Endoscopic visualization of the functional morphology of the ctenidia of the unionid mussel Pyganodon cataracta

Abstract

During reproduction, the outer demibranchs of the unionid mussel Pyganodon cataracta serve as marsupia, with incubation of the developing shelled glochidia larvae occurring within the water tubes. In this study, recently developed endoscopic video analysis techniques were employed to examine in vivo the dynamics of filter feeding and water transport in mussels during gravid and postgravid periods. Particles entering the mantle cavity and retained by the gills were transported to the palps in a complex mucus-bound cord by the ventral food groove of the medial ctenidia. Larval incubation and ctenidial swelling impeded flow around the lateral demibranchs, although marsupial ctenidia were still actively involved in suspension feeding. Cilia on the distended ventral edges of marsupial demibranchs were often observed transporting filtered particles to the frontal surface of the medial gills. Larvae within the brood chambers were morphologically isolated from the surrounding medium by dorsal brood caps on the primary water tubes. Direct observations of the secondary water tubes of marsupial gills constructed during periods of larval incubation confirmed their role as temporary lumina for water transport during gravid periods. Time-lapse video recordings revealed that mature larvae are released from the brood chambers via the suprabranchial cavity and exhalant siphon by rapid adductions of the valves and contractions of the brooding demibranchs.