Plesionida concava, Cabezas & Macpherson & Machordom, 2009

Cabezas, Patricia, Macpherson, Enrique & Machordom, Annie, 2009, Morphological and molecular description of new species of squat lobster (Crustacea: Decapoda: Galatheidae) from the Solomon and Fiji Islands (South-West Pacific), Zoological Journal of the Linnean Society 156 (3), pp. 465-493 : 482-484

PLESIONIDA CONCAVA SP. NOV. ( FIG. 8 )

Material examined: Solomon Islands. SALOMON 2. Stn 2260, 8°04.45′S, 156°55.87′E, 3 November 2004, 399– 427 m: 4 M, 10.5–12.0 mm (holotype M, 12.0 mm; MNHN-Ga6520); 3 ov. F, 11.3–12.6 mm; 1 F, 7.7 mm (paratypes, MNHN-Ga6519) .

Etymology: From the Latin concavus, meaning concave, referring to the concavity along the lateral side of the merus of the walking legs.

Description: Carapace as long as broad. Dorsal surface covered with numerous small spines. One epigastric spine, well-developed, behind each supraocular spine. Cardiac region slightly circumscribed. Frontal margins transverse. Rostrum 0.3 times the carapace length, compressed, slightly upturned, dorsally carinated, and not overreaching the cornea. Supraocular spines short, not reaching midlength of rostrum, and more slender than rostrum. Anterolateral spine large, reaching sinus between the rostral and the supraocular spines. Two or three small marginal spines before cervical groove. Branchial margins with four or five well- developed spines, and some small spinules. Posterior margin with numerous small spines ( Fig. 8A ).

Thoracic sternites smooth, without striae. Anterior part of fourth sternite narrower than third; median margin of third sternite contiguous with fourth sternite. Second to fourth abdominal somites with two transverse granulate ridges, lacking secondary transverse striae or scales; second somite without spines along anterior transverse ridge; third and fourth somites with two median spines on anterior ridge; one median spine on posterior ridge of fourth segment ( Fig. 8B ).

Eye large: maximum corneal diameter about 0.5 times the distance between the bases of the external orbital spines.

Basal segment of antennule (distal spines excluded) nearly reaching end of cornea, with distomesial spine shorter than distolateral spine; lateral border without spines. Anterior prolongation of first segment of antennal peduncle overreaching antennular peduncle; second segment with short distomesial spine, clearly not reaching end of third segment; third segment with small distomesial spine, clearly not reaching end of fourth segment. Second segment of antennal peduncle (spines excluded) about 1.5 times the length of the third segment, 1.5 times longer than wide; third segment as long as wide ( Fig. 8C ).

With Mxp 3 ischium slightly longer than the merus, and distoventrally bearing a spine. Merus with median well-developed spine on flexor margin; extensor margin unarmed ( Fig. 8D ).

With P1 subequal in length, between 2.5 and 3.0 times the carapace length, with a few setae; mesial margin of merus, carpus, and palm with spiniform crest, dorsal side with rows of small spines and some scattered granules. Merus shorter than carapace length, 1.5 times the length of the carpus, distomesial spine not reaching proximal fourth of carpus. Carpus 2.2–2.5 times as long as high, slightly shorter than hand. Palm slightly longer than fingers. Fingers with denticulated crest along mesial and lateral margins of movable and fixed finger, respectively, with longitudinal and rounded dorsal crest nearly reaching tips, distally curving and crossing, and ending in a sharp point; cutting edges with small teeth of various sizes ( Fig. 8E ).

With P2 about 2.5–2.7 times as long as carapace, with some iridescent uniramous setae, and numerous non-iridescent plumose setae, along dorsal margins of articles; merus slightly longer than carapace, about 4.5 times as long as high, nearly 3.5–4.0 times the carpus length, and 1.4–1.6 times as long as the propodus; propodus about 5.5–6.0 times as long as high, and 1.3–1.7 times longer than dactylus ( Fig. 8F ). Dorsal border of merus with row of spines, increasing in size distally, lateral side strongly concave; ventral border with row of short spines and well-developed distal spine. Carpus with some dorsal and ventral spines, increasing in size distally; distal margin slightly overreaching merocarpal articulation of P1. Propodus with 11–12 movable spinules along ventral margin, dorsal border serrated. Dactylus slightly curving, unarmed, dorsal border proximally slightly concave ( Fig. 8G ). P3 length slightly shorter than P2, with similar spination and proportions among articles ( Fig. 8H ). P4 length 0.9 times P2; merus 0.8–0.9 times the length of the P2 merus, lateral side less concave than in previous legs; ventral spines stronger than those of P2 and P3 meri ( Fig. 8I ). Merocarpal articulation ending at distal margin of corneae.

Remarks: The genus Plesionida Baba & de Saint Laurent, 1996 , at present, contains two species: Plesionida psila Baba & de Saint Laurent, 1996 , from the New Caledonia, and Wallis and Futuna islands ( Baba, 2005; Macpherson & Baba, 2006), and Plesionida aliena ( Macpherson, 1996) , from New Caledonia, Fiji and Tonga ( Baba, 2005). Plesionida concava sp. nov. is similar to P. aliena ( Macpherson, 1996) . However, they differ in the following features.

1. The carapace is covered by numerous small spines in the new species, whereas these spines are nearly absent in P. aliena , with only two epigastric spines and some granules, which are more numerous and acute on the hepatic and branchial regions.

2. The P1s are more spiny in the new species than in P. aliena . In the new species, the mesial margin of the merus, carpus, and palm has a spiniform crest, and some spines scattered on the dorsal side of the articles. In P. aliena , the dorsal side of the articles is unarmed, and the mesial margin of the merus, carpus, and palm is serrated.

3. The lateral side of the merus of P2–P4 is concave in the new species and convex in P. aliena .

Distribution: Solomon Islands, at a depth of between 399 and 427 m.

MOLECULAR ANALYSIS

Figures

Figure 8. Plesionida concava sp. nov., male holotype, 12.0 mm (MNHN-Ga6520). Solomon Islands. A, carapace and abdomen, dorsal view. B, sternum. C, left antennule and antenna, ventral view. D, ischium, and merus of right third maxilliped, lateral view. E, right cheliped, lateral view. F, right P2, lateral view. G, dactylus, right P2, lateral view. H, right P3, lateral view. I, right P4, lateral view. Scale: A, E, F, H, I = 2 mm; B, C, D, G = 1 mm.