Comparative ultrastructure of the spermatozoa of the M ajoidea ( C rustacea, D ecapoda, B rachyura) with new data on six species in five genera

Comparative ultrastructure of majoid spermatozoa belonging to 23 species, in 19 genera and five families, is considered, with new data on S chizophrys aspera ; S . rufescens ( M ajidae, M ajinae); C amposcia retusa ( I nachidae); P yromaia tuberculata ( I nachoididae); and H uenia heraldica and M enaethius monoceros ( E pialtidae, E pialtinae). The oregoniid C hionoecetes opilio , and inachids C yrtomaia furici , P latymaia rebierei , M acropodia longirostris and I nachus phalangium , possibly with C amposcia retusa , but not P odochela riisei , appear to form a group. Within the inachids, M acropodia and I nachus are especially close. A domed central acrosome zone, seen in most inachid sperm, in majines (both S chizophrys species), in pisines ( O xypleurodon orbiculatus and O . stuckiae ) and epialtines ( H uenia heraldica and M enaethius monoceros ), appears to be an autapomorphy of these majoids. A peripheral acrosome zone is seen in the inachid G rypacheus hyalinus , two inachoidids ( P . tuberculata and S tenorhynchus seticornis ) and the majid M aja squinado . P yromaia tuberculata differs from other inachoidids in having a slightly dome‐shaped operculum. The mithracine M acrocoeloma trispinosum ( M ajidae) sperm more closely resembles Inachoididae, than Inachidae. Spermatologically, the family M ajidae and the subfamily M ajinae are not homogeneous. Spermatozoal ultrastructure does not support a majoid–hymenosomatid relationship and is equivocal with regard to the placement of C ryptochiridae in either the T horacotremata or H eterotremata, the prominent operculum strongly differentiates cryptochirids from M ajoidea.