Reproductive characteristics of a primitive bivalve from a deep-sea reducing environment:giant gametes and their significance in Acharax alinae (Cryptodonta:Solemyidae)

Only 4 specimens of the cryptodont protobranch Acharax alinae have been found and collected to date, from the hydrothermal vent region of the Lau Basin (Fiji) in May 1989. The gonad organlzatlon and gametogenic cells of the 2 working specimens were investigated in the present study, using histology and transm~sslon electron microscopy, in order to enhance our understanding of the reproductive biology of bivalves from deep-sea reducing habitats. The a c ~n i of the female were elongated and closely appressed to the ellipsoid-shaped matu.re oocytes, glving a tubular appearance. In both male and female the structural characteristics of the gametogenic cells were similar to those previously described for littoral species. The presence of female gametes in all stages of development suggests that spawning is continuous or at least repeatedly partial in this species. The mature female and male gametes were extraordinarily large: the equivalent spherical diameter for the mature oocytes was up to 600 vm; correction for fixation shrinkage would increase this size to approx. 660 pm The mature sperrnatozoon presented a head+midpiece length of 28 pm and a flagellum length of approx. 100 vm. These dimensions for the male and female gametes are, to our knowledge, the greatest ever reported for any bivalve species. As the pericalymma larva of the Solemyidae is a non-feeding stage, the large oocyte size is probably an adaptation for an extended lecithotrophic strategy, which would favour either long-range dispersal or protracted benthic development. The unusually elongated spermatozoon is probably a consequence of the large oocyte size. Its morphology is distinct from those of the other bivalve subclasses, which however do present species showing some spermatozoon head elongation or curvature. More generally, we suggest that spermatozoon morphology in bivalve taxonomy is most useful in groups with homogeneous developmental strategies.