Molecular approaches to understanding salinity adaptation of estuarine animals

American Zoologist, Dec 1997 by Towle, David W

Molecular Approaches to Understanding Salinity Adaptation of Estuarine Animals1

DAVID W. TOWLE2

Department of Biology, Lake Forest College, Lake Forest, Illinois 60045 and

Mount Desert Island Biological Laboratory, Salsbury Cove, Maine 04672

SYNOPSIS. The molecular processes by which estuarine organisms adjust to salinity change are the central focus of this review, with emphasis on identifying the relevant mechanisms in euryhaline crustaceans using the techniques of molecular biology. This review is not intended to be complete with respect to ecological and physiological aspects but rather is an attempt to outline a molecular approach which other investigators may find useful as they address their own specific questions. Membrane transporters of sodium ions serve as the major focus, beginning with an examination of candidate transport systems in gill epithelial cells. Particular emphasis is placed on the recent identification and sequencing of a putative Na /H antiporter cDNA from gills of the green shore crab Carcinus maenas.

SALINITY VARIATION AS A PHYSIOLOGICAL LIMIT CONTROL

Salinity variation, both spatial and temporal, lies at the core of estuarine biology, serving as a physiologically limiting barrier for species which lack the necessary adaptive mechanisms. The Chesapeake Bay, for example, exhibits strong spatial salinity gradients which vary over seasonal and tidal time scales (Pritchard, 1952). Only a few specialized species of animals and plants are able to tolerate these salinity gradients, but those species often exhibit highly prolific populations within the estuary. Despite such success, extremes of salinity, particularly severe reductions resulting from massive influx of fresh water, may lead to severe population reductions of certain species (Boesch et al., 1976). Alternatively, salinity extremes may eliminate less tolerant competitors or predators, permitting proliferation of specific species including economically important species such as the blue crab Callinectes sapidus (Mangum and Towle, 1977).

RANGE OF ADAPTATIONS TO SALINITY VARIATION

Euryhaline animals exhibit several patterns of physiological adaptation to salinity change. Among the vertebrates, blood osmotic concentrations are regulated within a rather narrow range by controlling ion fluxes and the accumulation of organic osmolytes (Potts, 1976; Pequex et al., 1988). Among invertebrates, several adaptive patterns are evident but may be simplified to three major physiological themes: regulation of blood osmotic concentration, regulation of cell volume, or a combination of the two (Mantel and Farmer, 1983; Gilles, 1987).

EURYHALINE CRABS AS A PARADIGM OF SALINITY ADAPTATION

Examples of species which employ regulation of both blood osmolality and cell volume (at different salinites) are the green shore crab Carcinus maenas and the blue crab Callinectes sapidus. At salinities above 25 parts per thousand, both species permit the blood osmotic concentration to track that of the ambient water, employing volume regulatory measures in response to the salinity increase. In salinities below 25 parts per thousand, these animals activate physiological mechanisms which result in partial regulation of blood osmolality (Mantel and Farmer, 1983).

The major contributor to blood osmolality in euryhaline crabs is sodium chloride, and thus the regulation of the fluxes and permeabilities of these two ions is central to the animals' ability to tolerate salinity gradients. In Carcinus, such regulation is effective in salinities as low as 8 parts per thousand but then collapses (Siebers et al., 1982). In Callinectes, some populations are known to regulate blood osmolality effectively in environments which are essentially fresh water (Cameron, 1978; Mantel and Farmer, 1983) (Fig. 1).

Ion regulatory mechanisms in the green shore crab respond rapidly to salinity change, with blood Na levels reaching a new steady state within six hours following salinity reduction (Towle et al., 1994) (Fig. 2). The blue crab also possesses the ability to respond rapidly to broad tidal fluctuations of salinity, maintaining a nearly constant blood osmotic concentration under such conditions (Findley and Stickle, 1978). It has been suggested that specific "hair-peg" sensory organs on the legs of euryhaline crabs respond to ambient salinity gradients (Schmidt, 1989), possibly setting in motion a hormonal cascade which may involve atrial natriuretic peptide (Turrin et al., 1992) or dopamine (Sommer and Mantel, 1988).

GILL STRUCTURE IN RELATION TO ION TRANSPORT

The antennal glands of crabs are incapable of producing urine with a monovalent ion content different from that of the blood (Cameron, 1979). Although the midgut and its associated diverticula (hepatopancreas) have been considered as important ion-regulating tissues (Mantel and Farmer, 1983; Ahearn et al., 1990), most attention in relation to salinity adaptation has been given to the gills. A multifunctional organ, the gill of crabs is a multilamellar structure in which each lamella is perfused internally by blood and externally by the ambient water (reviewed by Taylor and Taylor, 1992). Each gill lamella consists of a cuticular envelope lined by a single layer of epithelial cells and separated by pillar cells (Copeland and Fitzjarrell, 1968; Goodman and Cavey, 1990). Two major types of epithelial cells are "thin" cells, believed to be particularly important in gas exchange, and "thick" cells, believed to be important in ion exchange. The two cell types vary dramatically in cuticle-to-hemolymph distance, the thick cells showing an elaborated basolateral membrane (Towle and Kays, 1986).