Diogenes minimus, Komai & Yoshida, 2020

Komai, Tomoyuki & Yoshida, Ryuta, 2020, A new species of the hermit crab genus Diogenes Dana, 1851 (Decapoda: Anomura Diogenidae) from shallow coastal waters in Japan, Zootaxa 4722 (6), pp. 571-582 : 572-580

publication ID	https://doi.org/ 10.11646/zootaxa.4722.6.4
publication LSID	lsid:zoobank.org:pub:58052602-8BB3-4CA5-9230-8765A0BB44A0
persistent identifier	https://treatment.plazi.org/id/5493C352-B351-453B-81A9-8CAB4DA81AAC
taxon LSID	lsid:zoobank.org:act:5493C352-B351-453B-81A9-8CAB4DA81AAC
treatment provided by	Plazi
scientific name	Diogenes minimus
status	sp. nov.

Treatment

Diogenes minimus n. sp.

[New Japanese name: Chigo-tsuno-yadokari]

( Figs. 1–6 View FIGURE 1 View FIGURE 2 View FIGURE 3 View FIGURE 4 View FIGURE 5 View FIGURE 6 )

Material examined. Holotype: CBM-ZC 15429, ovigerous female (sl 1.3 mm), off Kouyatsu, Tateyama , Chiba Prefecture, 34°58.58’N, 139°49.025’E, 5–10 m, sand, 21 June 2018, dredge, coll. R. Yoshida. GoogleMaps

Paratypes: CBM-ZC 15430, 4 males (sl 1.0– 1.1 mm), 2 juveniles (sl 0.7, 0.8 mm), off Kouyatsu, Tateyama , Chiba Prefecture, 34°58’N, 139°49’E, 16–19 m, sand, 19 March 2007, dredge, coll. T. Komai GoogleMaps ; CBM-ZC 15431, 1 ovigerous female (sl 1.3 mm), off Shimoda Port, Izu Peninsula, Shizuoka Prefecture, 34°40.03’N, 138°57.11’E, 18.6 m, sand, 25 October 2017, RV Tsukuba II (JAMBIO 13, stn 1), dredge, coll. T. Komai; CBM-ZC 15429, 1 male (sl 1.2 mm; DNA voucher), 1 ovigerous female (sl 1.3 mm), same data as holotype GoogleMaps

Description. Shield ( Fig. 1A View FIGURE 1 ) slightly longer than broad; anterior margin between obtuse rostral lobe and lateral projections slightly concave; anterolateral angles tuberculate or spinulose, lateral margins each with some small tubercles or spinules, posterior halves each cut with few transverse, granular or tuberculate ridges extending onto lateral face; posterior margin roundly truncate; dorsal surface with additional short transverse rows of spinules or granules. Lateral projections roundly triangular, exceeding rostral lobe, unarmed. Carapace lateral lobes ( Fig. 1A View FIGURE 1 ) each anteriorly with 3 or 4 conspicuous spines. Dorsal margin of branchiostegite with serration of 4 or 5 small spines.

Ocular peduncles ( Fig. 1A View FIGURE 1 ) 0.7–0.8 SL, moderately stout, broadened proximally; corneal diameter approximately 0.3 peduncular length, distinctly narrower than basal width of ocular peduncle. Ocular acicles subtriangular, anterior margin with 3–5 small spines increasing in size distally. Intercalary rostral process stout proximally, usually drawn out into small terminal spine, but not overreaching apices of innermost acicular spine; no ventral spine.

Antennular peduncles ( Fig. 1A View FIGURE 1 ) moderately slender, when fully extended, overreaching distal corneal margin by 0.8–0.9 length of ultimate article. Ultimate and penultimate articles each with few short to moderately long setae on dorsal surface; ultimate article slightly widened distally. Basal article with moderately inflated statocyst lobe. Dorsal flagellum aesthetasc-bearing portion consisting of 5 articles

Antennal peduncles ( Fig. 1A, C View FIGURE 1 ) overreaching distal corneal margins by 0.2–0.5 length of article 5; with supernumerary article. Article 5 with 2 rows of long setae ventrolaterally. Article 4 unarmed. Article 3 with small ventrodistal spine. Article 2 with dorsolateral distal angle produced into prominent spine, dorsomesial distal angle with small spine. Article 1 with small spine at dorsolateral distal margin; ventrodistal margin with minute spine. Antennal acicle moderately long and stout, overreaching midlength of article 4, terminating in simple or bifid spine; mesial margin with 2 or 3 small spines. Antennal flagellum ( Fig. 1D View FIGURE 1 ) not exceeding twice length of shield, reaching base of dactylus of left cheliped, consisting of less than 10 articles, each article with pair of long setae.

Maxilliped 3 ( Fig. 1E View FIGURE 1 ) with moderately slender endopod; basis with 1 minute spine on mesial margin ( Fig. 1F View FIGURE 1 ); ischium with crista dentata poorly developed, having 2 prominent, proximally directed spines ( Fig. 1F View FIGURE 1 ).

Left cheliped appreciably longer than right cheliped, sexually dimorphic. Male left cheliped ( Figs. 2 View FIGURE 2 , 6B View FIGURE 6 ) not particularly elongate. Chela 1.6 times as long as wide. Dactylus subequal in length to upper margin of palm, moderately curved, terminating in large calcareous claw, overlapping fixed finger; cutting edge with row of small calcare- ous teeth; outer surface generally flattened, proximally with short row of small tubercles adjacent to upper margin and with scattered setae; upper margin with single row of small tubercles extending to distal 0.2; inner surface weakly elevated in midline, usually with row of small tubercles at least in proximal half. Fixed finger broadly triangular, terminating in curved calcareous claw; outer and inner surfaces almost flat, with sparse setae; cutting edge oblique, with 2 large rounded tooth and few additional small teeth proximally. Palm compressed, slightly wider than long; outer surface convex with elevated midline, with scattered setation and sparse granules or minute tubercles and few conspicuous median spines proximally; upper margin with irregular double row of small tubercles; lower margin with single row of small tubercles extending onto fixed finger angle; inner surface with sparse, scattered low tubercles and few short setae. Carpus subequal in length to palm and shorter than merus, strongly widened distally; upper margin with row of small tubercles increasing in size distally and row of moderately long setae; outer face gently convex, with few small tubercles and scattered short to moderately long setae; lower margin not delimited, but with some tubercles; inner surface with covering of small, blunt tubercles. Merus distal margin unarmed; dorsal surface with row of low protuberances and sparse long setae; lateral face with few small granules, ventrolateral margin with 1 small subdistal spine and followed by 3 larger blunt spines somewhat distant from subdistal spine; mesial face smooth except for granules adjacent to ventromesial margin, ventromesial margin with 2 small distal spines followed by few smaller spines or tubercles; ventral surface slightly concave, with scattered setae. Ischium short, with short transverse row of small tubercles on lateral surface.

Female left cheliped ( Fig. 3 View FIGURE 3 ) generally more spinose than male left cheliped; setation similar to that in males. Chela about 1.7 times as long as wide. Dactylus upper margin armed with double row of small tubercles extending near tip. Palm upper margin with double row of small spines and tubercles; outer surface with irregular row of conspicuous spines in proximal half of midline and additional short rows of small to moderately large spines on either side of median row of spines; lower margin with double row of small spines extending onto fixed finger; inner surface with small spines or tubercles adjacent to lower margin. Carpus upper margin with single row of large spines increasing in size distally; outer surface with median row of small spines or tubercles and some scattered small spines inferior to median spine row; lower margin with 1 or 2 prominent spines; inner surface with scattered small tubercles. Carpus-merus articulation with some degree of counterclockwise rotation. Merus dorsal margin with row of small spines or tubercles; distal margin unarmed; lateral surface almost smooth, with few short setae, ventrolateral margin with row of large, forwardly curved spines; mesial surface also smooth, with some small tubercles or spines adjacent to ventromesial margin, ventromesial margin with 1 moderately large distal spine; ventral surface with few tubercles and sparse long setae. Ischium unarmed.

Right cheliped ( Fig. 4 View FIGURE 4 ) reaching proximal margin of left cheliped palm. Chela about twice as long as wide; dactylus and fixed finger without hiatus, both terminating in calcareous claws. Dactylus 1.5 times as long as palm, gently arched; dorsal surface with row of small spines or tubercles on midline and tufts of long setae; mesial surface with some small scattered tubercles; mesial and ventral surfaces unarmed; cutting edge unarmed. Palm widened distally; dorsal surface with irregular rows of small tubercles adjacent to lateral and mesial margins (lateral row extending onto fixed finger) and with scattered moderately long setae; dorsolateral and dorsomesial margins not delimited, dorsolateral margin gently convex in dorsal view; mesial face glabrous; ventral surface with scattered moderately short setae. Cutting edge of fixed finger with row of obtuse teeth in proximal 0.5–0.6. Carpus with row of moderately large spines on dorsomesial margin; dorsolateral margin not clearly delimited, but with row of small spines; lateral surface with few small, low tubercles; mesial surface glabrous; ventral margin unarmed. Carpusmerus articulation with some degree of clockwise rotation. Merus dorsal margin with row of small spines in distal half and row of moderately long setae; lateral surface with some low tubercles ventrally and few short setae, ventrolateral margin with row of small spines in distal half; mesial surface glabrous, ventromesial margin with few small spines; ventral surface with scattered moderately long setae. Ischium lateral surface sometimes with blister-like low tubercles; surfaces with sparse short setae.

Pereopods 2 and 3 ( Fig. 5A, E View FIGURE 5 ) slender for genus. Dactyli 1.1–1.2 times as long as propodi, 11.6–13.0 times longer than wide, gently arcuate, but not twisted; terminating in moderately small corneous claws; dorsal surfaces unarmed, but with row of moderately spaced long setae, ventral margins also with row of sparse long setae; lateral surfaces almost glabrous, each without median sulcus; mesial surfaces each with 2 rows of short to moderately long setae adjacent to dorsal and ventral margins, otherwise unarmed ( Fig. 5B, F View FIGURE 5 ). Propodi slightly arcuate, narrowing distally; dorsal margins each with row of moderately spaced long setae, but without conspicuous armature; lateral surfaces glabrous (second and right third) or some long setae distally (left third); ventral margin with few setae (sec- ond and right third) or with row of long setae (left third). Carpi each with row of short to long setae and 3–5 small spines (second; unarmed in juveniles) ( Fig. 5C, D View FIGURE 5 ) or only with dorsodistal spine (third; unarmed in juveniles) on dorsal margin ( Fig. 5G View FIGURE 5 ); lateral, mesial and ventral surfaces only with few setae. Meri unarmed; dorsal and ventral margins each with row of moderately spaced setae; lateral and mesial surfaces glabrous. Ischia also unarmed, with row of setae on dorsal and ventral margins.

Pereopods 4 ( Fig. 1G View FIGURE 1 ) semichelate, sparsely setose; dactylus overreaching distal margin of propodus (including rasp); propodus suboval, unarmed dorsodistally; propodal rasp consisting of 3 or 4 rows of small corneous scales extending to midlength of ventral margin; carpus without dorsodistal spine.

Pereopods 5 chelate.

Anterior lobe of thoracic sternite 6 ( Fig. 1H View FIGURE 1 ) not clearly divided, with row of sparse setae and pair of minute tubercle on anterior margin.

Male unpaired left pleopods 2–5 uniramous, marginally setose. Female with paired gonopores; unpaired left pleopods 2–4 well developed, biramous; pleopod 5 as in male.

Telson ( Fig. 1I View FIGURE 1 ) with minute median cleft, left lobe larger than right; left terminal margin with row of tiny spines, and several larger spines extending onto lateral margin; right terminal margin with row of tiny spines, not extending onto lateral margin.

Size. Maximal male sl 1.2 mm; maximal female (ovigerous) sl 1.3 mm.

Colour in life ( Fig. 6 View FIGURE 6 ). Shield generally pale brown, darker anteriorly and with 1 pair of darker brown spots posteriorly; ocular peduncles semi-transparent in distal half, brownish in proximal half; antennule and antenna generally semi-transparent; left cheliped generally whitish, dorsally with tinge of brown on palm, carpus and merus. Right cheliped also generally whitish, dorsally with brown spots on merus. Ambulatory legs generally whitish; dactyli each with broad median and narrower proximal transverse bands; propodi each with dark brown bands distal to midlength and proximally.

Distribution. Presently known only from Pacific side of central Japan: Boso and Izu Peninsula, at depths of 5–19 m, sand bottom.

Remarks. Diogenes minimus n. sp. is referred to the Diogenes edwardsii species group, diagnosed by the possession of a small intercalary rostriform process of which the margins are unarmed, the antennal peduncle being longer than the ocular peduncle, and the antennal flagellum bearing a pair of long setae on each article ( Asakura & Tachikawa 2010), and appears closest to D. holthuisi in sharing the following characteristics: (1) the proximally widened ocular peduncles; (2) the multi-spined carapace lateral lobe; (3) relatively slender ambulatory dactyli; and (4) relatively few scales consisting the propodal rasp of the pereopod 4. In particular, the multi-spined carapace lateral lobe is not known in the other congeneric species ( Asakura & Tachikawa 2010). It is distinguished from D. holthuisi by the following particulars: the antennal acicle is acuminate with two or three small spines on the mesial margin in D. minimus n. sp. ( Fig. 1A View FIGURE 1 ), rather than unequally bifid in D. holthuisi (cf. Asakura & Tachikawa 2010: fig. 1A); the male left cheliped is not particularly elongate in D. minimus n. sp. ( Fig. 2 View FIGURE 2 ), whereas it is distinctly elongate in D. holthuisi (cf. Asakura & Tachikawa 2010: fig. 3A, B); the carpus of the right cheliped is distinctly shorter than the chela in D. minimus n. sp. ( Fig. 4 View FIGURE 4 ), rather than subequal in the length to the chela in D. holthuisi (cf. Asakura & Tachikawa 2010: fig. 2H–J); the merus of the right cheliped is armed with a row of spines on the ventrolateral and ventromesial margins respectively and a few spines on the dorsal margin in D. minimus n. sp. ( Fig. 4B, D View FIGURE 4 ), whereas those margins are unarmed in D. holthuisi (cf. Asakura & Tachikawa 2010: fig. 2H–J); the carpi of the pereopods 2 are armed with one to four small spines in addition to the dorsodistal spine in D. minimus n. sp. in adults ( Fig. 5C, D View FIGURE 5 ), whereas they are armed only with a dorsodistal spine in D. holthuisi (cf. Asakura & Tachikawa 2010: fig. 5A); the left lateral margin of the telson bears small spines in D. minimus n. sp. ( Fig. 1I View FIGURE 1 ), whereas there are no spines on that margin in D. holthuisi (cf. Asakura & Tachikawa 2010: fig. 5I, J).

The new species is probably one of the smallest species in Diogenes alongside D. holthuisi (sl of the known maximal specimen attains only 1.3 mm in D. minimus n. sp. and 1.39 mm in D. holthuisi ) ( Asakura & Tachikawa 2010). With the addition of this new species, 14 species of Diogenes are now known from Japanese waters. Of these, six species ( D. edwardsii , D. izanamiae , D. minimus n. sp., D. nitidimanus , D. penicillatus and D. spinifrons ) are so far restricted to the warm temperate areas in Northeast Asia ( Miyake 1978; Asakura 2006; McLaughlin et al. 2007; Korn et al. 2008), including the Honshu to Kyushu main Japanese islands, whereas six species ( D. avarus , D. biramus , D. dorotheae , D. leptocerus , D. pallescens and D. tumidus ) are widely distributed in the tropical Indo- West Pacific ( Davie 2002; McLaughlin 2002 a, 2002b; Rahayu & Osawa 2009; Arima 2014). Diogenes holthuisi and D. heteropsammicola are so far known only from the Ogasawara Islands ( Asakura & Tachikawa 2010) and Amami Islands in the Ryukyu Islands ( Igawa & Kato 2017), respectively. Although some informal species groups are recognized in Diogenes on the basis of the development and armature of the intercalary rostriform process and the development and structure of the antenna (cf. Rahayu & Forest 1995; McLaughlin 2005; Asakura & Tachikawa 2010), 14 Japanese species are referred either to the D. edwardsii species group (nine species: D. avarus , D. dorotheae , D. edwardsii , D. heteropsammicola , D. holthuisi , D. minimus n. sp., D. nitidimanus , D. penicillatus , D. spinifrons ) or the D. pallescens species group (five species: D. biramus , D. izanamiae , D. leptocerus , D. pallescens and D. tumidus ).

The specific identity of the five unidentified species reported by Arima (2014) remains to be clarified, because no voucher specimens are available. There is no doubt that there are still many unknown species that await discovery in Japanese waters.

Etymology. The specific epithet is a Latin adjective, meaning “small”, in reference to the small body size of the new species for Diogenes .