Tidal migration patterns moderate thermal risk in the intertidal snail Nucella ostrina
Hayford, Hilary Anne
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Physical environmental characteristics place limits on when and where organisms can survive, thereby shaping distributions and abundances of species. A thermal performance curve is often used to describe the organism’s performance along a thermal environmental gradient. Behavior can moderate temperatures experienced by an organism, increasing the time spent at optimal temperatures that confer high performance. Much of what is known about behavioral thermoregulation in ectotherms is based upon fast-moving animals, but organisms with relatively slow mobility, such as snails, may navigate their thermal environments on fundamentally different temporal or spatial scales. The intertidal zone has characteristics of both marine and terrestrial habitats, alternating between these two extremes multiple times per day. The timing of low tides has important consequences for the thermal environment; extreme temperatures depend on both aspect of the shoreline and the time during the day or night when the substrate is exposed. The predictability of these tidal influences can be exploited by mobile animals living in the intertidal zone to access food resources during times of reduced thermal risk. I examined foraging and migration behavior of the intertidal snail Nucella ostrina in controlled field experiments and surveys of natural habitats, and estimated snail body temperatures from physical thermal mimics deployed in the field. I assayed snail performance across a range of temperatures and modeled the effect that behavior would have upon the temperatures snails experienced, performance under current conditions, and performance in a simulated future climate with 2°C increase. I found that snails migrate into high shore foraging areas for only 2-4 d per two-week tidal cycle, then retreat to locations lower on shore and in thermal refuges such as cracks. This periodic cycle of behavior, foraging on days with nighttime low tides that are reliably cool, is seen under both manipulated conditions and in natural populations. Furthermore, when given the choice between different substrate aspects, this species selects the cooler microhabitat. These temporal and spatial patterns of behavior lead to a reduction in the time spent at high temperatures (30°C and above). Snail performance begins to decline above 30°C, taking several hours to recover from a 2 h duration of exposure to aerial temperature of 34°C and resulting in death for 75% of individuals at 35°C. Using a generalized model derived from observed snail behavior as a filter for temperatures experienced, I found that migratory behavior drastically decreases the exposure of N. ostrina to temperatures that cause reduced performance or death. Simulated climate change increases the frequency of high temperature events, but the benefit of migratory behavior is maintained. Selection of foraging time and location confers a thermal benefit to N. ostrina, reducing this snail’s risk of exposure to detrimental temperatures that frequently occur in its habitat. Climate change is unlikely to directly affect this species as it simply is not present at times and in places where high temperatures occur, however, several indirect effects should be considered. Slow animals may be thermoregulating on a different temporal or spatial scale than we are accustomed to examining for faster moving species.
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