Leptathanas powelli Grave, 2008, sp. nov.

Leptathanas powelli sp. nov.

Figs. 1–5

Leptalpheus sp. Powell, 1985: 236

Material examined: Holotype: ovigerous female (cl 3.2 mm), Nigeria, Niger delta, Eagle Island, Bonny River system, leg. C.B. Powell, 20 September 1979, from burrows of Upogebia furcata (RMNH D52026). Paratypes: 13 specimens, males and females (cl 1.8–3.4 mm), from same lot/locality (RMNH D49914); 5 specimens from same lot/locality (OUMNH 2008-02-001).

Derivation of name: Named after the collector of the type series, C. B. Powell (1943–1998), in recognition of his considerable field collecting skills and his advancement of our knowledge of Nigerian Decapoda (see Williamson, 2003).

Description: Body moderately elongate (Fig. 1 A), not compressed laterally, glabrous. Orbital teeth absent, pterygostomial angle rounded, branchiostegal margin sparsely furnished with setae, cardiac notch well developed (Fig. 1 D). Frontal margin with shallow, triangular rostrum (Fig. 1 B, C), rostral carina weakly developed, extending to mid-length of carapace. Eyes concealed in dorsal or lateral view (Fig. 1 B, C), without anterior projections, cornea well developed. Ocellar beak not conspicuous. Epistomial sclerite unarmed.

Pleura of first through to fourth abdominal somites rounded, those of fifth and sixth somites somewhat angular (Fig. 1 E); sixth somite with subtriangular articulated plate at posteroventral angle.

Antennular peduncle moderately stout (Fig 1 C, 3A), second segment shorter than first; stylocerite reaching to mesial margin of second segment, distally acute; lateral flagellum biramous, accessory branch reduced, consisting of two segments furnished with groups of aesthetascs (Fig 3 A). Antenna with basicerite stout, armed with strong ventrolateral tooth; scaphocerite (Fig. 3 B) short, square, anterior margin of blade convex, not extending beyond distolateral tooth; carpocerite robust, exceeding to distal margin of scaphocerite.

Mouthparts fairly typical for family. Mandible (Fig. 2 A) with two-segmented palp; molar process as illustrated; incisor process with five subtriangular teeth, second ventral tooth largest. Maxillule (Fig. 2 B) with bilobed palp, dorsal lobe without setae, ventral lobe with single plumose seta; ventral endite with pappose setae. Maxilla (Fig. 2 C) with moderately broad scaphognathite; endopod small, unsegmented; dorsal endite not subdivided. First maxilliped (Fig. 2 D) with moderately expanded caridean lobe on exopod; endopod not segmented, terminally with single robust plumose seta, laterally with several less robust plumose setae; epipod large, ear-shaped, slightly subdivided. Second maxilliped (Fig. 2 E) not particularly modified, with elongate epipod. Third maxilliped (Fig. 2 F) pediform, moderately slender; lateral plate acutely produced; penultimate segment about 2.5 times as long as wide; tip of ultimate segment tapered, with three cuspidate setae on apex (Fig. 2 G); arthrobranch absent.

First pereiopods unequal in size, asymmetrical in shape (Fig. 3 C, D), carried flexed when not in use. Major cheliped (Figs. 3 D–H) robust; basis with minute exopod (Fig. 3 E); coxa, basis and ischium unarmed; merus about 2.5 times as long as wide, mesially excavated (Fig. 3 F), lateral margin distinctly crenulated (Fig. 3 G); carpus vase-shaped; chela robust, ventral margin of palm crenulated (Fig. 3 E); linea impressa and adhesive discs absent; fingers about 0.35 times length of palm, cutting edges of pollex and dactylus each with large tooth fitting in a hiatus on opposed margin (Fig. 3 H), finger tips blunt, broad. Minor cheliped (Fig. 3 C) much weaker than major cheliped; basis with minute exopod (Fig. 3 C); coxa, basis and ischium unarmed, merus about 2.7 times as long as wide, carpus slightly longer than propodus; fingers about as along as palm, with broad tips.

Second pereiopod (Fig. 4 A) relatively short, stout; ischium shorter than merus, mesially somewhat excavated; merus mesially depressed; carpus with five segments, segment length ratio approximately:; chela simple, fingers as long as palm, tips conical, without additional teeth (Fig. 4 B). Third pereiopod (Figs. 4 C) relatively stout; basis with slender cuspidate seta disto-dorsally; ischium unarmed ventrally, with slender cuspidate seta disto-dorsally; merus unarmed, about 3.5 times as long as ischium; carpus about 0.6 times as long as merus, with one ventrolateral cuspidate seta disto-ventrally; propodus subequal to carpus in length, with three ventral cuspidate setae and one disto-ventral cuspidate seta proximal to dactylus (Fig. 4 D); dactylus simple, subconical, curved, about 0.5 times as long as propodus. Fourth pereiopod (Fig. 4 E) similar to third pereiopod, with fewer ventral cuspidate setae on propodus. Fifth pereiopod (Fig. 4 F) more gracile than third and fourth; propodus with two ventral cuspidate setae; setal brush well developed (Fig. 4 G), with two rows of thick pappose setae; dactylus slender, curved, slightly more than 0.6 times length of propodus.

Male second pleopod (Fig. 5 F) with appendix masculina shorter than appendix interna, furnished with five distal and subdistal simple setae (Fig. 5 G); female second pleopod with appendix interna only.

Uropods slightly longer than telson (Fig. 1 E); lateral lobe of protopod distally concave, furnished with five-six stout cuspidate setae; endopod and exopod subequal in length, latter with incomplete diaeresis; distolateral spine submarginal (Fig. 5 C). Telson broad, approximately 0.7 times as wide as long, tapering distally; dorsal surface with two pairs of cuspidate setae, distant from lateral margin, anterior pair situated at about 0.25 of telson length, posterior pair at about 0.65 of telson length; posterior margin (Fig. 5 E) somewhat convex, each posterolateral angle with pair of cuspidate setae, mesial ones about twice as long as lateral ones; central portion of posterior margin between cuspidate setae with 10–12 plumose setae (Fig. 5 E); anal tubercles absent.

Ecology: All specimens of L. powelli sp. nov. were collected from burrows of the mudshrimp Upogebia furcata (Aurivillius, 1898) (Thalassinidea, Upogebiidae) from high and medium salinity mangrove creeks in the Bonny River system of the Niger delta. The general morphology of L. powelli sp. nov. also suggests that this shrimp may indeed be an infaunal “commensal” inhabitant of upogebiid burrows. At least two species of the presumably closely related genus Athanopsis (see above) are known to be associated with echiurans (see Anker et al. 2005 for review). This hypothesis is also reinforced by the fact that the non-related but morphologically very similar shrimps of the genus Leptalpheus are infaunal associates of various thalassinideans (e.g., Anker et al., 2006b).

Distribution: Presently known only from the Niger delta in Nigeria, West Africa. Powell (1985) provides no further details of this species’ possible wider distribution, beyond noting its occurrence in brackish water in the Niger delta.

Taxonomic relationships: See under generic discussion.