Unusual modes of oogenesis and brooding in bivalves: the case of Gaimardia trapesina(Mollusca: Gaimardiidae)

摘要

I describe an unusual case of follicular oogenesis in a bivalve, Gaimardia trapesina,a common marine bivalve from the Magellan Region and adjacent Sub-Antarctic waters,whose members brood their developing larvae. The gonad in G. trapesina is an acinus organthat infiltrates the perivisceral connective tissue; the walls of the acini are formed by tall,slender cells with distal nuclei, supported by a thin conjunctive tissue layer. At the onset ofvitellogenesis, each developing oocyte becomes surrounded by a one-cell-thick layer of folliclecells, which may originate from the wall of the acinus. The cells form a follicle that completelyencompasses single oocytes, except at the basal zone, where oocytes are in contact with theacinus wall. The follicle persists beyond the end of vitellogenesis and spawning. After gameterelease, the persistent follicle participates in the attachment of ova and developing embryos tothe interfilamental junctions of the inner and outer demibranchs of the gill, where embryosare incubated until hatching as late-stage pediveliger larvae. Ripe eggs are large (～250μmdiameter), suggesting that development is entirely lecithotrophic. The follicle cells that me-diate connections between developing embryos and the maternal individual probably have amechanical role only, providing support and possibly facilitating the accommodation of alarge number of embryos to maximize the branchial space available for brooding.