An endogenous SCP-related peptide modulates ciliary beating in the gills of a venerid clam, Mercenaria mercenaria.

The activities of both the lateral and frontal cilia of Mercenaria mercenaria were unaffected, either by the two endogenous SCP-related peptides AMSFYFPRMamide and YFAFPRQamide, or by FMRFamide (all at 10(-6) M). Dopamine (DA) inhibited the lateral cilia; the mean EC50 was 2 x 10(-6) M. The peptide YFAFPRQamide--but neither AMSFYFPRMamide nor FMRFamide--antagonized the inhibition induced by DA; this effect was dependent on both time and dose. At a DA concentration of 5 x 10(-7) M, the effect of YFAFPRQamide appeared within 20 min and became maximal within 40-60 min; the mean EC50 at these times was 4.7 x 10(-11) M. If the concentration of DA was increased to 10(-6) M, the maximal effect of the peptide was delayed to 50 min, and the mean EC50 increased to 1.1 x 10(-7) M. Particle transport by the frontal cilia was inhibited by 5-hydroxytryptamine (5HT); the mean EC50 was 5.7 x 10(-7) M. Again, only YFAFPRQamide had an antagonistic effect on the 5HT-induced inhibition. At a 5HT concentration of 10(-6) M, the effects of YFAFPRQamide did not appear until 45 min; the mean EC50 was 10(-6) M. When radioimmunoassayed with an SCP antiserum, the elution profile of a gill extract overlapped those of the SCP-related peptides that had previously been identified in extracts of whole animals. These data suggest that all three SCP analogs occur in the gill. Immunohistochemistry of the gill, carried out with a monoclonal antibody raised to SCPB, stained many varicose neuronal fibers. Most of these were associated with the gill musculature, but a sparse innervation of the filaments underlying the cilia was also observed. Some fluorescent nerve cell bodies were also seen in the gill tissue. Our results are consistent with the hypothesis that YFAFPRQamide modulates branchial activities--muscular as well as ciliary--that are associated with feeding.

The Physiological Effect of Aureococcus anophagefferens ("brown tide") on the Lateral Cilia of Bivalve Mollusks.

The "brown tide" alga, Aureococcus anophagefferens, had no effect on the activity of the lateral cilia of isolated gills of the bivalve mollusks Argopecten irradians, Geukensia demissa, and Mya arenaria. A. anophagefferens caused a significant decrease in the activity of lateral cilia of five other bivalve mollusks, Crassostrea virginica, Ostrea edulis, Mercenaria mercenaria, Modiolus modiolus, and Mytilus edulis. Exposure of isolated gills of M. edulis to the water from which A. anophagefferens had been removed, or to polystyrene beads at the same concentration as A. anophagefferens, had no effect upon the activity of the lateral cilia. Thus, the inhibition of the lateral cilia is not caused by a compound excreted into the water, nor is it the result of the high density of cells. The response of lateral cilia to dopamine was identical to the response to A. anophagefferens; lateral cilia that were inhibited by dopamine were also inhibited by A. anophagefferens. Pretreatment of the gills of M. edulis with the dopamine antagonist ergometrine blocked the inhibition of the lateral cilia by both dopamine and A. anophagefferens. A water-soluble inhibitory compound was released from A. anophagefferens by exposing the cells to amylase, and then removing the cells by filtration. The effect of this inhibitory compound was also blocked by ergometrine. We propose that the isolated gills are digesting the extracellular coat of A. anophagefferens releasing a water soluble dopamine-mimetic compound that causes inhibition of lateral cilia.