the lack of a division of the lateral face of the carpus of the left cheliped ( Komai & Osawa, 2006). It appears close to P. paulayi Komai & Osawa, 2006 , so far known only from Guam, Mariana Islands. Shared characters of the two species include: the palm of the right cheliped bears scattered coarse granules on the dorsal surface; the carpus of the male right cheliped is armed with some spines dorsomesially; the left pereopod has prominent tufts of setae on the dactylus; and the male left gonopore is absent. The new species, however, can be distinguished from P. paulayi by the somewhat depressed carpus of the right cheliped, which is more pronounced in the male ( Fig. 11A, B ). Such a condition is not seen in P. paulayi (cf. Komai & Osawa, 2006: fig. 34B–D), as well as in other congeneric species. Furthermore, the following features would seem to warrant the recognition of the new species: ventral setae on the ultimate article of the antennular peduncle are much fewer, arranged in series of tufts, in P. depressus , new species (cf. Fig. 9C versus Komai & Osawa, 2006: fig. 33B, C); there is no ventromesial protrusion on the coxa of the left pereopod 5 in the male of P. depressus , new species ( Fig. 9H ) (versus present in P. paulayi ; cf. Komai & Osawa, 2006: fig. 33G); and the anterior lobe of the thoracic sternite 6 is proportionally wider in P. depressus , new species, than in P. paulayi ( Fig. 9D versus Komai & Osawa, 2006: fig. 33F).

Figures

Fig. 9. Pagurixus depressus, new species. A–E, holotype, female (sl 1.4 mm), QM W29566; F, G, paratype, female (sl 1.0 mm), ZRC 2021.0637; H, I, paratype, male (sl 1.2 mm), QM W29567. A, shield and cephalic appendages, dorsal view (corneas artificially shrunk); B, ultimate peduncular article and flagella of left antennule, lateral view; C, same, ventrolateral view, showing ventral setation of ultimate article (dorsal flagellum omitted); D, coxae of pereopods 3 and thoracic sternite 6, ventral view; E, distal articles of left pereopod 4, lateral view; F, left pereopod 4, lateral view; H, coxae of pereopods 5 and thoracic sternite 8, ventral view; I, telson, dorsal view.

Fig. 10. Pagurixus depressus, new species, holotype, female (sl 1.4 mm), QM W29566. A, right cheliped, mesial view; B, same, lateral view; C, chela and carpus of right cheliped, dorsal view; D, left cheliped, mesial view; E, same, lateral view; F, chela and carpus of left cheliped, dorsal view.

Fig. 11. Pagurixus depressus, new species. A–C, paratype, male (sl 1.2 mm), QM W29567; D–G, holotype, female (sl 1.4 mm), QM W29566. A, right cheliped, mesial view; B, same, lateral view; C, chela and carpus of right cheliped, dorsal view; D, left pereopod 2, lateral view; E, same, dactylus, mesial view; F, left pereopod 3, lateral view; G, same, dactylus, mesial view.