Reproduction, dwarf males, sperm dimorphism, and life cycle in the commensal bivalve Peregrinamor ohshimai Shoji (Heterodonta : Galeommatoidea : Montacutidae)

The commensal brooding bivalve Peregrinamor ohshimai Shoji is dioecious. Th e female is up to 20 mm long and attached by a byssus to the sternum betwee n the pereiopods of species of the burrowing mud shrimp Upogebia. Only one female is found per mud shrimp. Initial attachment of the female bivalves o ccurs in April and May on small mud shrimps that have settled in mid or lat e winter. When the host moults, the female bivalve moves to the new exoskel eton, which allows it to grow proportionally together with the host. Shells of one year-old female bivalves are 11.9-12.9 mm long, but few individuals attain an age of 1.5 years, corresponding to a length of 14.0-16.8 mm. By depriving the mud shrimp of part of its food, the female bivalve retards th e growth of its host. Females greater than or equal to 3.2 mm are normally accompanied by several 360-mum-long dwarf males attached to the bases of th e host's pereiopods. The number of males per female increases during summer and autumn, but drops in December, and males are almost entirely absent du ring winter. Males are never transferred together with the female when the host moults, but left to perish on the old exoskeleton. Two types of sperm cells are produced. The typical sperm (euspermatozoa) are stored in paired pouch-like seminal receptacles, which were previously mistaken for the spec ies' testes. The breeding period of females < 3/4-years old is from July th rough November. When breeding starts for the second time in spring, the ova become fertilized by sperm that have survived the winter in the receptacle s. Peregrinamor ohshimai produces females only in spring, while males are t urned out throughout spring, summer and autumn. Atypical sperm (paraspermat ozoa) reside in the suprabranchial cavity outside the receptacle. Their rol e in reproduction is uncertain.