Is water temperature responsible for geographic variation in shell mass of Littorina obtusata (L.) snails in the Gulf of Maine?

Abstract

A latitudinal cline in length-standardized shell thickness and mass of Littorina obtusata in the Gulf of Maine (GoM) has been attributed to predation by the European green crab Carcinus maenas. However, latitudinal variation in water temperature may also contribute to this cline. To investigate this hypothesis, we grew snails from 2 thin-shelled and 2 thick-shelled populations in warm (16 °C) and cold (10 °C) water treatments, representing summer temperatures of southern and northern parts of the GoM, respectively. Snails grown in warmer water showed a markedly greater increase in shell length (50%) and shell mass (60%) compared to conspecifics grown in colder water, but shell mass standardized for length was only weakly (relative to clinal variation) and not consistently (among populations) affected by temperature. Our experimental snails made shells that were lighter, when standardized for length, than conspecifics grown in nature. This “shell thinning” was particularly apparent in warmer water, because experimental snails diverted the increased calcification afforded by warmer water parallel (shell elongation) rather than perpendicular (shell thickening) to the axis of shell coiling. Interestingly, we estimate that if 16 °C snails had only grown to the length of 10 °C snails and used the additional shell material they produced to make their shell thicker rather than longer, they would have produced a shell comparable in mass to that of snails from the same population and shell length grown in nature. We conclude that water temperature is not directly responsible for natural variation in size-standardized shell thickness of L. obtusata in the GoM, but that it may contribute to this pattern indirectly, for example by facilitating the snail's plastic phenotypic response (shell thickening) to predation risk.