Richalpheus Anker & Jeng, 2006

Anker, Arthur, 2011, Description of a third species in the alpheid shrimp genus Richalpheus Anker & Jeng, 2006 (Crustacea, Decapoda), Zootaxa 3059, pp. 59-68 : 60-61

Richalpheus Anker & Jeng, 2006

Richalpheus Anker & Jeng, 2006: 380 .

Emended diagnosis. Carapace glabrous; frontal margin without rostrum or orbital teeth; anterolateral suture feebly developed; pterygostomial angle rounded; branchiostegial margin with broad, somewhat membranous, ventral lip; cardiac notch deep. Abdominal somites with rounded pleura; sixth pleomere with more or less marked suture separating a triangular area near posteroventral angle, but without distinctly articulated plate. Telson with two pairs of strong dorsal spiniform setae and two pairs of posterolateral spiniform setae; posterior margin rounded; anal tubercles present, but feebly developed. Eyes concealed in dorsal and lateral views; anteromesial margin of eyestalk bluntly projecting. Antennular peduncles elongate, flattened dorsoventrally; first article with dorsomesial margin armed with small spiniform setae; ventromesial carina armed with blunt or acute tooth; stylocerite pressed tightly against first article, blunt distally, not exceeding distal margin of first article; second article distinctly longer than wide; lateral antennular flagellum with shorter ramus mostly fused to main ramus. Antenna with basicerite moderately robust, with distoventral tooth; carpocerite overreaching scaphocerite. Mouthparts not especially modified; mandible with two-articulated palp; first maxilliped with elongate palp and broad caridean lobe; second maxilliped with elongate epipod. Third maxilliped with lateral plate on coxa moderately produced dorsally; tip of ultimate article unarmed. First pereiopods (chelipeds) unequal in size, asymmetrical in shape, carried flexed; major cheliped elongate, with enlarged chela; basis and ischium unarmed; merus slender, unarmed, with depressed ventral surface; carpus short, cup-shaped; chela subcylindrical, elongate, palm ventromesially depressed, dorsal face with deep transverse constriction near propodo-dactylar articulation; pollex with rounded fossa or shallow longitudinal groove proximally; dactylus unarmed or with flat bulge or well-defined tooth fitting into fossa on pollex; adhesive discs present on distal margin of palm and base of dactylus. Second pereiopod with four-articulated carpus; chela with rows of stiff setae forming several dense brushes on fingers. Third pereiopod and fourth pereiopods compressed; ischium and merus unarmed, carpus with strong distoventral spiniform seta; propodus with strong spiniform setae on ventral margin; dactylus simple, conical. Fifth pereiopod not compressed, shorter and more slender than third and fourth pereiopods; propodus with several setal rows distolaterally. Uropod with lateral lobe of protopod rounded or feebly bifid distally; exopod distally truncated or incised, diaeresis with deep incision and triangular tooth near mesial margin of exopod. Gill/exopod formula: 5 pleurobranchs (above P1–5), 1 arthrobranch (at Mxp3), 2 lobe-like epipods (Mxp1–2), 0 podobranchs, 0 mastigobranchs, 0 setobranchs, 3 exopods (Mxp1–3).

Species included. R. palmeri Anker & Jeng, 2006 (type species), R. dahabensis Anker & Dworschak, 2007 , R. alpheoides sp. nov.

Distribution. Indo-West Pacific: Philippines, French Polynesia, Egypt (Red Sea).

Remarks. Richalpheus appears to be somewhat “intermediate” between the Leptalpheus – Fenneralpheus clade and the Amphibetaeus clade, differing from all of them by the absence of mastigobranchs on the coxae of the third maxilliped and first to fourth pereiopods and the associated setobranchs on first to fifth pereiopods (also known as epipod-setobranch complex), and the presence of several rows of very stiff setae, forming several setal brushes, on the chelae of the second pereiopod. These two features, i.e. the absence of the epipod-setobranch complex and the presence of setal brushes on the second pereiopods are clearly correlated. Setobranchs (held in place by mastogobranchs or hooked epipods) are responsible for passive cleaning of gills, mainly pleurobranchs. The epipod-setobranch complex is present in many other caridean families, as well as some other decapod groups, e.g. Dendrobranchiata, Astacidea etc. ( Bauer 1981). In carideans, in which the epipod-setobranch complex was secondarily lost, e.g., all Palaemonidae and some Hippolytidae , the pleurobranchs are cleaned by setal brushes present either on the first or the second pereiopod ( Bauer 1979). Most Alpheidae have a complete epipod-setobranch complex (mastigobranchs on Mxp3 + P1–4, setobranchs on P1–5), but in several species (or genera), this complex is reduced to only two or three mastigobrach-setobranch sets on the third maxilliped and the most anterior pereiopods (Anker et al. 2006). Only in four alpheid genera, including Richalpheus and the very speciose genus Synalpheus Bate, 1888 , the epipod-setobranch complex was completely lost and the gill cleaning is carried out by the setal brushes on the second pereiopod. These cleaning brushes are indeed very dense in both Richalpheus and Synalpheus ( Dardeau 1984; Anker & Jeng 2006; see also below).