Sperm dimorphism and spermatozeugmata in the commensal bivalve Pseudopythina macrophthalmensis (Galeommatoidea, Kelliidae)

The bivalve Pseudopythina macrophthalmensis (Galeommatoidea) is a commensal with the crab Macrophthalmus convexus (Ocypodidae) in Okinawa Prefecture, Japan. It is a protandric hermaphrodite which incubates the 65-mum large ov a in the suprabranchial cavity. The species produces two types of sperm, wh ich were studied with the electron microscope. The euspermatozoon has an el ongate 2.8-mum-long, pointed acrosome, a slender 12- to 13-mum-long nucleus and a middlepiece containing several closely packed mitochondria arranged as a 5.5- to 6.0-mum-long sheath around the basis of the flagellum. The par aspermatozoon is vermiform, 220-mum-long and up to 5-mum-broad. Anteriorly there is a ca 7-mum-long bullet-shaped acrosome followed by a subcylindrica l 3.0- to 4.7-mum-long nucleus. Adjacent to the nucleus occurs a bundle of 26-42 40-mum-long flagella. The cytoplasm is packed with spherical lipid dr oplets and ovoid granules of unknown composition. Sperm of both types aggre gate to form spermatozeugmata, which were found in the posterior mantle cav ity or in paired seminal receptacles. Within the receptacles the euspermato zoa dissociate themselves from the spermatozeugma and become attached to th e epithelial lining of the receptacle whereas the paraspermatozoa presumabl y disintegrate. The possible significance of the two types of sperm is disc ussed in the light of their presumed functions in gastropods.