FACTORS AFFECTING THE GROWTH AND FECUNDITY OF BULINUS TROPICUS (KRAUSS) (GASTROPODA)

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Abstract
The size structures of populations ofNodilittorina unifasciata were examined monthly for a year at three levels on two Tasmanian shores. Mean size increased with shore height, while density declined. Recruitment was greatest at the lower levels and occurred year-round, with some slackening from October to December. Larger animals were more tolerant of high temperatures and desiccation, but no differences could be detected between size classes in their ability to retain their hold on the surface in a water current. Marked snails released at the lowest tidal level migrated upshore; released in the middle zone, only large snails moved non-randomly, upshore. When released at the highest level, large snails moved short distances at random while medium and small animals moved downshore. The size gradients observed differ from those observed elsewhere in Australia; this may be related to more intense biological factors operating at more northerly sites.