Metabetaeus Borradaile, 1899

Anker, Arthur, 2010, Metabetaeus Borradaile, 1899 revisited, with description of a new marine species from French Polynesia (Crustacea: Decapoda: Alpheidae), Zootaxa 2552, pp. 37-54 : 38-39

Genus Metabetaeus Borradaile, 1899

Metabetaeus Borradaile 1899: 1014 ; Holthuis 1993: 204.

Diagnosis. Body moderately slender, not particularly compressed. Carapace smooth, somewhat gibbous (domed) dorsally, cardiac notch well developed. Frontal margin of carapace with short, proximally broad rostrum and acute, somewhat mesially directed orbital teeth. Pterygostomial angle not projecting anteriorly. Eyes concealed in dorsal and lateral views, visible in frontal view; corneas more or less reduced, in lateral position. Sixth abdominal somite lacking articulated plate at posteroventral angle. Telson with two pairs of strong dorsal spiniform setae and two pairs of stout posterolateral spiniform setae; posterior margin straight to slightly rounded medially; anal tubercles absent. Antennular peduncle with relatively long, distally acute stylocerite; lateral antennular flagellum with well-developed secondary ramus. Antenna with basicerite bearing stout distoventral tooth; scaphocerite with well-developed distolateral tooth and blade. Mouthparts typical for family; mandible with two-segmented palp, incisor process with large brown spot. Third maxilliped pediform; lateral plate of coxa not acutely produced; ultimate segment with small spiniform seta subdistally. First pereiopods (= chelipeds) more or less enlarged, equal in size, symmetrical or subsymmetrical in shape, carried extended with dactylus in lateral position; ischium with spiniform setae on dorsal margin, merus with spinules on ventromesial margin; carpus elongate, subcylindrical to vase-shaped, without comblike setal rows mesially; chela simple, smooth; linea impressa and adhesive discs absent; cutting edges of fingers without snapping mechanism, unarmed or armed with simple teeth. Second pereiopod with fivesegmented carpus, first segment longest, third segment longer than second or fourth. Third and fourth pereiopods slender; ischium usually with one or two spiniform seta(e) on ventrolateral surface; merus usually with one to three spiniform seta(e) on ventrolateral surface; propodus with small spiniform setae ventrally; dactylus simple, acute distally, with slightly thickened setae subdistally. Male second pleopod with appendix masculina greatly exceeding appendix interna; female second pleopod with appendix interna only. Uropodal exopod with diaeresis bearing strong sharp tooth adjacent to stout distolateral spiniform seta; endopod with triangular tooth on distolateral margin. Gill/exopod formula shown in Table 1.

Type species. Metabetaeus minutus ( Whitelegge, 1897) .

Other species included. Metabetaeus lohena Banner & Banner, 1960 ; Metabetaeus mcphersonae n. sp. (see below).

Distribution. Indo-West Pacific (see Fig. 1 ).

Taxonomic remarks. Metabetaeus can be recognised and separated from all other alpheid genera by the combination of the following features: (1) frontal margin with a basally broad, distally acute rostrum and acute, usually mesially directed orbital teeth; (2) more or less reduced lateral corneas; (3) incisor process of the mandible with a conspicuous dark-brown spot (visible in external view of the mouthparts, and sometimes in lateral view through the carapace); (4) equal and symmetrical or subsymmetrical chelipeds, with a nonsculptured simple chela, with fingers unarmed or armed with teeth, and with the ischium bearing a row of spiniform setae on the dorsal margin; (5) second pereiopod with a five-segmented carpus, the first segment being the longest, while the third being longer than the second or fourth; (6) third and fourth pereiopods with merus usually bearing one to three spiniform seta(e) on ventrolateral surface, and with a simple, conical dactylus bearing stiff setae at about 0.7 dactylar length; (7) sixth abdominal somite without articulated plate distoventrally; (8) uropodal exopod with a strong triangular tooth laterally on the diaeresis; (9) uropodal endopod with a more or less developed subtriangular tooth on the distolateral margin. Most of these features are not exclusive to Metabetaeus ; however, the presence of a very conspicuous dark-brown spot on the mandible (3), the second pereiopod with the third carpal segment being longer than second or fourth (5), and the presence of a more or less strong tooth on the distolateral margin of the uropodal endopod (9) appear to be unique to Metabetaeus and are therefore particularly useful for confirmation of the generic identity.

Mxp1 Mxp2 Mxp3 P1 P2 P3 P4 P5 Pleurobranchs + + + + + Arthrobranchs - / +

Podobranchs

Lobe-shaped epipods + +

Strap-like epipods (mastigobranchs) + + + + + Setal epipods (setobranchs) + + + + + Exopods + + +

According to Banner & Banner (1983), the presence of spiniform setae on the ischium and merus of the third and fifth pereiopods (P3–5) is a variable character. Indeed, Banner & Banner (1960) did not mention nor illustrate these spine-like setae in M. lohena . However, in the currently examined specimens of Metabetaeus , including M. lohena , the P3 and P4 merus is furnished with one to three, sometimes very conspicuous spiniform seta(e), on the ventrolateral surface. Another apparently variable character is the presence or absence of an arthrobranch at the base of the third maxilliped. This gill structure is absent in M. minutus according to Coutière (1899), which is confirmed by examination of three specimens of M. minutus . However, a rudimentary arthrobranch appears to be present in the new species (see below).

Phylogenetic remarks. Metabetaeus appears to have no obvious affinities to any other alpheid genus. Borradaile (1899) suggested that Metabetaeus may be closely related to Salmoneus Holthuis, 1955 and also briefly contrasted it with Alpheopsis Coutière, 1896 . Coutière (1899) suggested that the closest relative of Metabetaeus could be Alpheopsis aequalis Coutière, 1896 , because of general similarities in the frontal margin (except for the orbital teeth) and chelipeds. In the phylogenetic analysis of Anker et al. (2006), Metabetaeus was recovered in a weakly supported larger clade that contained several other alpheid genera, including Salmoneus (clade SD), and Athanas Leach, 1814 + closely related “athanoid” genera (clade AP). However, Metabetaeus does not share any exclusive synapomorphies with Athanas , Salmoneus and Alpheopsis . Therefore, its position within the Alpheidae remains unclear; it may well represent a relatively distinct lineage on its own.

Figures

FIGURE 1. Metabetaeus mcphersonae n. sp., paratype, ovigerous female from Moorea (FLMNH UF Arthropoda 23250): A, frontal region, dorsal; B, same, lateral; C, tooth on ventromesial carina of first segment of antennular peduncle, lateral; D, first pleopod with two embryos attached, lateral; E, second pleopod, lateral; F, uropod, dorsal; G, telson, dorsal.