Sex, seminal receptacles, and sperm ultrastructure in the commensal bivalve Montacuta phascolionis (Veneroida; Galeommatacea)

It is argued that the paired structures in adult Montacuta phascolionis pre viously claimed to be testes are seminal receptacles. The receptacles in 3. 0 mm long females are filled with sperm cells or spermatids. A 2.0-mm long specimen had a well-developed testes exhibiting ripe sperm. It is concluded that the species is a protandrous hermaphrodite and that the previously de scribed dwarf males are rather complemental males. The ultrastructure of th e sperm cells is described and compared with that of the sperm of related s pecies.