Maternal effects on reproduction and development in the mud snail, Ilyanassa obsoleta

Daniel Bryan Schwab, College of William and Mary


A maternal effect occurs when the phenotype of an organism is influenced by the phenotype of its mother. When the maternal environment is an accurate predictor of the offspring's environment, maternal effects can play an important role in enhancing offspring fitness. Maternal investment (e.g. egg size), which is often a function of maternal size, is one mechanism for the transmission of maternal effects. I used the mud snail, Ilyanassa obsoleta, as a model system for examining the effects of maternal size on offspring phenotypes. Females deposit egg capsules on blades of eel grass, and exhibit high variability in the number of egg capsules laid, the number of eggs per capsule, and the morphology of their egg capsules. During early ontogeny, encapsulated embryos suffer high levels of predation. I conducted two studies to test for the presence of inducible maternal effects in response to predation. First, I conducted a pilot study analyzing how maternal size affects egg capsule deposition in small (shell length = 14.5 - 18.0 mm) medium (18.1 - 21.0 mm) and large (21.1 - 26.0 mm) snails, both with and without the presence of a predator (the green crab, Carcinus maenas). In this experiment I measured egg size, the number of eggs per capsule, the number of egg capsules laid, and several egg capsule morphometrics. I then, in a more complete study, investigated how egg capsule deposition and embryonic development are influenced by maternal size in the presence of predatory green crabs by exposing small (15 – 19 mm) and large (21 – 25 mm) adult mud snails to C. maenas cue and measuring egg size, egg number, egg capsule number, and egg capsule morphometrics. Additionally, I measured larvae at hatching to test for effects of predator cues on intracapsular development. Across both studies, I found that large snails lay more egg capsules and eggs per capsule, and that large snails lay significantly larger eggs than small snails. Larval size at hatching increased significantly in the presence of green crabs, and there is a trend suggesting that these larvae hatch sooner. Egg capsules were longer and wider in large snails, and, in the presence of a predator, were wider and possessed significantly longer defensive spines. In a test of predator preference, capsules with short spines were preferentially preyed upon by hermit crabs, suggesting that spines may be an adaptive deterrent to predation on egg capsules. Altogether, these results suggest that maternal effects in I. obsoleta can be size- and context-dependent, play an important role in defending embryos from predators during early development, and may persist post-hatching.