Gustavus mecognathus, Ahyong, Shane T. & Ng, Peter K. L., 2009

Ahyong, Shane T. & Ng, Peter K. L., 2009, Aphanodactylidae, a new family of thoracotreme crabs (Crustacea: Brachyura) symbiotic with polychaete worms, Zootaxa 2289, pp. 33-47 : 40-44

Gustavus mecognathus n. gen., n. sp.

( Figs. 4 , 5 , 7A )

Material examined. ZRC, male holotype (cl. 6.1 mm, cw. 8.8 mm), female paratype (cl. 8.0 mm, cw. 13.9 mm), SW Cocos Barrier, Guam, near small pass on large terebellid worm, G. Paulay, 20 March 2000.

Description. Female: Carapace ovate, distinctly broader than long, width to length ratio 1.8; dorsal surface smooth, regions poorly demarcated, scattered shallow pits; surface finely pubescent on surface and margins; lower lateral margins with dense plumose setae. Front deflexed, medially emarginate in dorsal view, triangular in anterior view, inner orbital margin rounded in dorsal view. Orbital margins entire, slightly narrowed laterally; eye filling orbit, cornea simple; infraorbital margin terminating mesially as rounded angle, supraorbital margin terminating mesially in slender, ventrally directed tooth. Anterolateral margin entire, convex laterally, cristate, continuing onto posterolateral margin but not supraorbital margin. Epistome very short, medially sunken. Antennule folding obliquely, filling fossa. Antennae with stout basal antennal article, not reaching distolateral angle of carapace; antenna not excluded from orbit.

Maxilliped 3 ischium distinctly longer than merus, slender, triangular, distomesial angle produced to rounded lobe extending mesially to level of mesial meral margin; merus smaller than ischium, slightly wider than long, rounded-quadrate: palp articulating on distal margin of merus; exopod slender, overreaching base of merus, flagellum short, not reaching distal margin of merus.

Chelipeds equal, covered with fine, short, black setae; merus triangular in cross section, with plumose setae proximally; carpus smooth, inner angle rounded with sparse plumose setae; chela surfaces smooth, palm about 1.4 times longer than dactylus; pollex with convex occlusal margin, crenulate proximally; dactylus occlusal margin with low, blunt teeth proximally.

P2–5 relatively short, P3 longest, P5 shortest, dorsal to other pereopods; propodus and carpus covered with short, fine, black-brown tomentum; merus with plumose setae along flexor and extensor margins; merus about 3 times as long as high, slightly longer than carpus and propodus combined; dactyli short, stout, apices corneous. P2 and 3 meri with scattered granules on flexor margin. P4 and 5 ischia and meri with stout spines on flexor margins. P2–5 propodi with 1–3 small distoflexor spines opposing dactyli.

Male: Carapace subpentagonal, broader than long, width to length ratio 1.4; dorsal surface smooth, regions poorly demarcated, scattered shallow pits; surface with fine, scattered setae on surface and margins; lower lateral margins with dense, plumose setae. Front deflexed, medially emarginate in dorsal view, triangular in anterior view, inner orbital margin angular in dorsal view. Orbital margins entire, slightly narrowed laterally; eye filling orbit, cornea simple; infraorbital margin terminating mesially as rounded angle, supraorbital margin terminating mesially in triangular, ventrally directed tooth. Anterolateral margin entire, convex laterally, cristate, continuing onto posterolateral margin but not supraorbital margin. Epistome very short, medially sunken. Antennule folding obliquely, filling fossa. Antennae with stout basal antennal article, not reaching distolateral angle of carapace; antenna not excluded from orbit.

Maxilliped 3 covering buccal cavern when closed; ischium distinctly longer than merus, slender, triangular, distomesial angle produced to rounded lobe extending mesially beyond level of mesial meral margin; merus smaller than ischium, slightly wider than long, rounded-quadrate: palp articulating on distal margin of merus; exopod slender, reaching almost to midlength of merus, flagellum short, not reaching distal margin of merus.

Chelipeds equal, covered with fine, short, black setae; merus triangular in cross section, with plumose setae proximally; carpus smooth, inner angle rounded with sparse plumose setae; chela surfaces smooth, palm about 1.2 times longer than dactylus; pollex with convex occlusal margin, with blunt teeth proximally; dactylus occlusal margin with low, blunt teeth proximally.

P2–5 relatively short, P3 longest, P5 shortest, dorsal to other pereopods; propodus and carpus covered with short, fine, black-brown tomentum; merus with plumose setae along flexor and extensor margins; merus about 3 times as long as high or greater, slightly longer than carpus and propodus combined; dactyli short, stout, apices corneous. P2 and 3 merus with scattered granules on extensor margin. P4 and 5 ischia and meri with stout spines on flexor margins. P2–5 propodi with 1–3 small distoflexor spines opposing dactyli.

Etymology. Derived from the Greek mekos for length, and gnathos for jaw, alluding to the elongate ischium of maxilliped 3.

Remarks. Gustavus mecognathus n. gen., n. sp., is perhaps the most peculiar of the aphanodactylids in the triangular rather than subquadrate maxilliped 3 ischium, broad male abdomen in which somites 3–5 are fused, and in the finely pubescent body surface, with longer, plumose setae around the bases of the pereopods and pterygostomian surfaces of the carapace. In all other aphanodactylids, the maxilliped 3 ischium is subquadrate, the male abdomen (not known in Gandoa ) is narrowly triangular with all somites and telson freely mobile, and the carapace and most surfaces of the pereopods are glabrous. Gustavus differs from other aphanodactylids in numerous other features including: a sharply triangular instead of broadly angular front (in anterior view); strongly oblique articulation of the antennules (versus slightly oblique to near transverse); an acute process demarcating the supraorbital margin from the upper antennular fossa; and a rounded rather than angular mesial termination of the infraorbital margin.

Gustavus mecognathus has proportionally more slender ambulatory legs than other aphanodactylids. The ambulatory meri are about three or more times longer than high in Gustavus in comparison to about twice as long as high in Aphanodactylus and Gandoa . The P2–4 meri in Uruma are also about three times as long as high, though the legs are disproportionately larger in comparison to the carapace in Uruma than in Gustavus . Distribution. Presently known only from the type locality, Guam.

Figures

FIGURE 4. Gustavus mecognathus n. gen., n. sp., male holotype, cl. 6.1 mm, cw. 8.8 mm (ZRC). A, habitus. B, cephalothorax, anterior view. C, right maxilliped 3. D, right chela. E – H, right P 2 – P 5. I, thoracic sternum. J, abdomen. K, right G 1, abdominal view. L, right G 2, abdominal view. Scales: A = 2.9 mm; B, K – L = 1.0 mm; C = 0.7 mm; D – J = 2.0 mm.

FIGURE 5. Gustavus mecognathus n. gen., n. sp., female paratype, cl. 8.0 mm, cw. 13.9 mm (ZRC). A, habitus. B, cephalothorax, anterior view. C, right maxilliped 3. D, left chela. E – H, right P 2 – P 5. I, thoracic sternum. J, abdomen. Scales: A = 2.9 mm; B, C = 1.0 mm; D – I = 2.0 mm; J = 4.0 mm.

FIGURE 7. A, Gustavus mecognathus n. gen., n. sp., male holotype, cl. 6.1 mm, cw. 8.8 mm (ZRC) (lower) and female paratype, cl. 8.0 mm, cw. 13.9 mm (ZRC) (upper) (photo: G. Paulay). B, Uruma ourana Naruse, Fujita & Ng, 2009, male holotype, cl. 4.2 mm, cw. 9.2 mm (RUMF-ZC- 907) (after Naruse et al. 2009: fig. 1).