Size-specific maternal effects in response to predator cues in an intertidal snail

The role of phenotypic plasticity in the adaptation of natural populations is a key issue in evolutionary ecology. Maternal effects are one source of plasticity in early development, and refer to the role of the maternal phenotype in generating offspring phenotypes. Reproductive investment, often a function of maternal size, is one mechanism for transmitting maternal effects. However, it is unclear how maternal size interacts with other ecological factors, such as predation pressure, to influence female reproductive behavior and offspring phenotype. We investigated how reproduction is influenced by maternal size in the presence of predators by exposing large and small adult mud snails Ilyanassa obsoleta to waterborne cues from the predatory green crab Carcinus maenas and the non-predatory green sea urchin Strongylocentrotus droebachiensis and measuring reproductive output (i.e. egg size and number, egg capsule number), egg capsule morphology, and offspring time to and size at hatching. Large snails produced more eggs overall than small snails, but the presence of green crabs did not affect reproductive output for either size class. Unexpectedly, small snails laid fewer capsules in the presence of green sea urchins. In contrast, capsule spine length increased in the presence of green crabs but was unaffected by the presence of sea urchins. Larval size at hatching significantly increased in the presence of both crabs and urchins. We also demonstrated the effectiveness of spines in deterring predatory crabs in the lab, and that snails are distributed in a manner consistent with the avoidance of green crab predators in the field. Maternal effects in I. obsoleta, therefore, appear to be size-and context-dependent, play an important role in defending embryos from predators, and are induced by cues from multiple members of the benthic community, including both crabs and urchins.