Diogenes platyops Rahayu & Forest, 1995

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 : 337-340

Diogenes platyops Rahayu & Forest, 1995

( Figs. 5D–F , 6B , 7C, D )

Diogenes platyops Rahayu & Forest, 1995: 399 , fig. 4a–h (type locality: Pari Island, Jakarta Bay, Indonesia); Lemaitre & Ng, 1996: 330; Rahayu, 1996: 348.

Diogenes jubatus McLaughlin, 2005: 612 (part), fig. 3.

Material examined. 1 male, 4.9 mm ( ZRC 2021.0118 ), st. SD133, South of Kusu Island , hand collecting, dive, 11 m ,

31 May 2013; 1 female, 3.3 mm ( ZRC 2021.0119 ), TB142, east Johor Strait , 1°17.838′N 104°04.157′E, beam trawl, mud, gravel, dead shell, 28.8 m , 31 May 2013; 1 female, 2.5 mm, 3 males, 2.2–3.3 mm ( ZRC 2021.0120 ), st. DR174, near Kusu Island , 1°12.202′N 103°52.178′E, rectangular dredge, mud and gravel, 79.6–135 m , 4 June 2013; 1 female, 2.4 mm, st. TB185, Pulau Senang , 1°09.942′N 103°43.458′E, beam trawl, mud, 24.5 m , 5 June 2013; 2 males, 1.1–2.4 mm, 5 individuals in shell ( ZRC 2021.0121 ), st. DR184, beside Raffles light house, 1°09.280′N 103°44.049′E, rectangular dredge, rock, sand, shell fragment, 31.6‒35.4 m , 5 June 2014; 2 males, 0.9–1.6 mm, 1 female, 1.5 mm ( MZB Cru 5217), st. DR128, beside Eastern Boarding Ground , 1°09.942′N 103°43.458′E , 30 May 2013; 1 male, 2.4 mm, 1 female, 1.5 mm ( MZB Cru 5218), st. DR161, beside St John’s Island , 1°12.843′N 103°51.449′E, 44.4– 41.2 m , gravel, 13 June 2013; 1 female, 2.4 mm ( ZRC 2021.0122 ), st. TB113, South Sister Island , 1°09.942′N 103°43.458′E, rock, 29.3‒30.5 m , 29 May 2013; 1 female, 2.2 mm ( ZRC 2021.0123 ), st. TB91, near St John’s Island , 1°12.561′N 103°51.322′E, rock, 46.1‒72 m , 27 May 2013; 1 male, 2.3 mm ( ZRC 2021.0124 ), SEA8139, Singapore; 2 males, 2.8–4.0 mm, 1 ovigerous female, 3.5 mm ( ZRC 2021.0125 ), SEA8128, 01°14.803′N 103°54.173′E, 41–45 m , 13 May 2013; 1 female, 3 mm ( ZRC 2021.0126 ), 01°14.414′N 103°54.491′E, sand, rock, 61‒68 m , 13 May 2013; 2 males ( ZRC 2021.0127 ), st. TB91, Southern Fairway near St John’s Island, 1°12.561′N 103°51.946′E, rectangular dredge, sandy, broken shell, laterite gravel, 48.3−49.7 m , 27 May 2013.

Diagnosis. Shield ( Figs. 5D , 7C ) 1.1‒1.2 times as long as broad, with sparse tubercles and scattered long setae on dorsal surface; dorsal surface of branchiostegites with 4–5 small spines. Rostrum short, rounded, reaching level of lateral projections. Intercalary rostriform process vestigial, or reaching proximal 0.20 of ocular acicles. Ocular peduncles 0.7‒0.8 times as long as shield, dilated proximally, very slightly inflated distally, with long, dense plumose setae on dorsal surface subproximally; corneal diameter 0.1‒0.2 times as long as ocular peduncles; ocular acicles elongate, each with 3–5 strong spines. Antennular peduncles overreaching distal corneal margin by two thirds or entire length of ultimate segment. Antennal peduncles slender, reaching mid-length of ultimate segment of antennular peduncle, reaching beyond distal corneal margin by half length of fifth penduncular segment; antennal acicles relatively narrow, short, slightly overreaching mid-length of fourth peduncular segment, with 5‒6 strong mesial spines on oblique inner margin. Flagellum with very long and dense setae, primarily inserted ventrally. Left cheliped dactyl ( Figs. 6B , 7D ) broad, not recurved, slightly flattened, with row of calcareous-tip spines on upper margin, and few tubercles on outer surface; palm with row of strong spines on both upper and lower margins, continuing onto fixed finger, at ventroproximal angle row of spines parallel with carpal articulation curving distally along midline, terminating abruptly at about 0.2 proximal of palm; longitudinal furrow adjacent to upper margin, outer surface with scattered tubercles; long plumose setae on upper and lower margins of dactyl, palm, and fixed finger, outer surface pubescent, obscuring armature; carpus with numerous long setae but not obscuring armature, upper margin with row of strong spines, outer face with scattered tubercles. Right cheliped with chela entirely masked by long, plumose setae; dactyl with row of small spines on upper margin, palm with one dorsodistal spine followed by denticles; fixed finger with row of small spines on ventral margin; carpus with upper margin denticulate; longitudinal furrow adjacent to upper margin of palm and carpus. Left P3 more robust than P2, right P2, P3 ( Fig. 7C, D ) with dactyl about same length as propodi, unarmed except for minute dorsodistal spinule on each carpus, and occasionally two or three denticles on proximodorsal margin of carpus of left P2; dactyl and propodus of left P3 slightly compressed laterally, abundant long, plumose setae on dorsal and ventral margins, lateral surfaces pubescent ( Fig. 6B ); left P2, right P2, P3 with less dense setae on dorsal and ventral margin, lateral surface with few tufts of setae. Sternite of third pereopods (thoracic sternite 6) squarish ( Fig. 5E ), anterior lobe subsemicircular with median longitudinal depression, armed with spinules and/or with central protuberance on either side of depression. Telson ( Fig. 5F ) with left posterior lobe produced, right slightly shorter than left, median cleft barely indicated; terminal margin with series of small to moderately large spines, strongest at outer angle, on left extending onto lateral margin.

Colour in life. Shield cream or pinkish white mottled with brown and light pink, setae white. Ocular peduncle white with greenish brown blotch on half proximal. Antennular peduncle penultimale segment white, ultimate segment and proximal part of flagella blue, distal part of flagella orange. Antennal peduncle second segment brown, fourth, fifth segments and proximal part of flagella blue, rest of flagella light orange. Cheliped mottled with cream and dark brown; P2 and P3 cream with greenish brown blotch dorsoproximally on each segment ( Fig. 7C, D ).

Distribution. Indonesia and Singapore; at 24–135 m depth, on mud, sand, gravel substrates.

Remarks. McLaughlin (2005) demonstrated that the characters used to separate the species in the Troglopagurus group of Diogenes are mostly unreliable as they are variable depending or not, on the size and sex of individuals. These characters include the relative length of the ocular peduncles against the shield, the corneal diameter against the length of the ocular peduncles, the relative length of the antennular and antennal peduncles against the length of the ocular peduncles, the relative length of the dactyl against the propodus in the P2 and P3, the setation on the left P3 dactyl and propodus, and the armature on the ocular acicle. Based on the variability of these presumably diagnostic characters, she considered that D. platyops was a junior subjective synonym of D. jubatus . Furthermore, she gave three characters that are useful to distinguish species of the Troglopagurus group: the live colouration, the shape of the anterior lobe of the sternite of the third pereopods, and the asymmetry of the telson.

Examination of the specimens initially identified as D. jubatus in this study revealed two separable forms on the basis of the living colouration and the setation of the chelipeds and ambulatory legs. The first form is characterised by the following attributes: shield cream mottled with brown and dark brown; ocular peduncles dark brown or black with white band on proximal area and on area just proximal to cornea; ocular acicle white; penultimate segment of antennular peduncles, first to fourth segments of antennal peduncle, and antennal acicle dark brown with some white spot; penultimate segment of antennular peduncles, and fifth segment and flagellum of antenna cream; ground colour of chelipeds and pereopods cream to white with mottling of dark brown or light brown on proximal part of palms, most of carpi and meri of chelipeds, dactyls, median parts of propodi, carpi, and meri of P2 and P3 ( Fig. 7A, B ); left cheliped covered with long, sometimes plumose setae on margins and entire surface of fingers and palm, obscuring armature; dactyl and propodus of left P3 covered with dense, long plumose setae on dorsal, lateral, and ventral surfaces; carpus and merus having fewer setae ( Figs. 6A , 7A, B ). The second form is characterised by the following attributes: shield cream or pinkish white mottled with brown and light pink; ocular peduncle with distal half white, greenish brown blotch proximally; ocular acicle also white; antennular peduncles blue; antennal peduncles generally dark brown, third and fourth segments with blue tinge on brown; fifth segment blue, flagellum orange; antennal acicle white; ground colour of chelipeds and pereopods cream to white with mottling of greenish brown or light brown on proximal part of propodi, carpi, and meri ( Fig. 7C, D ); margins of fingers and palm of left cheliped, and margins of dactyl and propodus of left P3 bearing fringe of long dense plumose setae, outer surface with short pubescence, giving impression of flattened lateral surfaces of dactyls and propodus ( Figs. 6B , 7D ); carpus and merus covered with long, dense setae.

McLaughlin (2005) used the shape of the anterior lobe of the sternite of third pereopods to distinguish between D. jubatus and D. jousseaumei , while the asymmetry of the posterior lobes of the telson and the absence of row of spines on the branchiostegites were used to distinguish between D. jubatus or D. platyops and D. manaarensis .

The specimens of the two forms studied here have distinctly asymmetrical posterior lobes of the telson and the subquadrate-shaped anterior lobe of the sternite of P3, although it is slightly more trapezoidal in the first form specimens ( Fig. 5B, C, E, F ), while branchiostegite is unarmed or with few minute spinules in D. jubatus , in contrast to D. platyops which has a row of four to five small spines. In addition to the differences mentioned above, other distinct morphological characters were also observed. The first form has the following features: shield as long as broad, or slightly broader than long; antennal peduncles stout, and left cheliped palm with outer surface armed with scattered small spines or tubercles. The second form has the following features: shield slightly longer than broad, antennal peduncles slender, and left cheliped with outer surface of palm armed with short row of spines medially not reaching articulation with dactyl, and with shallow longitudinal furrow between midline and upper margin. These two forms of colouration and setation, and morphological differences are not influenced by size and sex and could represent morphological markers for species discrimination.

Therefore, there is little doubt that the two forms recognised herewith represent two distinct species. The first form is referred to Diogenes jubatus because it closely agrees with the redescription of Lemaitre & Ng (1996), particularly in the living colouration and the setation of the chelipeds and pereopods. Although no information on the living colouration was available for D. platyops , the second form is considered to represent D. platyops , resurrected from the synonymy of D. jubatus , because the setation of the chelipeds and ambulatory legs agrees well with that of the type specimens of the taxon.

Bathymetry range is overlapped between these two species, 24–135 m for D. platyops and 0.4–39.9 m for D. jubatus , but they do not live sympatrically.

Figures

Fig. 5. A–C, Diogenes jubatus (Nobili, 1903), male, 5.1 mm (ZRC 2021.0114); D–F, Diogenes platyops Rahayu & Forest, 1995, male, 4.9 mm (ZRC 2021.0118); G–I, Diogenes jousseaumei (Bouvier, 1897), ovigerous female, 5.8 mm (ZRC 2021.0128). A, D, G, shield and cephalic appendages; B, E, H, anterior lobe of sternite of third pereopods; C, F, I, telson. Scale = 1 mm.

Fig. 6. A, Diogenes jubatus (Nobili, 1903), male, 5.1 mm (ZRC 2021.0114); B, Diogenes platyops Rahayu & Forest, 1995, male, 4.9 mm (ZRC 2021.0118). A, B, left cheliped, P2 and P3.

Fig. 7. A, B, Diogenes jubatus (Nobili, 1903); A, ovigerous female, not collected, Johor Strait, 21 October 2012; B, male, 5.1 mm (ZRC 2021.0114); C, D,Diogenes platyops Rahayu & Forest, 1995; C, male (not collected), st. TB29, Singapore Strait,01°13.036′N 103°52.820′E, 22 May 2013; D, ovigerous female, 3.5 mm (ZRC 2021.0125); E, Diogenes jousseaumei (Bouvier, 1897), male, 2.2 mm (ZRC 2021.0129).