Diogenes spongicola, Komai & Ravinesh & Kumar, 2018

Komai, Tomoyuki, Ravinesh, Raveendhiran & Kumar, Appukuttannair Biju, 2018, A new species of the hermit crab genus Diogenes Dana, 1851 (Decapoda: Anomura: Diogenidae) from southern India, Zootaxa 4504 (2), pp. 243-252 : 244-251

Diogenes spongicola n. sp.

( Figs 1–5 )

Material examined. Holotype: male (sl 1.9 mm) (ZSI/ WGRC /IR-INV 11699), Vizhinjam , Thiruvananthapuram, Kerala, southwestern India, 08°22’N, 76°59’E, 1–5 m, 12 January 2018, snorkeling, coll. R. Ravinesh.

Paratypes: 2 males (sl 1.7, 1.8 mm) ( DABFUK / AR-AN98 , 99 ), 4 ovigerous females (sl 1.9, 2.0, 2.1, 2.2 mm) ( DABFUK / AR-AN100–103 ), same data as holotype ; 1 male (sl 1.2 mm) ( DABFUK / AR-AN104 ), 1 ovigerous female (sl 1.6 mm) ( DABFUK / AR-AN105 ), Vellapatti , Tuticorin, Tamil Nadu, southeastern India, 08°50'N, 78°10'E, 2–5 m, 8 February 2018, snorkeling, coll. R. Ravinesh.

Description. Shield ( Fig. 1A ) 1.1 times as long as broad; dorsal surface slightly rugose medially, with some short, transverse, marginally spinulose ridges laterally, and with few tufts of short setae. Rostral lobe broadly rounded, weakly produced. Lateral projections triangular, slightly exceeding tip of rostral lobe; lateral projections and slightly sloping anterolateral margins lined with minute spinulose granules, anterolateral angles each with small spine. Branchiostegite membranous, except for dorsal portion being well-calcified, dorsal margin unarmed; anterior margin rounded, unarmed.

Ocular peduncles ( Fig. 1A ) equal in length, 0.6–0.7 length of shield, inflated proximally, narrowing distally to rounded cornea, surfaces almost glabrous. Corneas not dilated, their diameter distinctly less than basal width of ocular peduncles, about 0.2 of peduncular length. Ocular acicles longer than broad, subtriangular, mesioproximal angles narrowly separated; terminating in small spine, and with row of 5 or 6 small spines on distolateral margin. Intercalary rostriform process simply acuminate at tip, not reaching tips of ocular acicles.

Antennular peduncles ( Fig. 1A ), when fully extended, overreaching distal corneal margins by 0.7 length of article 3. Article 3 1.2 times as long as article 2, slightly widened in lateral view, with few moderately long setae on dorsal surface. Articles 2 and 1 unarmed. Dorsal flagellum with aesthetasc-bearing portion short, consisting of 6 or 7 articles.

Antennal peduncles ( Fig. 1A, C ) overreaching distal corneal margins by half-length of article 5. Article 5 subcylindrical, unarmed, ventrally with 2 rows of long setae. Article 4 with triangular dorsodistal margin, otherwise unarmed. Article 3 short, with few setae ventrally, unarmed. Article 2 stout, with dorsolateral distal angle produced into prominent spine, dorsomesial distal angle unarmed; ventral surface convex. Article 1 unarmed, or at most with few tubercles on dorsolateral distal angle, with scattered short setae, mesially with very short plumose setae. Antennal acicle stout, nearly reaching distolateral margin of article 4, terminating in bifid spine, with row of 3 strong spines and short setae on mesial margin. Antennal flagella ( Fig. 1B ) about 1.8 times as long as shield, forming cast net-like structure, each article distally with long lateral and mesial setae, and sometimes with additional short seta dorsally.

Maxilliped 3 ( Fig. 1D ) moderately slender. Basis with 3 or 4 minute spiniform denticles on mesial margin ( Fig. 1E ). Ischium with crista dentata composed of 3 or 4 small corneous spines, of which proximalmost one strongest and recurved toward proximal ( Fig. 1E ). Merus to dactylus unarmed; dactylus distinctly shorter than propodus. Exopod reaching nearly to midlength of carpus, with well-developed flagellum.

Chelipeds unequal and dissimilar, left much larger than right, but not particularly elongate even in males. Left cheliped of male ( Fig. 2 ) with very narrow hiatus between dactylus and fixed finger; setation on chela and carpus quite sparse. Chela 1.3–1.4 times as long as wide. Dactylus gently curving, about 1.1 times length of palm; dorsal surface with rows of small spinulose tubercles along midline and adjacent to dorsomesial margin; dorsomesial margin with row of small, forwardly directed spines decreasing in size distally; cutting edge with 2 large blunt calcareous teeth and additional much smaller calcareous teeth, terminating in strong calcareous claw. Palm somewhat compressed, wider than long measured along mesial margin; dorsal surface markedly elevated proximomedially, with scattered, well-spaced, small tubercles (those clustered on proximomesial elevation spiniform); dorsolateral margin clearly delimited, weakly upturned, crested, sinuous in lateral margin, bordered with irregularly double or triple row of tubercles of various size; dorsomesial margin with row of small, forwardly directed spines accompanied by much smaller spinulose tubercles adjacent to dorsomesial margin; ventral surface gently convex, smooth, nearly glabrous; fixed finger short, stout, with tufts of short setae on dorsal and ventral surfaces; cutting edge with 2 large, blunt calcareous teeth, terminating in strong calcareous claw. Carpus noticeably becoming wider distally; dorsomesial margin with row of spines noticeably increasing in size distally; dorsal surface elevated in midline, with single row of strong spines, and with 3 strong spines adjacent to dorsodistal margin; dorsolateral surface sloping to lateral surface, with scattered, low tubercles; mesial and ventral surfaces almost smooth, ventrodistal margin bordered with minute granules. Merus almost as long as high, with very low tubercles or protuberances on dorsal margin accompanied by sparse short setae, dorsodistally with row of long setae; lateral surface with small, low tubercles adjacent to indistinctly delimited ventrolateral margin, ventrodistal margin with few minute spines; mesial surface nearly flat, with few small, low tubercles adjacent to indistinctly delimited ventromesial margin; ventral surface with small, low tubercles and scattered short setae. Ischium without conspicuous armature, with sparse short setae.

Left cheliped of female generally similar to that of male, but with weaker spines on dorsomesial margin of palm.

Right cheliped ( Fig. 3 ) reaching to midlength of fingers when fully extended. Dactylus gently curving, 1.4 times as long as palm measured along mesial margin; dorsal surface with scattered tufts of short to long setae, and with tiny tubercles or spinules along proximal half of midline, dorsomesial margin not clearly delimited, but with row of tiny spinulose tubercles; cutting edge with row of very small, blunt calcareous teeth, terminating in small corneous claw. Palm with tiny scattered tubercles or spines and tufts of short to long setae on dorsal surface, extending onto fixed finger, dorsomesial margin with row of small spines; lateral surface sloping without delimitation of dorsolateral margin; mesial to ventral surface glabrous, with few setae. Cutting edge of fixed finger with row of minute blunt calcareous teeth, terminating in small corneous claw. Narrow hiatus between fingers. Carpus widened distally, with scattered tufts of long setae dorsally; dorsomesial margin with row of small spines increasing in size distally, followed by few low protuberances; dorsolateral margin indistinctly delimited, with few small spines distally, followed by low protuberances; lateral surface with few small, low protuberances; mesial and ventral surfaces almost glabrous. Merus somewhat compressed laterally; dorsal margin with row of well-spaced tufts of short to long setae, dorsodistal margin with long stiff setae; lateral surface with few small, low protuberances ventrally, ventrolateral distal margin with 1 small subterminal spine; mesial surface almost glabrous, ventromesial margin with faint tiny protuberances; ventral surface narrow, with scattered long setae. Ischium unarmed, with sparse setae.

Pereopods 2 and 3 ( Fig. 4A, C ) generally similar, relatively slender. Dactyli subequal in length to propodi, slightly curved ventrally, but not twisted, terminating in long corneous claw; dorsal margins each with row of sparse moderately long setae; lateral and mesial faces each with few short setae, devoid of median sulcus ( Fig. 4B, E ); ventral margins each with row of sparse moderately long setae. Propodi distinctly longer than carpi, slightly curved; dorsal margin without conspicuous armature, each with row of sparse, moderately long setae; lateral and mesial faces almost glabrous; ventral surface with row of widely spaced, moderately long setae. Carpi each with 1 or 2 small dorsodistal spines and row of moderately long setae; lateral and mesial faces almost glabrous; ventral margin with few moderately short setae. Meri each with row of sparse, moderately long setae on dorsal and ventral margins; lateral and mesial faces almost glabrous; ventral margins unarmed. Ischia each with row of setae on dorsal and ventral margins. Female with paired gonopores on coxae of pereopods 3.

Pereopods 4 semichelate ( Fig. 1F ). Dactylus slightly curved distally, dorsodistally with long, dense setae. Propodi with small spine at dorsodistal angle; propodal rasp occupying lateral face of fixed finger and extending onto ventral one-third of palm, consisting of numerous rows of small oval corneous scales. Carpus unarmed at dorsodistal angle.

Anterior lobe of thoracic sternite 6 (pereopods 3) ( Fig. 1G ) slightly divided into 2 lobes, unarmed.

Pleon dextrally twisted. Male with 4 well-developed, unpaired left pleopods, pleopod 3 largest, pleopods 2–3 biramous, pleopod 5 uniramous. Female also with 4 unpaired left pleopods.

Telson with indistinct median cleft on posterior margin. In holotype, left posterior lobe smaller than right posterior lobe (rather abnormal; Fig. 1H ), while in paratypes, vice versa (normal; Fig. 1I ). Larger (normally left) posterior lobe generally rounded, terminal margin with row of small spines mesially and row of short to long setae laterally. Smaller (normally right) posterior lobe also rounded, terminal margin with single row of small spines extending onto posterolateral margin and increasing in size laterally, and with row of long setae extending onto lateral margin.

Colour in life ( Fig. 5 ). Shield generally cream, with patches of brown on posteromedian part; median part of posterior carapace (= cardiac region) cream, branchial regions pinkish. Ocular peduncles cream in proximal half, translucent in distal half, with 2 black spots arranged transversely at midlength and tinge of light brown proximally; cornea with mixture of black and white spots; ocular acicles light brown. Antennular peduncle semi- translucent, with few widely spaced black spots. Antennal peduncle and flagellum blue-gray. Left cheliped generally cream, with large brown blotch on palm; carpus and merus with black transverse bands. Right cheliped also generally cream; black transverse bands crossing proximal part of palm and subdistal part of merus. Ambulatory legs generally cream; dactyli each proximally with black ring; propodi each with 2 black transverse bands (one median and one proximal); carpi each with 1 black transverse medially and black spot proximally; meri each with 2 transverse bands medially and proximally).

Distribution. So far known only from southern India, Kerala and Tamil Nadu states; subtidal, 1–5 m deep.

Ecology. The specimens were all collected from cavities of the callyspongiid sponge Callyspongia diffusa . Gastropod shells used as housing include Euchelus asper (Gmelin, 1891) (Chirodontaidae) , Anachis terpsichore (Sowerby, 1822) (Columbellidae) and Reticunassa sp. (Nassaridae). Surfaces of those shells were often covered also with C. diffusa ( Fig. 5B ).

Remarks. The asymmetry of the telson in the holotype is reversed ( Fig. 1H ). In all paratypes, the telson is normal with the left posterior lobe being larger than the right one ( Fig. 1I ).

Diogenes spongicola n. sp. is referred to the D. edwardsii (De Haan, 1849) species group, as diagnosed by Asakura & Tachikawa (2010), in having a small, simple intercalary rostral process, and the antennal peduncles that exceed the ocular peduncle and have long flagella with paired rows of long setae ventrally. It appears close to D. takedai , known from Indonesia, in the following particulars (cf. Rahayu 2012): ocular peduncle inflated proximally and narrowing to non-inflated, small cornea; palm of left chela proximomedially elevated into blunt ridge and having scattered very small tubercles or granules on entire dorsal surface; and dorsal margins of dactyli to carpi of ambulatory legs unarmed except for dorsodistal spine(s) on carpi. Furthermore, the living coloration is generally similar between the two species, in particular the banded ambulatory legs. Nevertheless, there are many features differentiating the two species, as outlined below: (1) the anterolateral margins of the shield are spinulose in D. spongicola n. sp. ( Fig. 1A ), but smooth in D. takedai (cf. rahayu 2012: fig. 1A); (2) the dorsal surface of the shield bears some transverse spinulose ridges on the anterior half in D. spongicola n. sp. ( Fig. 1A ), but there are no such transverse ridges in D. takedai (cf. rahayu 2012: fig. 1A); (3) article 2 of the antennal peduncle does not have a spine other than the prominent dorsolateral distal spine in D. spongicola n. sp. ( Fig. 1C ), whereas it has three or four additional spines on the ventrolateral distal margin in D. takedai (cf. rahayu 2012: fig. 1B); (4) the antennal acicle nearly reaches to the distolateral margin of the article 4 of the antennal peduncle and is armed with three spines on the mesial margin in D. spongicola n. sp. ( Fig. 1A, C ), rather than only reaching to the midlength of the article 4 and being armed with only one spine on the mesial margin in D. takedai (cf. Rahayu 2012: fig. 1A, B); (5) the merus of the left cheliped does not have conspicuous spines in D. spongicola n. sp. (there are only a few minute spines on the ventrolateral margin; Fig 2C ), whereas there is row of some spines on the ventrolateral margin in D. takedai (cf. rahayu 2012: fig. 1D); (6) the palm of the right chela is armed with small spines on and adjacent to the dorsomesial margin in D. spongicola n. sp. ( Fig. 3A, B ), whereas such spines are absent on the palm of the right chela in D. takedai (cf. rahayu 2012: fig. 1ED). Furthermore, the living color of the left palm is different between the two species: in D. spongicola n. sp., the palm has a large dark brown blotch centrally ( Fig. 5A ), which is absent in D. takedai (cf. Rahayu 2012: 269).

Diogenes albimanus Landschoff & Rahayu, 2018 , recently described from South Africa, also has proximally inflated ocular peduncles and short antennal acicles. The presence of a row of spines on the dorsal margin of each carpus of the pereopods 2 and different armature of the left cheliped immediately distinguish D. albimanus from the new species. The outer surface of the palm of the left cheliped bears closely packed small tubercles in D. albimanus (cf. Landschoff & Rahayu 2018: fig. 1B), whereas in D. spongicola , there are fewer, sparse small tubercles ( Fig. 2A ); the merus is armed with a row of strong spines on ventrolateral margin in D. albimanus (cf. Landschoff & Rahayu 2018: fig. 1B), rather than unarmed in D. spongicola ( Fig. 2D ). The colouration in life is also very different between the two species. In D. albimanus , the left chela is entirely white; the ambulatory legs have orange longitudinal stripes (cf. Landschoff & Rahayu 2018: fig. 3). As illustrated in Fig. 5 , in D. spongicola , the left chela has a large dark brown blotch on the outer surface; there are no longitudinal stripes on the ambulatory legs.

As mentioned above, the examined specimens of D. spongicola n. sp. were all collected from cavities of Callyspongia diffusa , suggesting a possible association of this sponge species. The housing shells are often covered also with sponge ( Fig. 5B ), but it is unknown if those covering sponges are derived from the Callyspongia colonies that the hermit crabs inhabit.

Etymology. The name is a combination of the Latin spongia (= sponge) and - cola (= dweller), in reference to the possible association of the new species with the callyspongiid sponge Callyspongia diffusa .

Figures

FIGURE 1. Diogenes spongicola n. sp. A–H, holotype, male (sl 1.9 mm), ZSI/WGRC/IR-INV 11699; I, paratype, ovigerous female (sl 2.0 mm), DABFUK/AR-AN 105. A, shield (including carapace lateral lobes) and cephalic appendages (antennal flagella omitted), dorsal view; B, left antennal flagellum, dorsal view; C, left antennal peduncle, lateral view; D, left maxilliped 3, lateral view; E, same, ischium and basis, dorsal view; F, left pereopod 4, dactylus, propodus and carpus, lateral view; G, anterior lobe of thoracic sternite 6, ventral view; H, I, telson, dorsal view; same, setae omitted.

FIGURE 2. Diogenes spongicola n. sp., holotype, male (sl 1.9 mm), ZSI/WGRC/IR-INV 11699, left cheliped. A, chela, dorsal view; B, same, ventral view; C, entire left cheliped, mesial view; D, same, mesial view; E, carpus, dorsal view.

FIGURE 3. Diogenes spongicola n. sp., holotype, male (sl 1.9 mm), ZSI/WGRC/IR-INV 11699, right cheliped. A, chela and carpus, dorsal view (setae omitted); B, entire cheliped, mesial view; C, same, lateral view.

FIGURE 4. Diogenes spongicola n. sp., male (sl 1.9 mm), ZSI/WGRC/IR-INV 11699, ambulatory legs. A, right pereopod 2, lateral view; B, same, dactylus, mesial view; C, left pereopod 3, lateral view; D, same, dactylus, mesial view.

FIGURE 5. Diogenes spongicola n. sp., paratypes, showing colour in life. A, paratype, male (sl 1.7 mm), DABFUK/AR- AN98; B, paratype, ovigerous female (sl 2.2 mm) and housing shell covered with Callyspongia diffusa, DABFUK/AR-AN103.