ENDOSCOPIC VISUALIZATION OF THE FUNCTIONAL-MORPHOLOGY OF THE CTENIDIAOF THE UNIONID MUSSEL PYGANODON-CATARACTA

During reproduction, the outer demibranchs of the unionid mussel Pygan odon cataracta serve as marsupia, with incubation of the developing sh elled glochidia larvae occurring within the water tubes. In this study , recently developed endoscopic video analysis techniques were employe d to examine in vivo the dynamics of filter feeding and water transpor t in mussels during gravid and postgravid periods. Particles entering the mantle cavity and retained by the gills were transported to the pa lps in a complex mucus-bound cord by the ventral food groove of the me dial ctenidia. Larval incubation and ctenidial swelling impeded flow a round the lateral demibranchs, although marsupial ctenidia were still actively involved in suspension feeding. Cilia on the distended ventra l edges of marsupial demibranchs were often observed transporting filt ered particles to the frontal surface of the medial gills. Larvae with in the brood chambers were morphologically isolated from the surroundi ng medium by dorsal brood caps on the primary water tubes. Direct obse rvations of the secondary water tubes of marsupial gills constructed d uring periods of larval incubation confirmed their role as temporary l umina for water transport during gravid periods. Time-lapse video reco rdings revealed that mature larvae are released from the brood chamber s via the suprabranchial cavity and exhalant siphon by rapid adduction s of the valves and contractions of the brooding demibranchs.