Ultrastructure of the mature spermatozoon of the bivalve Estellarca olivacea (Mollusca: Bivalvia: Arcidae) and its phylogenetic implications.

Ultrastructure of mature spermatozoa of Estellarca olivacea was studied by transmission electron microscopy and its phylogenetic implications are discussed for the first time in this paper. The mature spermatozoon is composed of a head which contains a cone-shaped acrosome, a round nucleus and a tail region. The subacrosomal space is less electron dense which contains a homogeneous material. No axial rod and a basal plate were observed in subacrosomal space. No anterior invagination exists in the nucleus, but an inverted shallow V-shaped posterior invagination is visible. Nuclear lacunae could be seen clearly although the nucleus is highly condensed. Within the mid-piece of the spermatozoon there exist five spherical mitochondria while the long whip-like end portion is composed of an axoneme with the typical 9+2 structure. The spermatozoon of Estellarca olivacea is a product of the evolution of the reproductive system of the family Arcidae. Whether the particular acrosome, subacrosomal space, or the highly condensed nucleus might be adaptations of high fertilization rate in the particular environment of this species is discussed.