The Influence of Water Temperature on Predator-Induced Defensive Responses and Life-History Trade-offs in a Marine Intertidal Snail

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Evolutionary ecologists seek to understand (1) how organisms adapt to changing environments and (2) how broad scale patterns of morphological variation reflect such adaptation. Until recently, morphological variation within species has been interpreted as adaptation due to natural selection rather than environmentally induced (ecophenotypic) phenomena. However, it is increasingly clear that morphological variation within species can also reflect the developmental response of an organism to its environment (phenotypic plasticity). Among the best documented examples of adaptive phenotypic plasticity are predator-induced defensive responses in prey. Recent experiments have shown that snails can reduce their vulnerability to crab predators by altering shell form (e.g., thickening shells) in response to water-borne chemical cues (effluent) from crabs. Colder water temperatures, however, increase the rate at which shell material dissolves and, thus, could profoundly affect the degree, effectiveness, and cost of this defensive response. The PIs wish to determine how snail populations respond to the interacting environmental cues of crab effluent and water temperature. The historical and geographic relationship of the smooth periwinkle snail Littorina obtusata and the introduced green crab Carcinus maenas in the Gulf of Maine provides an outstanding system to test these objectives. The PIs earlier work has shown that smooth periwinkles in the northern Gulf have thicker shells than in the southern Gulf and that shell thickness of snails increases in response to green crab effluent. Water temperature and green crab abundances are both higher in the southern than in the northern Gulf of Maine and, thus, could influence snail shell form separately or in concert. To determine the relative contribution of crab effluent and water temperature to snail shell form, the PIs will conduct a field experiment that manipulates levels of each cue. Half of a group of juvenile northern snails will be moved to a southern site, while the remainder are left in their native site. Similarly, half of a group of juvenile southern snails will be moved to the northern site, while the remainder are left at the southern site. The PIs will compare shell thickness and body growth among snails transplanted between northern and southern populations and those reared in their native environments. For each population in each location, half of the snails will be exposed to crab effluent and the other half will not. These experiments will help us understand the effects of environmental and genetic influences on resulting shell and body form as well as the relative importance of each cue. In a second experiment, trade-offs between body size, growth rate, reproductive success and shell thickness will be examined in the laboratory using a similar reciprocal design. Egg masses will be cultured from 2 northern and 2 southern populations, and the hatchlings will be raised in temperature-controlled incubators. Each population will be subjected to 2 water temperatures and 2 crab treatments (with, without effluent). Upon reaching maturity, female snails will be allowed to mate and lay eggs. These experiments will identify life history trade-offs associated with predator-induced change in shell form at each temperature. In a third set of experiments, the effectiveness of the induced response in reducing vulnerability to crab predation will be tested by comparing handling times and techniques and foraging success of green crabs on the snails derived from the laboratory experiments. Finally, the PIs will survey green crab and smooth periwinkle populations at multiple sites in the Gulf of Maine to test for geographic and seasonal associations between snail shell thickness and crab abundance. Results from this project will provide a better understanding of (1) the relative importance of genetic vs. environmentally induced phenomena and (2) contributions of two ubiquitous environmental cues (predators, temperature) in producing broad scale morphological variation. These issues are particularly germane, because (1) anticipated changes in global climate will alter water temperatures locally and regionally and (2) temperature-related range expansions and human-mediated introductions will expose native prey populations to novel predators.