Observations on the biology of the trochid gastropod Austrocochlea constricta (Lamarck) (Prosobranchia). II. The effects of available food on shell-banding pattern

The trochid gastropod Austrocochlea constricta (Lamarck) shows a variable pattern of shell banding. The concentration of the major shell pigment, uroporphyrin I, has been shown to differ between stripe-classes and, within each stripe-class, between shores. This paper describes the relation between amount of available food and shell pigmentation on six shores.Qualitative and quantitative methods were used to determine the concentrations of chlorophyll pigments in the substratum of each shore. Thin-layer chromatography showed differences in the type of microalgae present. Quantitative, spectrophotometric determinations of the chlorophyll concentrations of acetone extracts of scrapings of the substratum showed significant differences in total chlorophyll between the six shores. A uroporphyrin index gives an estimate of the total amount of uroporphyrin/unit area on each shore, and enables the wet wt/animal, density/area, and proportions of the population in different stripe-classes to be related to the concentrations of uroporphyrin/g shell within the stripe-classes. There was a highly significant linear regression of the uroporphyrin index on total chlorophyll concentration. This provides evidence for the effect of amount of available food on shell pigmentation. The mean concentration of pigment/g shell increased as the chlorophyll content of the substratum increased.A hypothesis is proposed to explain the observed differences in banding-pattern frequencies between shores; this represents the frequency distribution of animals with different concentrations of pigment/g shell as a normal distribution, with increasing mean and standard deviation as the concentration of chlorophyll in the substratum increases. Theoretical distributions are given to show that similar banding pattern frequencies to those observed on the six shores can be produced under this hypothesis. Neither a more complex hypothesis, nor selective nor genetic mechanisms are necessary to explain the observed distributions of banding pattern in Austrocochlea.