Diogenes pisinnus, Rahayu, 2022

Rahayu, Dwi Listyo, 2022, Hermit crabs of Singapore (Crustacea: Decapoda: Anomura: Diogenidae, Paguridae), with description of two new species, Raffles Bulletin of Zoology 70, pp. 329-363 : 347-348

Diogenes pisinnus , new species

( Fig. 10 )

Material examined. Holotype, male, 1.3 mm ( ZRC 2021.0203 ), st. DR02, off Seringat Kias , Lazarus Island, coll. CMBS, 11 m, 23 April 2012.

Description. Shield ( Fig. 10A ) slightly longer than broad; rostrum broadly rounded; anterior margin between rostrum and lateral projections somewhat concave; lateral projections triangular, exceeding tip of rostral lobe, each with small marginal spine; anterolateral angle rounded; lateral margins glabrous; posterior margin slightly concave; dorsal surface with few small spines and tufts of sparse setae. Dorsal margins of branchiostegites each with row of 6 small spines.

Ocular peduncle ( Fig. 10A ) moderately stout, cylindrical, about 0.8 length of shield, slightly inflated proximally, with tufts of very sparse setae dorsally; cornea not dilated, its diameter about 0.3 of peduncular length. Ocular acicles broad, subrectangular, basally separated by approximately 0.2 width of 1 acicle, with row of spinules on terminal margin, mesialmost spine largest. Intercalary rostriform process stout, simple at tip, not reaching tip of mesial spine of ocular acicle, with few long setae; no ventral spine.

Antennular peduncle ( Fig. 10A ), when fully extended, overreaching distal corneal margin by whole length of ultimate segment. Ultimate and penultimate segments unarmed, but with few short setae. Basal segment with few long setae.

Antennal peduncle ( Fig. 10A ) overreaching distal corneal margin by 0.7 length of fifth segment; fifth segment with row of long setae laterally; fourth segment with triangular dorsodistal margin, unarmed; third segment short, stout, unarmed; second segment with dorsolateral distal angle produced into small but prominent spine, dorsomesial distal angle with or without small spine, mesial and lateral margins with few setae; first segment unarmed. Antennal acicle short, subtriangular, reaching midlength of fourth segment, with strong terminal spine; mesial margin with 1 large spine and few setae. Antennal flagellum long, overreaching tip of left cheliped; each article with three or four long plumose setae.

Third maxilliped with coxa and basis unarmed; crista dentata on ischium obsolete; merus subequal in length to carpus, both segments with long setae on mesial margin, lateral margin with fewer setae; propodus slightly shorter than carpus; dactyl shorter than propodus, both with dense setae ventrally.

Left cheliped ( Fig. 10B ) much larger than right, narrow hiatus between dactyl and fixed finger. Dactyl slightly longer than palm, arched; cutting edge with row of blunt calcareous teeth increasing in size proximally and few tufts of short setae, terminating in small calcareous claw overlapped by claw of fixed finger; outer surface with sparse short setae, smooth medially, longitudinal row of tubercles near upper margin, not reaching distal end of dactyl, and irregular row of tubercles near cutting edge; upper margin with row of small spines decreasing in size distally and long plumose setae; inner surface slightly convex, with longitudinal row of tufts of short setae. Palm upper margin with row of moderately small spines; convex outer surface with irregular longitudinal row of small spines on midline not reaching articulation with dactyl; additional row of large but widely spaced spines between midline and upper margin; lower outer surface of palm with sparse spines forming irregular longitudinal rows; lower margin delimited by row of subacute spines or tubercles; inner surface almost smooth and glabrous except for few setae. Fixed finger outer surface with sparse small blunt tubercles; lower margin delimited by row of moderately small spines, interspersed with long plumose setae; cutting edge with row of calcareous teeth, large tooth subdistally; terminating in large calcareous claw; inner surface with scattered small tubercles near cutting edge and few scattered tufts of short setae. Carpus slightly longer than palm; upper margin with row of prominent spines and sparse long setae; outer surface convex bearing scattered small tubercles; lower margin with large spine distally and few small spines medially; inner surface with few tubercles and sparse setae. Merus subtriangular in dorsal view; dorsodistal margin with strong spine; dorsal surface with rows of moderately small spines and tufts of long plumose setae; lateral face tuberculate, ventrolateral margin with row of large spines, distal spines strongest and accompanied by long setae; mesial face flattened, smooth, ventromesial margin with row of moderately large spines, distal spines strongest and accompanied by plumose setae; ventral surface tuberculate. Ischium unarmed, with sparse short setae on ventromesial margin.

Right cheliped ( Fig. 10C ) reaching proximal third of palm of left cheliped; no hiatus between dactyl and fixed finger. Dactyl slightly longer than palm; upper margin with row of evenly spaced small spines and numerous long setae; outer surface smooth but with moderately dense long setae; cutting edge with row of low, blunt calcareous teeth, terminating in moderately large calcareous claw overlapped by fixed finger. Palm with row of moderately small spines and tufts of long setae on upper margin; outer surface smooth but with long setae; fixed finger with sparse tubercles partially obscured by long setae on outer surface; lower margin smooth but with sparse long setae; cutting edge with row of small calcareous teeth, terminating in moderately large calcareous claw. Carpus with row of small spines on upper margin; outer face almost smooth, small dorsodistal spine and two small spines on distolateral margin. Merus with numerous long, simple or plumose setae; ventromesial (not illustrated) and ventrolateral margins each with 2 large spines distally and long, simple and plumose setae. Ischium unarmed, but with few long setae.

P2 and P3 ( Fig. 10D–G ) slender. P2 ( Fig. 10D, F ) with dactyls 1.2 times as long as propodi, somewhat curved ventrally, but not twisted in dorsal view, terminating in moderately small corneous claws; dorsal and ventral margins each with moderately dense mixture of short and long, plumose, and simple setae; mesial surfaces each with row of plumose setae near dorsal margin. Propodi 1.5 times as long as carpi, unarmed, each with moderately dense long setae on dorsal and ventral margins; mesial faces each with few setae. Carpi 0.7 times as long as meri; dorsal margins each with row of small spines and sparse, long setae; ventral margins unarmed. Meri each with sparse setae on each dorsal and ventral margin, otherwise unarmed. Ischia unarmed, each with sparse setae. P3 ( Fig. 10E, G ) with dactyls and propodi same as in P2; carpi 0.9 times as long as meri, each with prominent dorsodistal spine, and sparse setae, lateral faces and ventral margins unarmed; meri and ischia also unarmed but with few setae. P4 semichelate ( Fig. 10H ); propodal rasp consisting of three rows of calcareous scales, carpi each with dorsodistal spine.

Telson ( Fig. 10I ) without median cleft; left posterior lobe longer than right, left terminal margin with row of strong spines extending onto lateral margin, becoming smaller; right terminal margin also with row of small spines, but not extended onto lateral margin; long setae at outer angles continued onto lateral margins.

Colour. Unknown.

Etymology. The name is derived from Latin pisinnus , meaning small, alluding to the small antennal acicle which has only two spines.

Distribution. At the moment found only in Singapore Strait, at 11 m depth, on shell fragment substrate.

Remarks. One of the characteristics of the new species is the short antennal acicle, which barely reaches to the midlength of the fourth segment of the antennal peduncle. This feature is shared with only six congeneric species among the 31 known species of the D. edwardsii group: D. dorotheae Morgan & Forest, 1991 , D. guttatus Henderson, 1888 , D. holthuisi Asakura & Tachikawa, 2010 , D. tirmiziae Siddiqui & McLaughlin, 2003 , D. takedai Rahayu, 2012 , and D. heteropsammicola Igawa & Kato, 2017 . The antennal acicle of this new species is widely bifid, and in this regard, the new species resembles D. holthuisi and D. heteropsammicola . Diogenes pisinnus , new species, however, is readily distinguished from the latter two species by the subrectangular ocular acicle (subtriangular in D. holthuisi and D. heteropsammicola ), the presence of row of spines on dorsal margin of the carpus of P2 (only a distal spine in D. holthuisi and D. heteropsammicola ), the presence of a strong dorsodistal spine on P4 carpus (no spine in D. holthuisi and D. heteropsammicola ), and the asymmetrical lobe of the telson with row of small spines on the right lobe, larger spines on the left lobe (asymmetrical with one to five spines of various size on each lobe in D. holthuisi , symmetrical with strong spines on each lobe in D. heteropsammicola ) which separate these three species.

In the unique holotype, the ocular peduncle apparently tapers distally, though it is not yet ascertained if the condition is real or an artefact caused by preservation. Nevertheless, comparison with D. takedai , which normally has a tapering ocular peduncle, would be interesting. The two species differ in the armature of the chelipeds and the telson. In D. pisinnus , the chelipeds are only sparsely spinose (covered by small, drop-like tubercles in D. takedai ), and the telson with row of small spines on the right lobe, larger spines on the left lobe, and small spines continuing onto lateral margins (row of small spines on both lobes, interspersed by large spines and continued onto lateral margin of left lobe in D. takedai ).

Diogenes dorotheae is readily distinguished from the new species by having the antennal acicles armed with three strong spines, and the subtriangular telson with strong spines on the terminal margin of each lobe, while D. guttatus and D. tirmiziae are very different from the new species in having mushroom-shaped tubercles on the palm of the left cheliped, as well as the propodi and carpi of P2 and P3 with row of spines on the dorsal margin in D. tirmiziae and the row of spines only on the carpi of P 2 in D. guttatus .

Figures

Fig. 10. Diogenes pisinnus, new species. Holotype, male, 1.3 mm (ZRC 2021.0203). A, shield and cephalic appendages; B, chela, carpus and merus of left cheliped, dorsolateral view; C, chela and carpus of right cheliped, dorsolateral view; D, left second pereopod, lateral view; E, left third pereopod, lateral view; F, right second pereopod, lateral view; G, right third pereopod, lateral view; H, left fourth pereopod, lateral view; I, telson. Scale = 0.5 mm.