PARASPERMATOGENESIS IN GASTROPOD MOLLUSKS

Dimorphic, and sometimes polymorphic, spermatozoa are a feature of man y caenogastropod and some archaeogastropod (s.l.) taxa. The two types of sperm, named euspermatozoa (fertilizing) and paraspermatozoa (non-f ertilizing) by Healy and Jamieson (1981), are produced simultaneously in the same testicular acini. Paraspermatozoa develop from spermatogon ia which are similar in structure to those producing euspermatozoa. Fo rmation of parasperm from paraspermatocytes proceeds by atypical meiot ic divisions. Despite the variability in the structure of parasperm of prosobranchs, the morphological changes which occur during parasperma togenesis are remarkably similar between taxa. Multiple flagella devel op from numerous basal bodies which in turn originate from two parent and their satellite centrioles (procentrioles). In parasperm which loo se all the chromatin (apyrene sperm), the nucleus initially fragments into a number of vesicles. The chromatin in the vesicles gradually deg enerates, the remaining material being discharged from the cell by exo cytosis. In those parasperm in which a proportion of the chromatin is retained, the nucleus gradually decreases in size as the chromatin con denses. Electron-dense bodies (vesicles) which are produced either by granular endoplasmic reticulum or Golgi bodies begin to form in parasp ermatocytes. These vesicles gradually increase in size and coalesce to form larger dense glycoprotein blocks which form the bulk of the head region of the parasperm. It is suggested that the material which form s these electron-dense blocks is derived from nuclear degeneration.