Settlement in different-sized patches by the gregarious intertidal barnacle Chamaesipho tasmanica Foster and Anderson in New South Wales

Most models on settlement of open marine invertebrate populations are based on space-limitation. These models, however, do not recognise that free space may not drive the demography of populations when larval numbers are small or when larval supply varies along a gradient in the habitat. They also do not incorporate the effects of larval choice when settling. It has been hypothesised that, in gregarious barnacles, the effects of adult conspecifics, rather than available free space, may play a primary role in settlement. That is, cues from adults along perimeters of patches, rather than space available, may enhance colonisation. This study therefore aimed to distinguish between these separate influences on populations of Chamaesipho tasmanica, a gregarious barnacle characterised by relatively few larvae arriving to settle each year. Patches of 6, 3 and 1.5 cm diameter were cleared within aggregations of barnacles at three heights (Low, Mid, Upper) of Chamaesipho's distribution at two sites and during 2 years of settlement. Total numbers of settlers in each year were manipulated to determine the separate influences on settlement due to availability of substratum or the effects of conspecific adults. To test for the effects of available free space, numbers of settlers per unit area were analysed. To test for gregarious effects due to the presence of adults, numbers of settlers per unit perimeter were analysed. While available substratum was found not to affect settlement of this barnacle, gregarious settlement in response to adults at perimeters of patches was thought to be confounded by differential larval supply and differential conspecific cues among heights on the shore. Results from this study therefore have important implications for survival of gregarious populations following disturbances, especially in species where larval supply is poor.