The ultrastructure of the spermatozoa ofPetalomera lateralis(Gray) (Crustacea, Brachyura, Dromiacea) and its phylogenetic significance

Summary The dromiid sperm, as exemplified by Petalomera lateralis and Dromidia antillensis, differs markedly from spermatozoa of other crabs (the Oxystomata-Oxyrhyncha-Cancridea-Brachygnatha assemblage) in the discoid, relatively undifferentiated acrosome capping and not embedded in the nucleus (plesiomorphies); the capitate form of the perforatorium and the composition of this (autapomorphies); the greater, apomorphic, reduction of cytoplasm and organelles, including mitochondria and centrioles; and the absence (Petalomera) or brevity (Dromidia) of nuclear arms. In view of some similarities of the acrosome to those of Eubrachyura suggestive of relationship, brevity of arms may be secondary by reduction. Presence of well developed nuclear arms is a synapomorphy of all investigated non-dromiid brachyurans and of the Palinura, Astacidea and Anomura while absence is a symplesiomorphy of other Malacostraca. If brevity of arms in dromiids were plesiomorphic, the Dromiacea might be derived from early decapods before evolution of the Palinura-Astacidea-Anomura-Brachyura assemblage as suggested by their non-brachyuran, anomuran type larvae. If dromiids are monophyletic with true crabs, zoeal morphology would demand a basal position in the Brachyura. Exclusion of Raninoidea from the Dromiacea, or from a podotreme group consisting of the Dromiacea-Archaeobrachyura, is advocated in view of clear synapomorphies of raninoid sperm with those of the Oxystomata-Oxyrhyncha-Cancridea-Brachygnatha assemblage (e.g. spherical, embedded, more complexly zoned acrosome) which are not seen in dromiids.