PHYSIOLOGY OF THE ROCKY INTERTIDAL PREDATORNUCELLA OSTRINAALONG AN ENVIRONMENTAL STRESS GRADIENT

Abstract

To gain insight into suborganismal mechanisms underlying responses of predators to environmental stress, we examined the effects of temperature and food availability on the physiology and feeding activity of the predatory whelk Nucella ostrina (formerly N. emarginata). Studies were conducted in the mid-low transition zone on rocky reefs at two sites on the Oregon coast: Strawberry Hill (SH) and Boiler Bay (BB). SH is dominated by suspension-feeding invertebrates and their predators, while BB is dominated by macrophytes. In wave-exposed (EX) and wave-protected (PR) habitats at each site, we measured air and water temperatures, whelk body temperatures, and indices of physiological condition (malate dehydrogenase [MDH] activities and RNA : DNA ratios) for wild-caught whelks. The influence of food (±prey) and thermal conditions (±shelter) on prey consumption, physiological performance (oxygen consumption rate), and response to thermal stress (induction of Hsp70-class stress proteins) of whelks was determined in field experiments. Low-tide air temperatures, whelk body temperatures, and biochemical indices of whelk field condition were consistently different between sites (SH > BB) and wave exposures (temperatures: PR > EX, condition: PR < EX). Foraging activity, metabolic rates, and stress protein (Hsp) expression were consistently higher at SH than BB. Respiration rates were lower for all −prey treatments than for +prey treatments; however, whelks in −prey +shade treatments had lower Hsp expression and higher respiration rates (an indication of better physiological condition) than whelks in −prey −shade treatments. Most measures of performance suggested that wave-exposed individuals were more physiologically robust than wave-protected individuals. Temporal physiological responses to stress suggested that high levels of heat stress compromise whelk performance later in the summer. These data are consistent with the hypothesis that physiological stress interacts with prey abundance to modify activity of whelks. Detailed studies of the effects of environmental stress on predator–prey interactions are needed to elucidate the relationship between physiological stress and predator–prey interactions. However, present evidence suggests that whelks alter activity (behavioral modification) and physiology (biochemical modification) in ways that lead to differential impacts by these predators on prey populations at different sites.