Spermatozoan morphology of four species of bivalve (Heterodonta, Veneridae) from Taiwan.

Using transmission and scanning electron microscopy, the mature spermatozoa of four bivalves of the family Veneridae--Gafrarium tumidum and Circe scripta (Circinae), Pitar sulfureum (Pitarinae) and Gomphina aequilatera (Tapetinae)--are described for the first time and compared with those of other bivalves, particularly other heterodonts. As our observations show, the spermatozoa of these four species are of the primitive type or ect-aquasperm form. The head contains a slightly curved nucleus with a short cone-shaped acrosome. The structure of the acrosome is typical of heterodont bivalves and two major components of the acrosomal vesicle material can be distinguished. The midpiece exhibits four or five mitochondria which surround the proximal and the distal centrioles. Variation in the shape and dimensions of the acrosomal vesicle and nucleus is substantial in these four Veneroidea species.