Triacanthoneus Anker, 2010

Anker, Arthur, 2020, Taxonomic remarks on the alpheid shrimp genus Triacanthoneus Anker, 2010 with description of a second eastern Pacific species (Malacostraca: Decapoda), Zootaxa 4772 (3), pp. 450-468 : 453-456

Genus Triacanthoneus Anker, 2010

Triacanthoneus Anker 2010a: 48 .

Emended diagnosis. Body slender, not compressed. Dorsal mid-line of carapace with one strong, sharp, anteriorly directed tooth arising at carapace mid-length or more or less posterior/anterior to it; dorsal carina typically present as continuation of rostral carina, rarely absent (only in T. armatus ). Carapace flanks with or without simi- larly strong, sharp, anteriorly directed teeth (one on each side) in hepatic or post-hepatic position. Anterolateral margin of carapace with or without sharp teeth arising directly from postorbital margin or slightly posterior to it (possibly homologous to orbital teeth). Rostrum well-developed, distally acute, ventral margin with small sharp tooth typically in subdistal position, rarely near rostral mid-length (only in T. akumalensis ). Pterygostomial angle not produced anteriorly. Cardiac notch variously developed, either deep or reduced to small incision. First to fourth pleonites posteroventrally rounded; fifth pleonite posteroventrally angular or more or less projecting; sixth pleonite without articulated plate. Telson typically slender, with two pairs of cuspidate setae on dorsal surface; posterior margin straight or with variously developed median incision, with two pairs of stout spiniform setae; anal tubercles absent. Eyes typically partly or largely exposed in dorsal view, largely exposed in lateral view, rarely completely concealed in dorsal view and partly visible in lateral view. Antennular peduncle with small sharp tooth on mesioventral carina of first article; stylocerite well developed, with acute tip reaching well beyond distal margin of first article, sometimes with thin dorsal longitudinal carina terminating anteriorly in acute or subacute, dorsally pointing tooth; lateral flagellum with well-developed accessory ramus. Antenna with well-developed, ovate scaphocerite, blade broad and reaching beyond distolateral tooth; carpocerite short, reaching at most half-length of scaphocerite. Mouthparts typical for family; mandible with palp subdivided into two articles; maxillular palp with both lobes furnished with setae. Third maxilliped with lateral plate rounded or subacutely produced; ultimate article tapering into corneous tip, without robust spiniform setae. Chelipeds strongly unequal and asymmetrical, carried flexed when not in use. Major cheliped elongate, slender; ischium with two or three cuspidate setae; merus flattened ventrally; carpus subcylindrical, slender, widening distally; chela smooth, finger cutting edges typically serrated with small subtriangular teeth, rarely (only in T. akumalensis ) with raptorial teeth intercalated between smaller teeth. Minor cheliped slender, slightly elongate, much shorter than major cheliped; ischium with two or three cuspidate setae; chela simple, finger cutting edges unarmed. Second pereiopod slender; ischium armed with one to three cuspidate seta(e); carpus with five subdivisions, first always longest. Third and fourth pereiopods slender; ischium with two or three cuspidate setae; merus and carpus unarmed; propodus with small slender spiniform setae; dactylus simple, conical, slender. Fifth pereiopod with ischium unarmed; propodal brush either well developed or reduced. Second pleopod with appendix masculina and appendix interna in all specimens, including ovigerous ones. Uropod with slender exopod and endopod; exopod with small distolateral tooth and slender distolateral spiniform seta, diaeresis present, more or less sinuous. Gill/exopod formula: 5 pleurobranchs (above P1–5) [with spiny process above P 5 in T. akumalensis ]; 1 arthrobranch (at Mxp3); 0–1 podobranchs (at Mxp2) [typically 0, but with 1 well-developed podobranch in T. akumalensis ]; 2 lobe-shaped epipods (Mxp1–2); 5 mastigobranchs (Mxp3, P1–4), 5 sets of setobranchs (P1–5); 3 exopods (Mxp1–3).

Type species. Triacanthoneus toro Anker, 2010 , by original designation ( Anker 2010a).

Other species included. Triacanthoneus alacranes Anker, 2010 ; T. pacificus Anker, 2010 ; T. armatus (Anker, 2010) , comb. nov.; T. akumalensis Alvarez, Iliffe, Gonzalez & Villalobos, 2012 ; T. chapelianus Alvarez, Iliffe & Villalobos, 2014 ; T. blanca sp. nov. (see below).

Distribution. Western Atlantic (southern Gulf of Mexico and western and central Caribbean Sea) and eastern Pacific ( Panama and Colombia).

Ecology. Western Atlantic : Coral reefs and associated rubble-rich habitats; sand flats with abundant coral rubble and sea grass, often close to mangroves; two taxa recorded from anchialine and marine caves, although only one of them appears to be restricted to caves; depth range: 0.5–15 m. Eastern Pacific : intertidal mud-sand flats with abundant rocks, subtidal sand-rubble flats with rocks and coral rubble; depth range: 0–8 m.

Remarks. The inclusion of T. chapelianus in Triacanthoneus by Alvarez et al. (2014), followed by the present inclusion of T. armatus , result in a certain heterogeneity in the genus, especially in the presence or absence of the strong, sharp, anteriorly directed teeth on the hepatic or post-hepatic area of the carapace. Based on this feature alone, Triacanthoneus can be subdivided into two distinct species groups. The first group includes all species, in which these teeth arise from the hepatic or post-hepatic area of the carapace, viz. T. toro , T. alacranes , T. akumalensis , T. pacificus and T. blanca sp. nov. (see below). The second group includes two species, viz. T. armatus and T. chapelianus , which do not have strong teeth on the hepatic or post-hepatic area of the carapace, but have smaller sharp teeth arising either directly from the anterolateral margin of the carapace (in the former species) or just posterior to it (in the latter species), i.e., in post-orbital position. The homology of these teeth with the orbital teeth of other alpheid shrimp genera, including Salmoneus , appears to be more likely than the hypothesis of their hepatic origin and posterior “migration” towards the post-orbital area (Alvarez et al. 2014), especially in T. armatus , in which they truly arise from the anterolateral margin of the carapace. However, only a detailed study of the presently unknown larvae and post-larvae, combined with Hox genes analyses, could shed more light of this subject.

At least two evolutionary scenarios are possible to explain the general similarity of T. armatus and T. chapelianus, In the first scenario, their great resemblance could be result of a convergent evolution, i.e. these two taxa may not be related at all or only distantly so (in which case, the herein proposed species transfer would obviously be unnecessary). In the second scenario, T. armatus and T. chapelianus could form a small clade on its own, for instance, in an intermediate position between Salmoneus and the other species of Triacanthoneus , or nested somewhere else within Salmoneus . The latter genus is becoming large and increasingly complex morphologically and may well paraphyletic in respect to at least some of the allied genera (including Triacanthoneus ). However, this topic cannot be discussed further without a comprehensive morphological and molecular analysis of several alpheid genera, focusing specifically on Salmoneus , Triacanthoneus , Deioneus and Caligoneus . For the time being, it seems most reasonable and taxonomically convenient to retain Triacanthoneus as a valid genus with a new composition, awaiting such an analysis.

All members of Triacanthoneus , as redefined above, are morphologically distinct from all species currently assigned to Salmoneus by the presence of a strong sharp tooth on the dorsal mid-line of the carapace, situated well posterior to the rostral base (ranging from 0.3 length of the carapace in T. armatus to almost 0.7 length of the carapace in T. alacranes ). However, it must be noted that in the presently monotypic Caligoneus , one strong tooth or two strong teeth is/are present on the carapace mid-line, adjacent to the rostral base (Komai & Fujita 2018: 2A–C), i.e. in a rather different position (much more anterior and also more elevated relative to the rostrum) from the more posterior mid-dorsal tooth in the species of Triacanthoneus . Additionally, Caligoneus differs from Triacanthoneus in the much deeper cleft on the posterior margin of the telson and in the general shape of the chelipeds (Komai & Fujita 2018: figs. 2D, 4C–E). Therefore, these two genera seem to be more distantly related, in addition to be geographically widely separated. Interestingly, within the non-closely related alpheid genus Parabetaeus Coutière, 1897 , one recently described species ( P. acanthus Anker, 2015 ) also has a sharp, anteriorly directed tooth in postrostral position, in contrast to the other two species of the genus, in which the post-rostral area is unarmed ( Nomura & Anker 2001; Anker 2015). This example shows that the armature in the form of a strong, sharp, anteriorly directed tooth on the mid-dorsal line of the carapace is not limited to Triacanthoneus and Caligoneus and that is also may be variable within a genus.