OBSERVATIONS ON BIOLOGY OF TROCHID GASTROPOD AUSTROCOCHLEA-CONSTRICTA (LAMARCK) (PROSOBRANCHIA) .1. FACTORS AFFECTING SHELL-BANDING PATTERN

The intertidal trochid gastropod A. constricta (Lamarck) has a variable pattern of banding, the shell ranging from white with fine black stripes, to completely black. The patterns may be classified into 6 arbitrarily chosen types. Populations were studied on 3 shores chosen to represent the range of habitat of the species (from estuarine to open coast localities) to determine the relative importance of environmental factors on shell-banding. The densities and proportions of the population in different stripe-classes were estimated from quadrat samples on the 3 shores. Analysis of size-frequency distributions indicates recruitment to be continuous. In all months, juveniles were at greater densities at the upper level of the distribution of the species on open coast and estuary-mouth platforms; the estuarine population was unimodal in size-frequency distribution. Growth rates of caged animals were higher on the open coast than in the estuary and changed in animals transferred between the 2 localities, but did not differ between animals of different banding pattern. The proportion of the population in each stripe-class was constant on each shore, suggesting large scale selection processes do not cause the observed differences in frequencies of each banding pattern. The concentrations of the major shell pigment, uroporphyrin I, varied between stripe-classes and within classes, between shores, with a trend for more densely pigmented shells on the open coast, a trend also evident for the proportion of the population in the darker stripe-classes. Caged animals transferred from the open coast to the estuary showed changes in banding pattern which were consistent with the differences in pigmentation of the 2 populations. Animals transferred from the estuary to the open coast changed pattern at the same frequency as controls from the open coast; a result of severe storms which caused the open coast population to show changes of banding pattern. Shell-banding patterns in A. constricta are primarily governed by environmental factors.