AN ULTRASTRUCTURAL STUDY OF EUSPERMATOZOA, PARASPERMATOZOA AND NURSE CELLS OF THE COWRIE CYPRAEA ERRONES (GASTROPODA, PROSOBRANCHIA, CYPRAEIDAE)

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Abstract
The sperm duct of the cowrie Cypraea errones Linne contains euspermatozoa, paraspermatozoa and nurse cells, the latter bearing clumps of attached euspermatozoa. Nurse cell/euspermatozoa associations have been described in certain littorinacean gastropods, but are previously unrecorded in the Cypraeacea. Euspermatozoa of C. errones resemble those of many other mesogastropods and some neogastropods (for example members of the Strombidae, Epitoniidae, Naticidae, Volutidae; shared features include structure of the acrosome, nucleus, glycogen piece, and helically coiled midpiece elements). However in C. errones, radially arranged paracrystalline fibres partly occupy the space between adjacent midpiece elements. Paracrystalline material is only rarely observed in spermatozoa of prosobranch gastropods and in all cases (including C. errones), is readily discernible from the complex paracrystalline layers present in the mitochondrial derivative of opisthobranch and pulmonate spermatozoa. Paraspermatozoa of C. errones are elongate, vermiform cells containing: multiple axonemes (bunched anteriorly, peripherally distributed posteriorly); granulated deposts (anteriorly); numerous dense vesicles (posteriorly) and scattered mitochondria (elongate, with parallel cristae). The axonemal attachment complexes fuse apically to form a sharp, conical structure. Comparison with available electron microscopic and light microscopic accounts of prosobranch euspermatozoa and paraspermatozoa suggests close ties between the higher Mesogastropoda (with which the Cypraeidae should be included) and the Neogastropoda. Further research will be required to determine whether the paracrystalline fibres observed in euspermatozoa of C. errones are characteristic of the Cypraeidae or perhaps shared with the Ovulidae or Triviidae.