Leptathanas, Grave, De, 2008

Grave, De, 2008, Leptathanas powelli gen. nov., sp. nov, a new infaunal alpheid shrimp associated with upogebiid mudshrimps in Nigeria (Crustacea, Decapoda), Zootaxa 1750, pp. 43-52 : 44-45

Leptathanas gen. nov.

Diagnosis: Carapace glabrous; branchiostegial margin without notch or ventral lip; cardiac notch well developed. Frontal margin with short, broadly triangular rostrum, without orbital teeth; rostral carina weak, reaching to middle of carapace. Pterygostomial angle rounded. Eyes concealed in dorsal and lateral views. Antennular peduncle robust; first segment without ventromesial tooth; stylocerite not appressed, distally acute; second segment as long as broad; lateral antennular flagellum with poorly developed accessory branch. Antenna with robust basicerite; carpocerite not over-reaching scaphocerite. Mouthparts typical for family; mandible with two-segmented palp; first maxilliped with expanded caridean lobe; second maxilliped with large epipod. Third maxilliped pediform; coxa with styliform lateral plate; tip of ultimate segment with cuspidate setae. First pereiopods (chelipeds) asymmetrical in shape, unequal in size, not sexually dimorphic, carried folded with propodus flexed against merus; basis with rudimentary exopod; merus of major cheliped mesially excavated; carpus vase-shaped; chela simple, with ventrally crenulated palm; linea impressa and adhesive discs absent; cutting edges of fingers armed with teeth, finger tips simple. Second pereiopod with five-segmented carpus, chelae simple. Third pereiopod with unarmed ischium and merus; carpus with single distoventral cuspidate seta; propodus mesially with cuspidate setae; dactylus simple. Fifth pereiopod with well developed setal brush on propodus. Second pleopod with appendix interna and appendix masculina in males, with appendix interna only in females. Telson with two pairs of dorsal and two pairs of distolateral cuspidate setae; posterior margin rounded; anal tubercles absent. Gill/exopod formula: 5 pleurobranchs (P1-5), 0 arthrobranch, 0 podobranch, 2 lobe-shaped epipods (Mxp1-2), 4 mastigobranchs (strap-like epipods) (Mxp 3, P1-3), 4 sets of setobranchs (P1-4), 4 exopods (Mxp1-3, P1[rudimentary]).

Type species: Leptathanas powelli sp. nov., by monotypy and present designation.

Etymology: The generic name is a combination of Leptalpheus Williams, 1965 and Athanas Leach, 1814 in reference to the morphological similarity of Leptathanas gen. nov. with Leptalpheus and relatedness to Athanas (see below). Gender is masculine.

Generic relationships: Leptathanas gen. nov. is superficially similar to Leptalpheus , e.g., by the general shape of the frontal margin, chelipeds (P1) and walking legs. However, Leptathanas gen. nov. can be distinguished from Leptalpheus by the absence of an arthrobranch at the base of Mxp3; the presence of a rudimentary exopod on the basis of P1; the absence of a mesioventral tooth on the first segment of the antennular peduncle; the absence of a deep incision and a mesial tooth on the uropodal diaeresis; the presence of cuspidate setae on the uropodal protopod; the presence of mastigobranchs on coxae of Mxp3 and P1-3 (vs. Mxp3 and P 1-4 in Leptalpheus ); the minor chela with short stout fingers (vs. long and slender in Leptalpheus ); and the ultimate segment of the Mxp3 without distally thickened setae, but with cuspidate setae on the tip (vs. with dense, distally thickened setae and unarmed tip in Leptalpheus ). All species of Leptalpheus and related genera (= leptalpheoid lineage or ALF clade in Anker et al., 2006) possess a hyper-developed arthrobranch; a deep incision and a strong mesial tooth on the uropodal diaeresis; the ultimate segment of Mxp3 with dense rows of distally thickened setae and without apical cuspidate setae; and a minor chela with long slender fingers, usually with toothed cutting edges. The absence of these features in Leptathanas gen. nov. suggests that the new genus is not closely related to Leptalpheus .

On the other hand, several features of Leptathanas gen. nov., such as the absence of an arthrobranch on Mxp3; the presence of a rudimentary exopod on P1; the presence of a cuspidate seta on the dorsal margin of the ischium of P1; and the absence of a mastigobranch on P4, are also present in members of the generic complex around Athanas Leach, 1814 (= athanoid lineage or clade AP in Anker et al., 2006). Among these, Athanopsis Coutière, 1896 with five species all in the Indo-West Pacific ( Anker & Ahyong, 2007b) appears to be morphologically closest to the new genus, especially in the general shape and size ratio of the major and minor chelipeds. For instance, the major cheliped in both genera is characterized by one or several cuspidate setae on the ischium; a distally broadened, ventrally excavated and laterally rugose merus; a tuberculate ridge on the ventral margin of the palm; and the cutting edges of the dactylus and pollex each armed with one or a few blunt teeth. However, Leptathanas gen. nov. can be easily separated from Athanopsis by the reduced rostrum (vs. well developed with rounded tip in Athanopsis ); the absence of orbital teeth (present and usually well developed in Athanopsis ); the absence of a mesioventral tooth on the first segment of the antennular peduncle; the five-segmented carpus of the P2; the absence of cuspidate setae on the ischium of P3; and finally, by the presence of a row of cuspidate setae on the uropodal protopod, a feature that appears to be unique within the family (A. Anker, pers. obs.). This last feature, in combination with other characters (see above) enables to separate Leptathanas gen. nov. from all other alpheid genera. The remarkable convergence between Leptalpheus from the leptalpheoid lineage and Leptathanas gen. nov. from the athanoid lineage may be explained by very similar infaunal life styles of these shrimps (see below).