Aphanodactylidae

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 : 34-35

Aphanodactylidae n. fam.

Diagnosis. Carapace strongly sexually dimorphic (where both are known); female carapace transversely ovate to rectangular; male carapace narrower than that of female, ovate to subquadrate; surface glabrous or finely setose; smooth or punctate; regions poorly defined. Front deflected ventrally, faintly sinuous to bilobed in dorsal view. Orbital margins entire, well defined, infraorbital margin terminating mesially as angular tooth or rounded corner. Antennules folding transversely or obliquely. Antennae not excluded from orbit. Cornea pigmented. Maxilliped 3 ischium longer than merus, subquadrate or triangular; merus subquadrate; palp articulating at distolateral margin of merus, segments articulating end-to-end, decreasing in size distally; exopod with flagellum. Chelipeds equal; segments unarmed; surfaces smooth, finely to sparsely setose. Ambulatory legs relatively short, stout, P3 longest, P5 shortest, dorsal to other pereopods; P3–4 merus with row of spines or teeth on flexor margin; propodus distoflexor angle with 1 or more spines opposing dactylus. Dactyli short, claw-like, apices corneous. Female gonopore (vulva) on sternite 6, between suture 5/6 and 6/7. Male gonopore (genital papilla) emerging near anterior margin of sternite 8, distinctly mesial to coxa 8. Abdomen of female with all somites and telson distinct, mobile; males with somites and telson mobile or with somites 4–6 fused. Male abdomen narrowly triangular or linguiform.

Included genera. Aphanodactylus Tesch, 1918 [type genus], Gandoa Kammerer, 2006 , Gustavus n. gen., and Uruma Naruse, Fujita & Ng, 2009 .

Remarks. Members of the Aphanodactylidae n. fam. all have a rather uniform appearance: transversely rectangular or ovate carapace in females, narrower carapace in males; unornamented carapace surfaces with very little evidence of regionalisation; short, stout pereopods 2–4 with short, claw-like dactyli opposing spines on the distal propodal margins. These features of Aphanodactylidae are reminiscent of many Pinnotheridae sensu lato and are essentially the basis of earlier classifications placing aphanodactylids there. Some features remain to be confirmed for some genera; male Gandoa and female Uruma are, unfortunately, not yet known.

Aphanodactylids, however, differ significantly from all known Pinnotheridae in lacking the chief synapomorphy of pinnotherids, namely, the highly modified maxilliped 3. In pinnotherids the merus of maxilliped 3 is either considerably larger than, or fused with the ischium, forming a single unit (e.g., Ahyong & Ng 2007: fig. 1C); and the dactylus usually articulates proximally to the distal end of the propodus, and may be considerably enlarged or significantly reduced. Conversely, the maxilliped 3 of aphanodactylids is of the more typical, plesiomorphic thoracotreme form: the merus is smaller than the ischium and the palp is not enlarged. They resemble those found in a number of thoracotremes such as ocypodoids, and are what might be expected in stem-lineage Pinnotheroidea. Comparisons of the position of the male gonopores of aphanodactylids with specimens of other thoracotreme families revealed another noteworthy pattern. The position of the male gonopore in aphanodactylids, being distant from the base of coxa 8, closely resembles the condition of other pinnotherids (e.g., Nepinnotheres , Arcotheres and Viridotheres ) as well as ocypodoids such as members of the Macrophthalmidae and Ocypodidae (see also Guinot 1979). As such, the overall pinnotherid-like habitus of aphanodactylids suggests that they are close to pinnotherids, though the plesiomorphic form of maxilliped 3 excludes them from placement within Pinnotheridae . Aphanodactylidae is therefore tentatively assigned to Pinnotheroidea, alongside Pinnotheridae . If our alignment of Aphanodactylidae with Pinnotheridae is correct, then the ocypodoid-like maxilliped 3 and male gonopore position might point towards some type of ocypodoid ancestry for Pinnotheroidea. Molecular investigations of the phylogenetic position of Aphanodactylidae and Pinnotheridae are currently underway by several investigators, although a close relationship between pinnotherids and ocypodoids has already been suggested by Wetzer et al. (2009) and to a lesser extent by Palacios-Theil et al. (2009).

All known members of the Aphanodactylidae , for which hosts are recorded, are associated with tube building polychaete worms ( Terebellidae ). The unusual distal propodal spines of the ambulatory legs, which oppose the dactyli forming a subcheliform structure, and teeth present on the flexor margins of some of the legs, possibly assists the crabs in holding onto the tube walls or the surface of their polychaete hosts. The four aphanodactylid genera can be distinguished by the key below.