Decaphyllus tenuis, Komai & Rahayu, 2013

Komai, Tomoyuki & Rahayu, Dwi Listyo, 2013, Six New Species Of The Hermit Crab Genus Decaphyllus De Saint Laurent, 1968 (Crustacea: Decapoda: Anomura: Paguridae) From The Boholo Sea, The Philippines, And The Ryukyu Islands, Japan, Raffles Bulletin of Zoology 61 (2), pp. 589-620 : 614-619

Decaphyllus tenuis , new species

( Figs. 20–22 )

Material examined. — Holotype: male (sl 1.8 mm), PANGLAO 2004, stn T5 , Bohol Island , W of Baclayon, 09°35.3'N, 123°52.2"E, 84–87 m, coarse sand and mud, 2 Jun.2004, NMCR 39091.

Paratypes: 2 females (sl 1.3, 1.3 mm), PANGLAO 2004, stn T18 , Cortes, Bohol Island , 09°41.8'N, 123°49.9'E, 80–100 m, muddy bottom with sponges, 19 Jun.2004, ZRC 2013.0679 ; 1 male (sl 1.9 mm), same data, CBM-ZC 11709; stn T41 , 2 males (sl 1.0, 1.1 mm), Cervera shoal, West Pamilacan Island , 09°29.7'N, 123°50.2'E, 110–112 m, 6 Jul.2004, ZRC 2013.0680 .

Description. — Ten pairs of biserial phyllobranchiae (no pleurobranchs). Two very small arthrobranchs above base of third maxilliped, anterior gill simple, posterior gill bilobed ( Fig. 20C ).

Shield ( Fig. 20A ) approximately as long as wide; anterior margin between rostral region and lateral projection concave; anterolateral margins sloping; posterior margin roundly truncate; dorsal surface with anteromedian part poorly calcified, with some tufts of short setae laterally. Rostrum rounded. Lateral projections moderately well developed, exceeding as far as rostral lobe, each with terminal spinule.

Ocular peduncle ( Fig. 20A ) approximately as long as shield, faintly constricted distal to midlength; dorsal surface with mesial row of tufts of moderately short to long setae directed mesially and some individual setae laterally, and prominent tuft of moderately long setae at base of cornea; cornea slightly dilated, its width slightly more than 0.2 of length of ocular peduncle; basal part not inflated, its width narrower than corneal width. Ocular acicle drawn out distally into acute spine, mesial margin with some long setae; separated basally by width of less than 1 acicle. Interocular lobe visible in dorsal view, anteriorly slightly concave.

Antennular peduncle ( Fig. 20A ) overreaching distal corneal margin by 0.4–0.5 length of ultimate segment. Basal segment with prominent spine on lateral margin of statocyst lobe, mesial face unarmed. Penultimate and ultimate segments unarmed, almost glabrous except for thin short setae at dorsomesial distal angle of ultimate segment.

Antennal peduncle ( Fig. 20A ) slightly falling short of base of cornea of ocular peduncle. Fifth and fourth segments with few setae. Third segment with prominent spine on ventromesial distal margin. Second segment with dorsolateral distal angle strongly produced, terminating in bifid spine, dorsomesial distal angle with small spine. First segment with strong spine on ventrodistal margin; lateral surface unarmed. Antennal acicle reaching or overreaching distal margin of fifth peduncular segment, or not reaching to reaching corneal base, terminating in small spine; mesial surface with row of sparse setae; lateral margin unarmed. Antennal flagellum with 2–4 short to moderately long setae on distal margin of each article.

Third maxilliped with merus armed with strong dorsodistal spine; crista dentata on ischium consisting of 4 or 5 triangular teeth ( Fig. 20B ); basis with acute denticle on mesial face. Exopod long, reaching nearly to distal margin of carpus.

Chelipeds ( Fig. 21 ) slightly unequal in length; right slightly longer but appreciably stronger. Right cheliped ( Fig. 21A–D ) slender, with chela elongate subovate in dorsal view, about 3.0 times longer than wide. Dactylus set at slightly oblique angle to palm ( Fig. 21C ), subequal in length to palm; dorsal surface with 1 or 2 tiny spines or tubercles proximally; all surfaces with scattered moderately short to long setae, particularly numerous on mesial surface; cutting edge with row of numerous small, blunt or acute calcareous teeth in proximal 0.8 and row of spaced minute corneous teeth in distal 0.2, terminating in small calcareous claw. Palm ( Fig. 21A, C ) distinctly shorter than carpus; dorsomesial margin with row of small spines, dorsal midline with row of moderately small spines not extending onto fixed finger, dorsolateral margin not delimited, dorsal surface lateral to midline with scattered small slender spines or low tubercles; lateral and mesial surfaces with scattered short to long setae; ventral surface gently convex, smooth, with sparse setae. Fixed finger ( Fig. 21A, C ) with cutting edge bearing row of numerous blunt to acute calcareous teeth, terminating in small calcareous claw. Carpus ( Fig. 21A–C ) subequal in length to merus, slightly widened distally, 2.3–2.9 times longer than wide; dorsomesial margin with row of 4–7 small spines, dorsolateral margin with row of 4–12 small spines (distalmost spine at dorsolateral distal angle minute); all surfaces with scattered short to long setae, subdistal transverse row of setae particularly prominent; both ventrolateral distal and distomesial angles unarmed. Merus ( Fig. 21A, B, D ) with 1 small spine on dorsodistal margin mesially; dorsal surface with sparse short setae; lateral surface without conspicuous spines or tubercles, ventrolateral margin unarmed or with 1 or 2 small, widely spaced spines or tubercles; mesial surface with 1 small proximoventral tubercle, ventromesial margin with 2 small, widely spaced spines or tubercles in distal half; ventral surface without conspicuous spine. Ischium ( Fig. 21D ) with 1 or 2 widely spaced small spines on ventromesial margin, distal spine, if present, directed distally, proximal spine also directed distally; lateral surface with spinule ventrally. Spines or spinules on merus and ischium sharper in small specimens than in large specimens.

Left cheliped ( Fig. 21E–H ) without hiatus between dactylus and fixed finger. Dactylus ( Fig. 21E, G ) about 1.3 times longer than palm, unarmed; all surfaces with numerous short to long setae, on mesial surface; cutting edge minutely denticulate, with row of minute corneous teeth in distal half. Palm ( Fig. 21E–G ) about half-length of carpus; dorsal surface with irregular longitudinal rows of small spines or tubercles, including stronger spines on midline; all surfaces with scattered short to long setae. Fixed finger with row of small calcareous teeth decreasing in size distally, several proximal teeth spine-like. Carpus ( Fig. 21E–G ) very slightly widened distally, about 3.8 times longer than wide; dorsolateral margin with 4–7 minute to moderately large spines, dorsomesial margin with 3–5 minute to moderately large spines; ventrolateral distal angle unarmed or with minute denticle, distomesial angle unarmed; all surfaces with scattered short to long setae. Merus ( Fig. 21E, F, H ) with sparse setae on dorsal surface; dorsodistal margin with 1 small spine; lateral surface with 1 spinule adjacent to ventral margin, ventrolateral margin with 1 subdistal spine; mesial surface with small distally curved spine proximoventrally, ventromesial margin with 1 small spine distal to midlength; ventral surface unarmed, with scattered long setae. Ischium ( Fig. 21E, H ) with 2 widely spaced small spines on ventromesial margin, distal spine directed mesially, proximal spine curved proximally; lateral surface unarmed.

Ambulatory legs ( Fig. 22 ) overreaching tip of right cheliped. Dactyli ( Fig. 22A, B, D ) 1.4–1.5 times longer than propodi, 14.5–16.7 times longer than broad, gently curved ventrally; all surfaces unarmed, but with numerous setae, particularly longer and stronger on dorsal margins (several distal setae on dorsal margins bristle-like). Propodi ( Fig. 22A, D ) unarmed, but with row of sparse setae on dorsal and ventral margins and scattered short setae on lateral and mesial faces. Carpi each with dorsodistal spine (second; Fig. 22A ) or without dorsodistal spine (third; Fig. 22D ), and with 2 additional small spines in proximal 0.3 (second, Fig. 22C ) or no additional spines (third, Fig. 22E ). Meri ( Fig. 22A, C–E ) each with 2 small spines (one located slightly distal to midlength and another at proximal 0.2) on dorsal margin; dorsal and ventral margins with sparse long setae, latter unarmed. Ischium with distal spinule on ventral margin mesially (second, Fig. 22C ) or unarmed (third, Fig. 22E ).

Fourth pereopods ( Fig. 20D ) non-chelate, with claw of dactylus entirely masked by tufts of short, dense setae; propodus with sparse setae on dorsal margin and distal half of ventral margins; 2 minute corneous scales present ventrodistally. Fifth pereopods semichelate.

Male with right sexual tube ( Fig. 20F ) long, directed from right to left across ventral body surface and curved anteriorly, reaching to level of coxa of left second pereopod; distal part somewhat flattened. Left sexual tube ( Fig. 20F ) directed from left to right, reaching to anteromesial part of coxa of right fifth pereopod, slightly twisted, slightly broadened distally, with sparse setae terminally and posteriorly. Female with unpaired left gonopore.

Anterior lobe of thoracic sternite 6 (third pereopods, Fig. 20E ) subsemicircular, slightly skewed to left, bearing numerous long setae anteriorly. Sternite of thoracic sternite 8 (fifth pereopods) in male ( Fig. 20F ) asymmetrical, anteriorly concave, with few setae ventrally; that in female transversely subovate.

Pleon dextrally twisted. Male with 4 unpaired pleopods, second, fourth and fifth uniramous, third unequally biramous; ramus of second pleopod bearing numerous marginal setae. Female with 4 unpaired, unequally biramous pleopods.

Telson ( Fig. 20G ) with shallow median cleft; terminal margin with prominently produced, spinose left exterior angle separated from less produced, but also prominent, spinose right exterior angle, and with 1 or 2 spinules on either side of median cleft; lateral margins forming chitinous plate.

Colouration. — In life ( Fig. 23B ). Carapace generally reddish, shield slightly mottled. Ocular peduncles pinkish, with scattered white spots. Antennal peduncles reddish, without conspicuous markings; antennal flagellum whitish in proximal half, blue in distal half. Chelae generally whitish, carpi generally reddish with whitish distal part; meri whitish, with reddish blotches.Ambulatory legs indistinctly banded by red and white; dactyli entirely whitish; propodi with distal part white and proximal part white, remainder alternated with red (distal to midlength) and white (proximal to midlength); carpi reddish; meri reddish with white distal parts. Pleon reddish.

In preservative. Chelipeds and ambulatory legs with iridescent sheen.

Variation. — Acuteness of dorsal spines on the right palm, scattered lateral to the midline, is substantially variable, although it seems to be size related. In small specimens (sl 1.0– 1.3 mm), those spines are slender and acute, but in large specimens (sl 1.8, 1.9 mm), they are reduced to blunt, rounded tubercles. The number of dorsolateral and dorsomesial spines on the carpi of chelipeds seems to increase with the increase of the body size.

Distribution. — Known only from Panglao Islands, 82– 200 m.

Remarks. — Decaphyllus tenuis , new species, is very similar to D. janquai in the general proportion of the cephalic appendages and the armature of the right chela. In particular, the dorsomedian row of spines on the right palm is clearly recognisable only in the proximal portion in both species. Decaphyllus tenuis is distinguished from D. janquai by the relatively shorter antennular peduncle overreaching the distal corneal margin by about half length of the ultimate segment (versus about 0.8 length) and the relatively slender right chela (about 3.0 times as long as wide versus 2.0 times as long).

As discussed above, D. brevis , new species, and D. spinicornis also superficially resemble D. tenuis . Differentiating

characters between the three species are discussed under Remarks of D. brevis .

Decaphyllus barunajaya (cf. McLaughlin, 1997) has a right chela being armed similarly to D. tenuis , but this species is readily distinguished from D. tenuis by the complete loss of arthrobranch gills on the third maxilliped, the stouter and less elongate right cheliped, the presence of a hiatus between fingers of the left cheliped, and the possession of a row of small calcareous teeth on the cutting edge of the dactylus of the left chela.

Etymology. — From the Latin tenuis [= narrow], in reference to the slender, elongate right cheliped of this new species.

Figures

Fig. 20. Decaphyllus tenuis, new species. A–D, F, G, holotype, male (sl 1.8 mm), PANGLAO 2004, stn T5, NMCR 39091; E, paratype, female (sl 1.3 mm), PANGLAO 2004, stn T18, ZRC 2013.0679. A, shield and cephalic appendages, dorsal view; B, ischium of left third maxilliped, ventral view; C, arthrobranchs on left third maxilliped; D, left fourth pereopod, lateral view; E, sixth thoracic sternite, ventral view; F, coxae of fifth pereopods, sexual tubes, and eighth thoracic sternite, ventral view; G, telson, dorsal view. Scale bars = 0.5 mm (A, B, D–G), 0.25 mm (C).

Fig. 21. Decaphyllus tenuis, new species, holotype, male (sl 1.8 mm), PANGLAO 2004, stn T5, NMCR 39091. A, right cheliped, mesial view; B, same, lateral view; C, same, chela and carpus, dorsal view; D, same, merus, dorsal view; E, left cheliped, mesial view; F, same, lateral view; G, same, chela and carpus, dorsal view; H, same, merus, dorsal view. Setae omitted. Scale bar = 1.0 mm.

Fig. 22. Decaphyllus tenuis, new species, holotype, male (sl 1.8 mm), PANGLAO 2004, stn T5, NMCR 39091. A, right second pereopod, lateral view; B, same, dactylus, mesial view (only mesial setae shown); C, same, carpus to ischium, mesial view (setae omitted); D, right third pereopod, lateral view; E, same, carpus to ischium, mesial view (setae omitted). Scale bars = 1.0 mm.

Fig. 23. A, Decaphyllus proprius, new species, holotype, male (sl 1.4 mm), PANGLAO 2004, stn T39, NMCR 39089, entire animal in dorsal view, showing living colouration; B, Decaphyllus tenuis, new species, holotype, male (sl 1.8 mm), PANGLAO 2004, stn T5, NMCR 39091, entire animal in dorsal view, showing living colouration.