Microstructural differences in the reinforcement of a gastropod shell against predation

Gastropod shells are important antipredator structures that vary morphologically in response to predation risk, often increasing in thickness when the risk of predation is greater. Because the shell is composed of different microstructures that vary in energetic cost and strength, shell thickness may be increased in different ways. We tested whether the common intertidal snail Nucella lapillus differs in microstructure between shores with different predation risk, and whether any differences in microstructure affect shell strength. Predation risk varies with degree of wave exposure, so we compared shell microstructure and strength between snails from different exposure regimes. N. lapillus shells are made of a strong but energetically expensive crossed lamellar microstructure and a weaker but less energetically expensive homogeneous microstructure. Independent of exposure regime, the homogeneous microstructure was used to thicken the shell as snails increase in size. The thickness of the stronger crossed-lamellar microstructure changes little with snail size or predator risk. Whelks from wave-protected shores, where predation risk is high, have much thicker shells than conspecifics from exposed shores, where predation risk is low. The greater thickness is largely due to a disproportionate increase in the thickness of the homogeneous layer, and this increase translates into a much stronger shell. The advantage of using the weaker microstructure may lie in the fact that it is energetically cheaper and can be deposited more quickly, allowing snails to grow more rapidly to a size refuge. Reinforcing the shell with weaker and energetically cheaper materials that allow rapid growth to a size refuge may be a more important strategy for reducing the risk of predation than simply maximizing strength.