Salinity—stress and Desiccation in Intertidal Worms

Abstract

SYNOPSIS. Intertidal worms (oligochaetes, polychaetes, sipunculids) inhabiting beaches and tidal fiats both on the open sea coast and in estuaries may be exposed to significant tidal as well as seasonal variations in salinity. However, there are very few measurements of the actual variations in salinity encountered by worms in nature. Behavior (irrigation of burrow, vertical movements in burrows, migration in gradients of salinity)may be important in determining to which of the available salinities in a tidal cycle the worms may be exposed. In response to rapid lowering of salinity, worms gain water and lose salts, these processes combining in diluting the internal fluids. Internal ilution occurs more slowly in euryhaline species than in stenohaline species. Worms fully adapted to salinities lower than 30 % sea water may be hyperosmotic (demonstrated for six species of Nereidae ). Mechanisms involved in hyperosmotic regulation include active transport of salts(demonstrated in Nereis diversicolor), reduction ofthe permeability of the body surface to salts and perhaps to water, and perhaps production of hypo—osmotic urine. Sipunculids can tolerate considerable loss of water rom dehydration and concomitant increases in osmotic concentration of the body fluids. It is suggested that worms exposed to significant tidal variations in salinity may seldom be in osmotic equilibrium with their external medium.