Uropod loss and transformation
The presence of a tailfan consisting of the telson and the paired appendages of the last pleomere is considered an ancestral decapod structure (Paul et al. 1985; Paul 1989). Uropods represent a pair of larval appendages on the sixth pleomere. As has been reported by Williamson (1974, 1988) and confirmed for some lithodids by Crain & McLaughlin (2000b), when uropods are present they appear in the third zoeal stage, whether or not that is the ultimate zoeal stage. Williamson (1974) suggested that lack of uropod development in the higher Brachyura resulted from ancestral stock that had only two zoeal stages. In most lithodid genera where uropods are entirely lacking, the species do have only two zoeal stages. Nonetheless, uropods never develop in the lithodid genera Acantholithodes and Cryptolithodes, each having four zoeal stages (Jensen pers. comm.; Hart 1965; Kim & Hong 2000), which certainly suggests the lack of uropod stage dependency. Uropods, when present in lithodids, are uniramous and are lost with the molt to crab stage 1 in all genera studied except Placetron (cf. Crain & McLaughlin 2000b). Since adults of P. wosnessenskii lack uropods, this is another example of heterochronic loss. In contrast, with the molt to crab stage 1 in all non-lithodids studied, the biramous megalopal uropods remain biramous but become substantially modified by the development of corneous scales on the dorsal surfaces. This development occurs whether the species is a shell-dweller or not (Forest 1987; McLaughlin et al. 1989, 1992, 1993; McLaughlin 2000). Uropod asymmetry, as suggested above, may be under genetic and/or environmental control.