Temperature-related diversity of shell colour in the intertidal gastropod Batillaria

The intertidal snail Batillaria exhibits remarkable variationin the shell colour within and among populations. Field studywas conducted to determine the factors in maintaining observedshell colour polymorphism. Geographical variations in shellcolour polymorphisms in B. attramentaria were significantlycorrelated with the temperature of the locality of the population.Darker morphs were predominant in colder regions, whereas lightermorphs increase their proportion in warmer regions. A consistentassociation was also found in B. multiformis that co-existedwith B. attramentaria. Strong predatory pressure imposed bydigenean trematode parasites was observed in B. attramentaria.However, it is unlikely to affect the colour variations, becauseno correlation exists between colour morphs and trematode parasitism.Although visual selection may also contribute to colour variationin Batillaria, no evidence is found for the existence of visualpredators that affect colour patterns of these snails. The deficitof variation in cold regions is possibly due to selection againstbrighter morphs, because bright colours reflect heat. Althoughdark shells absorb sunlight and may therefore be exposed tothe risks of overheating and drying up in a hot habitat, thedarkest morph was frequently observed in the warmer regions,suggesting that physical selection on the colour morphs canbe relaxed in the warmer environment. Our results suggest thatclimatic selection is one of the significant factors maintainingshell colour polymorphism in these intertidal snails. (Received 13 December 2006; accepted 14 April 2007)