Shell colour ontogeny and tubeworm mimicry in a marine gastropod Littorina mariae

The intertidal prosobranch Littorina mariae has an adult shell colour polymorphism and also exhibits variation in the ontogeny of shell colour. Shells may be uniform in colour throughout their life (yellow or reticulate) or exhibit a discrete white-spiral juvenile phase prior to acquisition of the highly cryptic adult pigmentation. Laboratory reared clutches have the same juvenile colour phases and non-random associations with adult shell colour as those observed in the field. Juvenile shell colour may be controlled at a separate locus to that for adult colour and appears to show linkage disequilibrium with the yellow shell in some populations. There is a visual resemblance between the white juvenile phase of L. mariae and the tubes of the polychaete Spirorbis which are found cemented to algal fronds. Several lines of evidence are consistent with a postulated mimetic relationship. Although L. mariae reaches an adult size of 14 mm, the white phase is rarely expressed on shells greater than 3.5 mm diameter; this approximates to the maximum size of the tubeworms. Spirorbis is found predominantly in the same intertidal zone as L. mariae and is found principally on Fucus serratus which is the major algal substrate of L. mariae. Individuals with the white phase were absent from a wave-exposed locality where the tubeworm was also absent; it occurred only in the yellow morph in habitats where the tubeworm was moderately common and occurred in both yellow and reticulate morphs where the tubeworm was very abundant (5-20 tubeworms/cm²). Predation experiments with the intertidal fish Blennius pholis indicated that individuals with the white phase had lower predation rates than yellow and reticulate shells, and that the difference was greater when Spirorbis was present. This represents the first reported example of gastropod-polychaete mimicry and provides further evidence for the overwhelming role of visual predators and natural selection in the evolution of shell colour among intertidal littorines.