D
173 DESICCATION STRESS STEVEN C. HAND AND MICHAEL A. MENZE Louisiana State University The threat of desiccation for organisms inhabiting the intertidal zone occurs during emersion at low tides or when organisms are positioned in the high intertidal zone, where wetting occurs primarily by spring tides, storm waves, and spray. Drying due to evaporative water loss is the most common mechanism for dehydration, although during winter in northern temperate regions freezing can also occur, which reduces the liquid water in extracellular fluids and can lead to intracellular dehydration in multicellular organisms. Freezing tolerance has been reported and characterized for a number of intertidal invertebrates, including gastropods such as an air-breathing snail and a periwinkle, and bivalve genera including the common and ribbed mussels. BIOLOGICAL RESPONSES TO THE THREAT OF DESICCATION Behavioral defenses against stress and injury from the loss of water from cells, tissues, or body fluids are seen across all organisms resident in the rocky intertidal zone: microorganisms , animals, and plants. For organisms that are highly mobile, the first response to water stress is generally behavioral: to leave the area or to seek microhabitats that afford some degree of protection against dehydration (Figs. , ). Such refugia include crevasses in the substratum, animals aggregated into clumps, cover and shade underneath macroalgae, and accumulated organic detritus. Other, less mobile organisms (Figs. , ) restrict various activities such as filter feeding and irrigation of respiratory epithelia (e.g., ectoprocts, barnacles, bivalves), constrict to reduce surface area and attach gravel/shell debris to body wall (sea anemones), and adhere more tightly to the rocky substratum (e.g., chitins, limpets, snails)—all behaviors that retard water loss. Behavioral responses to emersion may also include the synchronization of gamete release or hatching of embryos. FIGURE 1 Selected inhabitants of the intertidal zone in the northern Gulf of California, where tidal amplitudes are among the largest in the world (6–9 m of vertical displacement). (A) The colonial zoanthid anemone, Palythoa ignota, tightly constricted during emergence and pictured with the brown bubble gum alga Colpomenia; (B) a field of acorn barnacles, Chthamalus, occupied by the prosobranch snails Cerithium and (inset) Acanthina angelica, whose apertural spine (arrow) is used to pry open opercular plates of barnacles on which it feeds; (C) the gulf sun star, Heliaster kubiniji, tightly adhered to rock surface and with restricted locomotion during emersion; (D) the thatched barnacle, Tetraclita stalactifera, from the higher mid intertidal zone; (E) the sea hare, Aplysia californica, trapped at low tide in a rock crevasse; and (F) the green chiton Chiton virgulatus. The behavioral and physiological mechanisms possessed by these organisms for avoiding or tolerating water loss during emersion are varied (see text). Photographs by the author. D Physiological and biochemical features important for resisting or tolerating water loss are numerous in organisms of the intertidal zone. These include (a) the deployment of desiccation-resistant egg cases for embryonic development, (b) reduction in water permeabilities of body coverings and epithelia (with the unavoidable disruption of gas exchange), (c) accumulation of metabolic end products as a result of concomitant oxygen limitation , (d) short-term, facultative depression in metabolic and developmental rates, (e) maintenance of intracellular osmolytes for water retention and macromolecular protection at low-water activities, (f) differential gene expression for the production of protective macromolecules, and, in extreme cases, (g) global downregulation of gene expression and metabolism during estivation and anhydrobiosis . Energy conservation, as in options (d) and (g), is critical during extended periods when nutrient acquisition is offset by the potential for severe water loss. DEPRESSION OF METABOLISM Water loss during emersion is increased by active processes such as locomotion, feeding, and ventilation of respiratory surfaces. Sessile organisms, which often dominate significant areas of the intertidal zone, respond to desiccation stress by reducing the exposed surface areas across which water loss takes place. Thus, when these processes and surface areas are restricted, overall metabolism is commonly depressed, either by active downregulation or “automatically” by impeded gas exchange (e.g., collapsed gills). Different strategies in response to aerial exposure and water loss can be distinguished across species. Depending on the species, some sea anemones store significant seawater within the body cavity (coelenteron) during emersion. Intertidal anemones become quiescent during exposure, and overall metabolism declines as a function of time in air. For intertidally acclimatized individuals of some species, aerobic metabolic pathways (ones requiring oxygen) predominate, as evidenced by the lack of an oxygen debt upon reimmersion and by the observation that rates of respiration and heat release by...
OR
