Evol Ecol Res 2: 803-822 (2000)     Full PDF if your library subscribes.

Predator-induced plasticity and morphological trade-offs in latitudinally separated populations of Littorina obtusata

Geoffrey C. Trussell

Department of Biological Sciences, School of Marine Science, College of William & Mary, Gloucester Point, VA 23062 and Marine Science Center, Northeastern University, Nahant, MA 01908, USA

Adddress all correspondence to Geoffrey C. Trussell, Department of Ecology and Evolutionary Biology, Box G-W, Brown University, Providence, RI 02912, USA.
e-mail: geoffrey_trussell@brown.edu


Predation by shell-crushing predators is thought to be a principal force driving the evolution of gastropod shell form. However, recent evidence suggests that phenotypic plasticity in response to risk stimuli associated with predators may also be important. Thus, morphological co-evolution between predators and their gastropod prey may be driven by natural selection on reaction norms rather than genetically fixed phenotypes. In this study, I examined whether geographic variation in morphology and its plasticity in the intertidal snail (Littorina obtusata) were associated with both historical and present-day differences in the abundance of one its principal crab predators (Carcinus maenas). C. maenas has been well established in the southern Gulf of Maine for about 100 years, but has been present in the northern Gulf of Maine for at most 50 years. The shells of snails from the northern Gulf were thinner, weighed less and were weaker in compression than those of southern Gulf conspecifics. These geographic patterns in shell form may reflect, in part, either selection on genetically fixed phenotypes or environmentally induced phenotypes in response to geographic differences in C. maenas effluent concentrations.

 A laboratory experiment raising snails from the same field populations in the presence and absence of C. maenas effluent was conducted to test whether differences in the duration of contact with C. maenas influences plasticity in shell form. When raised in the presence of crabs, snails from all populations produced significantly thicker and heavier shells than conspecifics raised without crabs. These results support the hypothesis that geographic differences in shell form may partly reflect geographic differences in the abundance of C. maenas and, thus, the concentration of effluent indicating a risk of predation. In other words, the thinner shells of northern snails may reflect non-induced phenotypes, whereas the thicker shells of southern snails are an induced defence in response to C. maenas. Interestingly, despite their different periods of contact with C. maenas, both northern and southern snails showed similar shell thickness plasticity in the laboratory, suggesting that reaction norms in each region have evolved similar slopes. However, laboratory data coupled with comparisons of field populations suggest that there has been an evolutionary shift in reaction norm intercept; southern snails, regardless of treatment, consistently produced thicker shells and showed less plasticity relative to their counterparts in the field. Predator-induced increases in shell thickness were accompanied by significant reductions in body mass (defined by soft tissue mass) and body growth. These trade-offs probably reflect geometric constraints imposed by shell form on body mass and may explain the existence of micro- and macro-geographic variation and the evolution of inducible defences in marine gastropod shell form.

Keywords: Carcinus maenas, geographic variation, Gulf of Maine, Littorina obtusata, natural selection, phenotypic plasticity, trade-offs.

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