Spermatozoon Ultrastructure of Scapharca subcrenata and Barbatia virescens and Its Implications for Evolutionary Relationship in Arcidae

The spermatozoa ultrastructure of Scapharca subcrenata and Barbatia virescens was comparatively investigated by transmission electron microscope.The acrosome of Scapharca subcrenata spermatozoon is conical in shape,about half the length of the nucleus;there is no front nuclear pocket,but a posterior nuclear pocket exists;five (occasionally four) mitochondria are seen surrounding the distal centriole of the mid-piece;the end-piece of the sperm consists of the axoneme and its surrounding cytoplasm,and the axoneme has a typical "9+2" structure.The longitudinal section of the acrosome of Barbatia virescens is umbrella-like in shape,and the acrosome covers the front of nucleus,and is about 1/3 the length of the nucleus.The nucleus has both front and posterior nuclear pockets;five (occasionally six) mitochondria are seen surrounding the distal centriole in the cross sections;the structure of the end-piece of Barbatia virescens is the same as that of Scapharca subcrenata.In Arcidae,the morphology of acrosome,the presence of the front and posterior nuclear pockets,and the number of mitochondria in the mid-piece are the clues for understanding of the interspecifically evolutionary relationship.