Triacanthoneus, Anker, Arthur, 2010

Anker, Arthur, 2010, A new genus and three new species of alpheid shrimps (Crustacea, Decapoda, Caridea) from the tropical American coasts, Zootaxa 2652, pp. 47-63 : 48-49

Genus Triacanthoneus n. gen.

Diagnosis. Carapace with three strong, sharp, anteriorly directed teeth, one mediodorsal tooth situated posterior to carapace mid-length, and two teeth situated in post-hepatic position, anterior to mediodorsal tooth; frontal margin of carapace with or without orbital teeth, with well-developed rostrum, latter distally acute, with ventral margin bearing subterminal tooth; pterygostomial angle not produced anteriorly; cardiac notch deep. Eyes partly or largely visible in dorsal and lateral views. First to fourth abdominal somites rounded posteroventrally; fifth somite angular or slightly projecting posteroventrally; sixth somite without articulated plate. Telson slender, with two pairs of spiniform setae on dorsal surface; posterior margin straight, with two pairs of spiniform setae; anal tubercles absent. Antennular peduncle with strong mesioventral tooth on first segment; stylocerite reaching beyond distal margin of first segment, acute distally; lateral flagellum with well-developed accessory ramus. Antenna with scaphocerite ovate, with broad blade reaching beyond distolateral tooth; carpocerite short, not reaching half-length of scaphocerite. Mouthparts typical for family; mandible with bisegmented palp; maxillular palp with both lobes furnished with setae. Third maxilliped with lateral plate rounded or subacutely produced; ultimate segment tapering into corneous tip, without robust spiniform setae. Chelipeds strongly unequal and asymmetrical, carried folded; major cheliped elongate, slender; ischium with at least two spiniform setae; merus flattened ventrally; carpus subcylindrical, widening distally; chela smooth, finger cutting edges serrated; minor cheliped not elongate; ischium with at least two stout spiniform setae; chela simple, finger cutting edges straight. Second pereiopod with ischium bearing spiniform seta(e); carpus five-segmented, first segment longest. Third and fourth pereiopods very slender, with flexible ischio-meral articulation; ischium with spiniform setae; merus and carpus unarmed; propodus with small slender spiniform setae; dactylus simple, conical, relatively slender. Fifth pereiopod with propodal brush well developed or reduced. Second pleopod 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 P1–5); 1 arthrobranch (at Mxp3); 0 podobranch; 2 lobe-shaped epipods (Mxp1–2); 5 mastigobranchs (Mxp3, P1–4), 5 sets of setobranch (P1–5); 3 exopods (Mxp1–3).

Type species. Triacanthoneus toro n. sp.

Other species included. Triacanthoneus pacificus n. sp., T. alacranes n. sp.

Etymology. Name derived from the Latin adjective triacanthus (= with three spines) and the second part (- oneus) of Salmoneus , a closely related genus. Gender masculine.

Distribution. Transisthmian: tropical Western Atlantic ( Panama, Gulf of Mexico) and tropical Eastern Pacific ( Panama, Colombia).

Phylogenetic position. Triacanthoneus n. gen. shares numerous features with Salmoneus , including the presence of a well-developed rostrum; absence of an articulated flap on the sixth abdominal somite; a complete gill-exopod formula; unequal and asymmetrical chelipeds, with the major cheliped carried folded; serrated cutting edges of the major chela fingers; and the presence of an appendix masculina in ovigerous individuals. More specifically, Triacanthoneus n. gen. has similarities with some species of the S. gracilipes Miya, 1972 group, especially S. armatus , which also has a small mediodorsal tooth on the carapace, apically bifid rostrum, partly exposed eyestalks, and the cheliped ischia armed with two stout spiniform setae ( Anker 2010). However, in S. armatus , the small mediodorsal tooth is distinctly more anterior compared to the much stronger mediodorsal tooth of Triacanthoneus n. gen., and therefore, these two teeth do not appear to be homologous. In addition, all three species of Triacanthoneus n. gen. possess a strong post-hepatic tooth on each side and anterior to the mediodorsal tooth. The ischio-meral articulation of Triacanthoneus n. gen. appears to be flexible (see Figs. 4 G, I, 9G, I), which is not the case in Salmoneus (see figures in Anker 2003a, 2003b; Anker & Marin 2006; Anker 2010). The presence of three sharp teeth on the carapace and a facilitated ischio-meral articulation at once separate Triacanthoneus n. gen. from Salmoneus and all other alpheid genera. It remains clear that a taxonomic revision of Salmoneus sensu lato, combined with a molecular analysis, is becoming more and more necessary; some of the species groups (e.g. S. jarli Holthuis, 1951 and S. gracilipes groups) may indeed need to be elevated to generic rank.

Other remarks. Curiously, all known specimens of Triacanthoneus n. gen., seven in total, are both ovigerous and have a well-developed appendix masculina, reminiscent of the situation in Salmoneus ( Carvacho 1989; Anker 2010) and suggesting some form of hermaphroditism. Furthermore, all three species of the new genus are characterised by the relatively small number (usually no more than 20, cf. Figs. 1 , 7 ) large eggs or embryos, indicating advanced larval development.

Figures

FIGURE 1. Triacanthoneus toro n. gen., n. sp., paratype, ovigerous female (CL 3.4 mm) from Bocas del Toro, Caribbean coast of Panama, USNM 1145376: habitus, lateral.

FIGURE 4. Triacanthoneus toro n. gen., n. sp., paratype, ovigerous female (CL 3.4 mm) from Bocas del Toro, Caribbean coast of Panama, USNM 1145376: A, right (major) cheliped, lateral; B, same, carpus and chela, mesial; C, same, chela fingers; D, left (minor) cheliped, lateral; E, same, chela, mesial; F, second pereiopod, lateral; G, third pereiopod, lateral; H, same, dactylus; I, fifth pereiopod, lateral.

FIGURE 7. Triacanthoneus alacranes n. gen., n. sp., holotype, ovigerous female (CL 3.7 mm) from Alacranes Reef, Yucatán, Mexico, UNAM-CNCR 26177: habitus, lateral.