CUE SENSITIVITY, INDUCIBLE DEFENSE, AND TRADE-OFFS IN A MARINE SNAIL

Abstract

Despite considerable progress in the study of inducible defenses, our understanding of the role of cues and costs to their ecology and evolution remains limited. This study focused on inducible defenses in the intertidal snail, Littorina obtusata, in response to the invasive green crab, Carcinus maenas, in the Gulf of Maine. We used widely separated (∼400 km) snail populations from northern Maine and Massachusetts because they differ in their extent of historical and present-day contact with C. maenas. Crabs are presently more abundant in the southern Gulf and have been there for twice as long (∼100 yr). We examined whether these geographic differences in contact history and abundance may contribute to geographic differences in risk cue sensitivity, the amount of induced defense, and the scaling of trade-offs accompanying the induced defense. Risk cues from both crabs and damaged conspecifics induced the greatest increases in snail shell thickness, whereas exposure to just crab cues induced intermediate levels of thickening. Interestingly, our results suggest that the effects of risk cues are additive and that geographic differentiation in cue sensitivity only occurs in response to crab cues. While northern snails showed greater shell thickness plasticity in response to crab cues, there was no geographic differentiation in sensitivity to cues from damaged conspecifics. We also found that the scaling of trade-offs in body size vs. shell thickness was similar for both regions. This result contrasts with theory because one would expect the more intense selection regime caused by increased crab predation in the southern Gulf to diminish the impact of trade-offs on southern snails. However, this result is consistent with the hypotheses that (1) trade-offs in body size are likely mediated by architectural constraints imposed by the shell and (2) circumventing these constraints via changes in shell shape would likely result in a shell that is more vulnerable to crab predation. Analysis of aperture area and spire height for field populations revealed no geographic differentiation in aperture area, but taller spires in northern snails. Hence, more intense selection via crab predation in the southern Gulf may prevent changes in shell shape that reduce the impact of trade-offs. Finally, despite the large trade-offs accompanying increases in shell thickness, our study suggests that the functional value of increased shell strength in reducing vulnerability to crab predation outweighs these costs.