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

The intertidal snail Batillaria exhibits remarkable variation in the shell colour within and among populations. Field study was conducted to determine the factors in maintaining observed shell colour polymorphism. Geographical variations in shell colour polymorphisms in B. attramentaria were significantly correlated with the temperature of the locality of the population. Darker morphs were predominant in colder regions, whereas lighter morphs increase their proportion in warmer regions. A consistent association was also found in B. multiformis that co-existed with B. attramentaria. Strong predatory pressure imposed by digenean trematode parasites was observed in B. attramentaria. However, it is unlikely to affect the colour variations, because no correlation exists between colour morphs and trematode parasitism. Although visual selection may also contribute to colour variation in Batillaria, no evidence is found for the existence of visual predators that affect colour patterns of these snails. The deficit of variation in cold regions is possibly due to selection against brighter morphs, because bright colours reflect heat. Although dark shells absorb sunlight and may therefore be exposed to the risks of overheating and drying up in a hot habitat, the darkest morph was frequently observed in the warmer regions, suggesting that physical selection on the colour morphs can be relaxed in the warmer environment. Our results suggest that climatic selection is one of the significant factors maintaining shell colour polymorphism in these intertidal snails.