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 spa ce may not drive the demography of populations when larval numbers are smal l or when larval supply varies along a gradient in the habitat. They also d o not incorporate the effects of larval choice when settling. It has been h ypothesised that, in gregarious barnacles, the effects of adult conspecific s, rather than available free space, may play a primary role in settlement. That is, cues from adults along perimeters of patches, rather than space a vailable, may enhance colonisation. This study therefore aimed to distingui sh between these separate influences on populations of Chamaesipho tasmanic a, 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 number s of settlers in each year were manipulated to determine the separate influ ences on settlement due to availability of substratum or the effects of con specific adults. To test for the effects of available free space, numbers o f settlers per unit area were analysed. To test for gregarious effects due to the presence of adults, numbers of settlers per unit perimeter were anal ysed. While available substratum was found nor to affect settlement of this barnacle, gregarious settlement in response to adults at perimeters of par ches was thought to be confounded by differential larval supply and differe ntial conspecific cues among heights on the shore. Results from this study therefore have important implications fur survival of gregarious population s following disturbances, especially in species where larval supply is poor . Crown copyright (C) 2000 Published by Elsevier Science BN. All rights res erved.