THE OSPHRADIUM IN PLACOPECTEN-MAGELLANICUS AND PECTEN-MAXIMUS (BIVALVIA, PECTINIDAE) - HISTOLOGY, ULTRASTRUCTURE, AND IMPLICATIONS FOR SPAWNING SYNCHRONIZATION

The bivalve osphradium is a band of putatively sensory tissue located in the gill axis, whose function is uncertain. In the present study, e xtending from 1987 to 1994, anatomical, histological, and electron mic roscopical techniques were used to elucidate the structure and ultrast ructure of the osphradium in hatchery Pecten maximus L. and Placopecte n magellanicus (Gmelin) (collected from Passamaquoddy Bay, New Brunswi ck, Canada). The osphradium consists of two distinct regions which run longitudinally on both sides of each gill axis: the osphradial ridge, and the dorsal tuft cilia region. The osphradial ridge was largely de void of cilia other than those of the few free nerve fibres. The dorsa l tuft cilia region contained free nerve fibres and ciliary tufts, sep arated by undifferentiated epithelial cells. No paddle cilia were obse rved under isosmotic fixation conditions, although under hypotonic con ditions such cilia were quite common, suggesting an artefactual nature . Most of the cells of the osphradial ridge were highly secretory, the principal products being large pigment granules (in Pecten maximus) d irectly secreted by the Golgi bodies, and numerous small, electron-den se vesicles. These vesicles were arranged along extensive microtubule arrays in the basal region, indicative of axonal transport. These data support and extend Haszprunar's hypothesis of the role of the osphrad ium in the reception of chemical spawning cues and in the synchronizat ion of gamete emission. Together with independent data on nerve pathwa ys, osphradial sensory modalities, and monoamine localisation, an anat omical pathway and neurophysiological mediator are postulated.