Observations on Osmotic Relationships in the Holothurian Opheodesoma spectabilis

Abstract

The means whereby Opheodesoma spectabilis, a holothurian confined to the quiet waters of Hawaii, can tolerate a dilution of 80% sea water were investigated. The animal is notable for the ratio of coelomic to tissue fluid, about 4:1. Weight changes, osmometry, titrations, and sodium analyses on fluids before and after immersion of the animal in diluted sea water were some of the methods employed. The animal was found to reduce its content of coelomic fluid when placed in dilute sea water for 24 hr. There is no rhythmic circulation or pumping of fluids. Water exchange and regulation of volume are by way of mouth and anus, as the body wall proved to be impermeable to water. Soluble salts of body fluids in both starved and feeding animals are below the concentrations found in sea water; but in feeding specimens, the coelomic fluid demonstrated osmotic activity approaching that of sea water. It is suggested that tolerance to fresh water by the tissues is incidental, and derives from (1) the slow replacement of the comparatively large volume of coelomic fluid with environmental water, and (2) the habit of steady ingestion of variable quantities of organic material, leading to changing levels of digestive end-products in the coelomic fluid. Cells within the small volume of tissue fluid exchanging with the coelomic compartment can tolerate and have time to equilibrate with a wide range of concentrations, out of adaptive necessity. Volume regulation occurs in the gradual exchange between coelomic and environmental water to further advance osmotic equilibrium.