Effect of salinity on integumental transport in marine bivalves.

Abstract

The transport of organic solutes from sea water by the molluscan integument occurs via a Na⁺-dependent cotransport process. Although much is known about the mechanism of this transport, little attention has been given to the influence of environmental parameters on this process. One highly variable parameter to intertidal animals is salinity. Intertidal habitats are routinely exposed to periodic changes in external salinity from ∼0 to ∼425 mM Na⁺, representing a range of ∼0 to ∼1000 mOsM osmotic concentration. This study examined the effect of salinity on organic solute transport and cell volume regulation in the gill of two representative intertidal animals, Mytilus edulis and Mytilus californianus. Integumental organic solute transport was found to adapt to reduced salinity in a 2-stage response. There was an initial, steep inhibition of transport that was far greater than predicted from the reduction of external Na⁺ alone. However, within 10 min. of acclimation to the dilute medium, transport recovered to the level predicted by the external Na⁺ concentration. Long-term acclimation to reduced salinity resulted in no upregulation of integumental transport. Thus, the degree of adaptation of integumental organic solute to reduced external salinity was always defined by the availability of external Na⁺ for the transport process. The exposure to reduced salinity also resulted in a change of cell volume. The gill preferentially lost organic solutes but spared inorganic ions following long-term acclimation to reduced salinity. Short-term acclimation of gill tissue to reduced salinity resulted in a rapid Volume Regulatory Decrease (VRD) which served to restore cell volume toward the control value. This short-term VRD did not involve the substantial loss of organic solutes, but was probably mediated by the efflux of K⁺ (and a counter anion) across the basolateral membrane. The loss of inorganic ions during brief exposures to reduced salinity conserves metabolically expensive organic compounds. The loss of organic solutes during long-term acclimation to reduced salinity may be due to the decreased Na⁺ gradient which reduces the ability of integumental transport to maintain the high solute concentrations in gill tissue of control animals.