Acanthodromiinae

Guinot, Danièle, 2008, A re-evaluation of the Dynomenidae Ortmann, 1892 (Crustacea, Decapoda, Brachyura, Podotremata), with the recognition of four subfamilies, Zootaxa 1850, pp. 1-26 : 6-8

Acanthodromiinae n. subfam.

( Figures 1 A, 2, 5O–P)

Dynomeninae A. Milne-Edwards & Bouvier 1899 pro parte: 9; Alcock, 1900 pro parte: 127, 133; 1901 pro parte: 34, 74; A. Milne-Edwards & Bouvier 1902 pro parte: 22.

Type genus. Acanthodromia A. Milne-Edwards, 1880 (type species by monotypy: A. erinacea A. Milne- Edwards, 1880).

Diagnosis. Carapace longer than wide, oblong; dorsal surface convex, ornamented with close-set spines. Cervical, branchial grooves not well visible, branchiocardiac groove crescent- shaped. Anterolateral margins poorly defined, joining corners of buccal cavity, obscured by numerous spines. Posterior margin markedly concave. Frontal margin forming narrow projection; supraorbital margin oblique, continuous above orbits, eave-like, rimmed, not notched, spinous, prolonged in straight groove delineating subhepatic region; infraorbital margin concave, ornamented with spines. Orbits obliquely located, elongated, clearly visible from dorsal view; eyestalks short. Antenna with urinal article extended transversely, not medially beaked; second article with firmly fixed exopod. Proepistome short. Epistome narrow. Anteroventrally, front, inflated subhepatic pterygostomial portion forming together with merus of mxp3 a weak “face”. Anterior border of endostome forming raised wall, laterally notched by exhalant orifices. Mxp3 operculiform, angled; basis separated from ischium by nearly complete suture; ischium narrow at base, merus not extended laterally, narrow. Pleural line as wide, decalcified zone; branchiostegite of normal texture. Thoracic sternum extremely narrow, entirely (apart from sternites 1, 2) covered by male abdomen, abdominal margins close to P2, P3 coxae. Sternites 1, 2 fused into small, narrow shield, inserted between bases of mxp3; sternite 3 distinct but short, expanded laterally, delimited posteriorly by convex crest, corresponding to suture 3/4; sternites 3–8 not exposed laterally when abdomen closed; sternite 8 tilted. Sterno-coxal depressions very deep. Female sutures 7/8 ending well apart, along internal border of P3 sterno-coxal depression; spermathecal aperture small, slightly behind level of female gonopore. Sterno-abdominal depression narrow, deep, with steep sides, medially a flat floor. Abdomen broad, curved, extending over entire sternum (sternites 3–8) and reaching mxp3; first somite dorsal, with same concave curvature as posterior margin of carapace in which it is inserted; subsequent somites narrow, subequal in width; telson long, broadly triangular, slightly wider at base; somite 4 with a prominent, pearlshaped double tubercule. Abdominal somites 3–6 fused in males, probably also in females; sutures obscured by spines, only partially visible; suture between somites 5, 6 absent in males, more clearly visible in females (at least in Acanthodromia margarita ); in females somites 3–5 each with lateral portions produced as flanges overlapping following somites; flange of somite 6 more pronounced, raised. Male uropod small, showing as narrow transverse plate, slightly mobile. Abdomen tightly locked in both sexes by mechanism involving coxae of 4 thoracopods: coxa of mxp3 with spinules overhanging posterior part of telson; coxae of P1–P3 with several spinules or granules overhanging telson at P1, P2 levels, somite 6 with uropods, and somite 5 at P3 level (in Acanthodromia margarita all spinules better developed in females than in males). Chelipeds equal, more robust than P2; dactylus strongly curved downwards; fixed finger almost straight; both fingers spoon-tipped. P2–P4 ornamented with spines; dactyli curved, bearing numerous spines; 4 or 5 spines on inferior margin. P5 conspicuously reduced, sexually dimorphic; basis-ischium fused to merus, basis-ischiummerus proportionally thicker than distal articles; dactylus rudimentary ending in subchelate mechanism, obsolete in males, more noticeable in females. P5 coxa of males modified, extended to enclose penis. Pl1 vestigial in females. Vestigial pleopods on somites 3–5 in males. Gonopod 1 stout, forming half-rolled tube. G2 needlelike, with row of small distal spines.

Remarks. The subfamily is monotypic, and Acanthodromia comprises only two species ( Table 1). The morphology is masked by the spines which cover the whole body, so the precise outline and the grooves on the dorsal surface of carapace are not evident, which implies that potentially related fossil taxa are not easily assigned to this subfamily. The initial assignment of Acanthodromia to the Dromiidae ( A. Milne-Edwards 1880; Alcock 1899) and subsequent transfer to the Prosopidae , subfamily Pithonotinae Glaessner, 1933 ( Wright & Collins 1972), illustrates well the “curious mixture” ( McLay 1999: 534) of characters, already pointed out by A. Milne-Edwards & Bouvier (1902). The present study has shown that there is strong support for the interpretation that the Recent Acanthodromia retains ancestral characters and is the most primitive of all extant dynomenids.

Figures

FIGURE 1. Representative species of the four subfamilies of Dynomenidae, overall view. A, Acanthodromiinae n. subfam.: Acanthodromia margarita (Alcock, 1899), Balicasag Island, Panglao, Bohol, Visayas, Philippines (ZRC) (see McLay & Ng 2005: 18); B, Paradynomeninae n. subfam.: Paradynomene tuberculata Sakai, 1963, Balicasag Island, Panglao, Bohol, Visayas, Philippines (ZRC) (see McLay & Ng 2004: 4); C, Metadynomeninae n. subfam.: Metadynomene tanensis (Yokoya, 1933) with its undulating tomentum, Balicasag Island, Panglao, Bohol, Visayas, Philippines (ZRC) (see McLay & Ng 2005: 25); D, Dynomeninae Ortmann, 1892: Hirsutodynomene vespertilio McLay & Ng, 2005, Balicasag Island, Panglao, Bohol, Visayas, Philippines (ZRC) (see McLay & Ng 2005: 21). E, Paradynomene tuberculata Sakai, 1963, New Caledonia, frontal view showing the characteristic “ face ” (ZRC). (All photographs courtesy of P. K. L. Ng).