Caligoneus, Komai & Fujita, 2018

Komai, Tomoyuki & Fujita, Yoshihisa, 2018, A new genus and new species of alpheid shrimp from a marine cave in the Ryukyu Islands, Japan, with additional record of Salmoneus antricola Komai, Yamada & Yunokawa, 2015 (Crustacea: Decapoda: Caridea), Zootaxa 4369 (4), pp. 575-586 : 576-577

Genus Caligoneus n. gen.

[New Japanese name: Kurayami-teppou-ebi-zoku]

Type species. Caligoneus cavernicola n. sp.

Diagnosis. Carapace with 1 or 2 mid-dorsal spines at postrostral position; frontal margin with extra-orbital spine on either side; rostrum well developed, triangular in dorsal view, elevated in dorsal mid-line, cross-section trigonal; pterygostomial angle somewhat produced anteriorly; cardiac notch small, shallow. Eyestalks fully exposed, dorsomesial angle slightly produced and pointed distally; cornea located distolaterally. Pleomeres 1 to 4 with pleura rounded, pleomere 5 with pleuron angular posteroventrally; pleomere 6 without articulated flap. Telson slender, with 2 pairs of dorsolateral spiniform setae; posterior margin bifurcate with very deep V-shaped cleft, each angle with 2 unequally long spiniform setae, mesial much longer than lateral; anal tubercle absent. Antennular peduncle with stylocerite terminating in small spine reaching distal margin of article 1; lateral flagellum with welldeveloped accessory ramus. Antenna with scaphocerite ovate, with broad distal blade reaching well beyond distolateral spine; carpocerite short, reaching mid-length of scaphocerite. Mouthparts typical for family; mandible with bi-articulated palp; maxillule with bilobed palp, both lobules furnished with apical setae. Maxilliped 2 exopod with long, stout plumose setae on lateral margin. Maxilliped 3 with coxal lateral plate rounded; ultimate article without robust spiniform setae on tip. Pereopods 1 (= chelipeds) short, symmetrical, both carried extended or loosely folded; ischium with at least 4 spiniform setae; carpus subcylindrical, widened distally; chela much shorter than carpus; occlusal margin of fixed finger with few minute teeth on proximal half; occlusal margin of dactylus unarmed. Pereopod 2 with ischium devoid of spiniform seta(e); carpus 5-articulated, first article longest, more than half-length of entire carpus. Pereopods 3–5 slender, increasing in length toward posterior, with flexible ischiomeral articulation; ischium with 0–2 spiniform setae (pereopod 3) or unarmed (pereopods 4 and 5); merus and carpus unarmed; propodus with widely spaced, small, slender spiniform setae; dactylus simple, slightly flattened. Pereopod 5 with propodal grooming apparatus consisting of row of short stiff setae evenly spaced. Pleopod 2 with appendix masculina and appendix interna, including ovigerous individuals. Uropod with slender exopod and endopod; diaeresis more or less sinuous. Gill/exopod formula: 5 pleurobranchs (above pereopods 1–5); 1 arthrobranch (maxilliped 3); no podobranch; 2 lobe-shaped epipods (maxillipeds 1 and 2) and 5 strap-like epipods (maxilliped 3, pereopods 1–4), 5 sets of setobranchs (pereopods 1–5); 3 flagellum-like exopods (maxilliped 1–3).

Composition. Presently monotypic.

Etymology. Name derived from the Latin adjective caligos (= dark) and the second part (- oneus) of Salmoneus , a putatively closely related genus, in reference to the aphotic habitat of the new species. Gender masculine.

Distribution. Presently known only from one marine cave off Ie Island, Okinawa Islands in the Ryukyu Islands, Japan.

Remarks. The type species of the proposed new genus shares several taxonomically important morphological features with Salmoneus , including the well-developed rostrum and extra-orbital spines, the absence of an articulated flap at the posteroventral angle of the pleomere 6, and, most significantly, the possession of an appendix masculina in ovigerous individuals. The gill formula also agrees well with that of Salmoneus . In particular, it is similar to Salmoneus armatus Anker, 2010 in having a postrostral median spine on the carapace, representing a rather unusual feature within the Alpheidae ( Anker 2010a, 2010b). On the other hand, Caligoneus cavernicola n. sp. has a few distinctive features ( Table 1), which are not present in any species of Salmoneus , such as the rostrum being dorsally elevated and trigonal in cross-section (versus dorsoventrally flattened) and the subequal and symmetrical pereopods 1 (= chelipeds) of “minor form” (versus greatly dissimilar with an enlarged major cheliped showing various degree of modification according to species in Salmoneus ).

Furthermore, Caligoneus cavernicola n. sp. has some features shared with particular species of Salmoneus . The anteromesially pointed eyestalk as seen in C. cavernicola n. sp. is rather unusual for Alpheidae , being also found in some infaunal species of Salmoneus (cf. Anker & Marin, 2006), as well as in Yagerocaris cozumel Kensley, 1988 , another marine cave dweller from Yucatan Peninsula, Mexico ( Kensley 1988; Anker et al. 2006; Anker 2008). The very deep median cleft on the posterior margin of the telson is also characteristic to C. cavernicola n. sp., although a shallow notch or cleft is present in several species of Salmoneus (e.g., Miya 1972; Banner & Banner 1973; Anker 2007, 2011; Anker & Marine 2006; Komai & Anker 2012). The flexible ischiummerus articulation of the pereopods 3–5 is also seen in Salmoneus antricola (cf. Komai et al. 2015), although in vast majority of species of Salmoneus the articulation is not so flexible (cf. Anker 2010b).

Triacanthoneus is also similar to the new genus. According to Anker (2010b), Triacanthoneus differs from Salmoneus and all other alpheid genera by the presence of three conspicuous spines on the carapace (one middorsal spine, in post-gastric or cardiac position, and one pair of submedian spines, in post-hepatic position). Triacanthoneus and Caligoneus n. gen. also share the flexible ischium-merus articulation of the pereopods 3–5 and the presence of appendix masculina in all individuals, this latter feature shared with Salmoneus and possibly indicating some form of simultaneous hermaphroditism (cf. Anker 2010b). However, Caligoneus is lacking submedian spines on the carapace and also differs from Triacanthoneus by several other characters, such as the presence of a deep median cleft on the telson. In species of Triacanthoneus , the posteromedian cleft of the telson is absent or shallow if present (cf. Anker 2010b; Alvarez et al. 2012, 2014).

It is interesting to note that Caligoneus cavernicola n. sp. has some apparently plesiomorphic features within Alpheidae , such as the dorsally elevated rostrum, the fully exposed eyestalks and the symmetrical chelipeds of “minor form” with non-serrated dactyli (Anker et al. 2006). Nevertheless, at present, it is unknown if the condition exhibited by the new taxon is really plesiomorphic or result of a secondary reduction or reversal.