Abstract
The intertidal snail Nucella lapillus generally has thicker shells at sites sheltered from wave action, where crabs are abundant and pose a high risk of predation, than at exposed sites where crabs are rare. We studied two populations showing the opposite trend. We reciprocally transplanted snails between field sites and measured shell length, width and lip thickness of those recaptured 12 months later. Snails transplanted to the sheltered site grew larger than sheltered-site residents, which in turn grew larger than transplants to the exposed site. Relative shell-lip thickness was greater in residents at the exposed site than at the sheltered site. Transplants from shelter to exposure developed relatively thicker shells than their controls and relatively thinner shells from exposure to shelter. Progeny of the two populations were reared for 12 months in a common garden experiment presenting effluent from crabs feeding on broken conspecifics as the treatment and fresh sea-water as the control. The crab-effluent treatment decreased foraging activity, concomitantly reducing cumulative somatic growth and reproductive output. Juveniles receiving crab-effluent grew slower in shell length while developing relatively thicker shell lips than controls, the level of response being similar between lineages. F2 progeny of the exposed-site lineage showed similar trends to the F1s; sheltered-site F2s were too few for statistical analysis. At sexual maturity, shell-lip thickness was greater in snails receiving crab-effluent than in controls, indicating plasticity, but was also greater in the exposed-site than in the sheltered-site lineage, indicating heritable variation, probably in degree of sexual thickening of the shell lip. Results corroborate hypotheses that ‘defensive’ shell thickening is a passive consequence of starvation and that heritable and plastic control of defensive shell morphology act synergistically. Shell thickening of juveniles was similar between lineages, contrary to hypotheses predicting differential strengths of plasticity in populations from low- or high-risk habitats.
Highlights
It is widely reported that certain rocky shore gastropods develop thicker-walled shells at sites sheltered from wave action, where crabs are often abundant and pose a high risk of predation, than at exposed sites where crabs tend to be scarce (e.g. [1,2,3])
Because crabs tend to be more numerous at sites sheltered from wave action, or forage throughout the longer periods of tidal immersion at lower shore levels, defensive shell morphology tends to be more pronounced in such environments
In some cases, induced defensive shell morphology is more pronounced in populations from crabinfested habitats than in those from crab-free habitats [21], but the opposite may be true of other cases [8]
Summary
It is widely reported that certain rocky shore gastropods develop thicker-walled shells at sites sheltered from wave action, where crabs are often abundant and pose a high risk of predation, than at exposed sites where crabs tend to be scarce (e.g. [1,2,3]). It is widely reported that certain rocky shore gastropods develop thicker-walled shells at sites sheltered from wave action, where crabs are often abundant and pose a high risk of predation, than at exposed sites where crabs tend to be scarce Because crabs tend to be more numerous at sites sheltered from wave action, or forage throughout the longer periods of tidal immersion at lower shore levels, defensive shell morphology tends to be more pronounced in such environments. At sites exposed to wave action or at higher shore levels, adaptive shell morphology increases resistance to dislodgement or to physiological stressors while trading-off defensive attributes [1,3,15]. In certain taxa, sexual maturation may involve thickening of the aperture lip [22], confounded with any relationship between risk of crab predation and lip thickness of the adult shell. The induction of defensive shell morphology involves a complex of factors requiring investigation over a range of populations within and among taxa in order to reach better understanding
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