UPTAKE OF ORGANIC MATERIAL BY AQUATIC INVERTEBRATES. IV. THE INFLUENCE OF SALINITY ON THE UPTAKE OF AMINO ACIDS BY THE BRITTLE STAR, OPHIACTIS ARENOSA

Abstract

Ophiactis is capable of removing amino acids from very dilute solution in the ambient sea water. The amino acids obtained in this way enter oxidative pathways and are assimilated also into alcohol-insoluble compounds. Rough calculations indicate that the rate of uptake of glycine at ambient concentrations of approximately 10-5 moles/1 is sufficient to maintain the free glycine pool in the face of drains to synthesis and to energy metabolism. Production of C14O2 from glycine-C14 supplied to the animals indicates that glycine is a secondary energy source accounting for less than half the reduced carbon needed to support oxygen consumption. Assimilation is stimulated at salinities lower than normal sea water. As is the case with a number of other marine invertebrates, the free amino acid pool of Ophiactis is decreased at lower salinities. The stimulation of assimilation is sufficient to account for this osmoregulatory response. Both the increase in rate of assimilation of glycine from the free amino acid pool and the decrease in size of the pool are slow responses appearing 24 to 48 hours after a decrease in salinity in the environment. Consequently, survival of the organism presented with a salinity stress depends on its capacity for tolerance of unfavorable conditions followed rather slowly by regulatory responses.