Microstructural differences in the reinforcement of a gastropod shell against predation

Abstract

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.

An inducible morphological defence is a passive by-product of behaviour in a marine snail

“…As a consequence, the resulting shells of these snails were thicker and better defended against crushing forces. Although the homogeneous layer is mechanically weaker than the crossed-lamellar layer (Currey & Taylor 1974), its low organic content and passive production make it an inexpensive material for shell thickening (Palmer 1992;Avery & Etter 2006). To my knowledge, this is the first example of a predator-induced morphological defence that is a passive by-product of a behavioural response to predators.…”

“…Total shell thickness and microstructure layer thickness were measured to the nearest 0.01 mm from light microscope images (Image Pro Plus). This technique provides similar estimates of shell layer thickness as scanning electron microscopy (Avery & Etter 2006). To ensure microstructural layers were quantified from shell growth that occurred during the experiment, measurements were taken from the main body whorl at the leading edge of the aperture.…”

“…Shells were lightly sanded to ensure a smooth surface. Homogeneous layer thickness was calculated as the difference between total thickness and crossed lamellar layer thickness (Avery & Etter 2006).…”

Shell use by the hermit crab <i>Calcinus californiensis</i> at different levels of the intertidal zone

“…Polymorphism and variation in shell thickness in intertidal gastropods has been commonly reported in the literature, where shell thickness trends to parallel the gradients of wave exposure and predation intensity (Menge, 1978;Trussell, 1996;Carlson et al, 2006). Thicker-shelled morphs, which are resistant to being broken by predatory crushers (Avery and Etter, 2006), could be advantageous for gastropods and crabs at sites with high predator intensity; nevertheless, shell thickness is especially important for snails inhabiting wave-protected sites because of the large abundance of predatory gastropods in this area, which do not prey on crabs (Lam, 2002). However, for hermit crabs, the antipredatory benefit of occupying heavy shells might be counterbalanced by the higher energetic costs of locomotion, because the overall cost of moving would be higher for the more active hermit crabs than the commonly slow-moving snails (Donovan et al, 1999).…”

Cue reliability, risk sensitivity and inducible morphological defense in a marine snail

“…Apertural teeth and thickened apertural lips function as predator deterrents by reducing vulnerability to shell-peeling attacks by crabs and decreasing the area of the aperture through which crabs insert their claws (Vermeij 1987). Shell mass, which is a good indicator of overall shell thickness and resistance to crushing (Trussell and Nicklin 2002;Avery and Etter 2006) and body mass, were measured nondestructively following the protocol of Palmer (1982).…”