COMPARATIVE STUDIES ON THE FUNCTION OF GILLS IN SUSPENSION FEEDING BIVALVES, WITH SPECIAL REFERENCE TO EFFECTS OF SEROTONIN

Abstract

Observations were made at Elat (Red Sea, Israel) on intact gills (ctenidia) and gill fragments of the suspension-feeding bivalve mollusks Anomia achaeus, Juxtamusium maldivense, Pteria macroptera, Pycnodenate hypotis, P. numisma, Crassostrea (Saccostrea) cuccullata, C. lugubris, Tridacna maxima and T. squamosa. Serotonin (5-HT [5-hydroxytryptamine]) relaxed contracted gills and reduced response to mechanical stimulation in all but Tridacna. Concertina-like movements of gill plicae in Juxtamusium were associated with ciliary inactivity; when cilia were active plicae were inflated and movement ceased. Exposed gills or fragments continued to transport water but were inefficient in particle retention; 5-HT enhanced water transport and reduced collecting efficiency even more; it did not restore normal feeding activity. Laterofrontal cirri were not seen in Anomia, Pteria and Juxtamusium. In Crassostrea, Pycnodonte and Tridacna the laterofrontal cirri were medium-sized or small, and did not normally meet to span the interfilamentary spaces. 5-HT activated frontal cilia (shown by creeping rate of gill fragments) but arrested laterofrontal cirri in the vertical position. Most of the Tetraselmis cells used in feeding studies were transported freely in water currents, untrapped in mucus even though mucus was present. Three functional states were recognized in bivalve gills: a non-retentive state characteristic of disturbed animals and gill preparations; a cleaning state characterized by copious secretion of mucus and elicited by high concentrations of suspended matter; and a feeding state characterized by high rates of water transport and effective particle retention, found only in undisturbed bivalves. The mechanisms for combining high water transport and effective particle retention were not clear; and may involve functional integration of ciliary tracts on frontal and lateral surfaces of the gill filaments.