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

进化生态学家试图了解(1)生物体如何适应不断变化的环境，以及(2)形态变化的大小模式如何反映这种适应。直到最近，物种内的形态变化一直被解释为自然选择导致的适应，而不是环境诱导的(生态表型)现象。然而，越来越清楚的是，物种内的形态变化也可以反映生物体对其环境的发育反应(表型可塑性)。最有文献记载的适应性表型可塑性的例子是捕食者在猎物中诱导的防御反应。最近的实验表明，蜗牛可以通过改变壳的形状(例如，加厚的壳)来响应来自螃蟹的水中化学线索(流出物)，从而减少它们对螃蟹捕食者的脆弱性。然而，较低的水温会增加贝壳材料的溶解速度，从而可能对这种防御反应的程度、有效性和成本产生深远的影响。私人投资者希望确定蜗牛种群如何对螃蟹排泄物和水温的相互作用的环境线索做出反应。在缅因州海湾，光滑的长春蜗牛Littorina obtusata和引进的青蟹Carcinus maenas的历史和地理关系为测试这些目标提供了一个出色的系统。指数早前的工作显示，北部湾光滑的螺壳比南部湾的贝壳厚，螺壳厚度会因青蟹流出物而增加。缅因州南部的水温和青蟹丰度都高于北部，因此可能单独或共同影响蜗牛壳的形状。为了确定螃蟹排泄物和水温对蜗牛壳形状的相对贡献，PIS将进行现场实验，控制每条线索的水平。一群北方幼螺的一半将被转移到南部地点，而其余的将留在它们的原产地。同样，一群南方幼螺的一半将被转移到北部地点，而其余的则留在南部地点。PI将比较在北方和南方种群之间移植的蜗牛和在其本土环境中饲养的蜗牛的外壳厚度和身体生长。对于每个地点的每个种群，一半的蜗牛将暴露在螃蟹排泄物中，另一半则不会。这些实验将帮助我们了解环境和遗传影响对所产生的贝壳和身体形状的影响，以及每条线索的相对重要性。在第二个实验中，体型、生长速度、繁殖成功率和贝壳厚度之间的权衡将在实验室中使用类似的互换设计进行检验。将从2个北方和2个南方种群中培养卵块，并在温控孵化器中饲养孵化的幼虫。每个种群将受到2个水温和2个螃蟹处理(有或没有污水)。成年后，雌性蜗牛将被允许交配和产卵。这些实验将确定与捕食者在每种温度下引起的贝壳形状变化相关的生活史权衡。在第三组实验中，将通过比较处理时间和技术以及青蟹在实验室实验中获得的蜗牛的觅食成功来测试诱导反应在降低螃蟹捕食脆弱性方面的有效性。最后，PI将调查缅因湾多个地点的青蟹和长春花种群，以测试蜗牛壳厚度和螃蟹丰度之间的地理和季节关联。该项目的结果将更好地理解(1)遗传和环境诱导现象的相对重要性，以及(2)两种无处不在的环境线索(捕食者、温度)在产生大范围形态变化方面的贡献。这些问题尤其密切相关，因为(1)全球气候的预期变化将改变当地和区域的水温，(2)与温度有关的范围扩大和人类中介的引入将使本地猎物种群暴露在新型捕食者面前。

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.