Longevity determines sizes of an adult intertidal barnacle

The small honeycomb barnacle Chamaesipho tasmanica Foster and Anderson often forms continuous sheets covering the substratum at mid-shore levels of sheltered rocky shores but also occurs in sparse distributions on exposed shores, and on higher levels of sheltered shores, in south-eastern Australia. Larger barnacles are generally found in more exposed and higher areas. Effects of each site on ultimate sizes of barnacles were therefore examined by measuring growth of barnacles of the same cohort at sheltered and exposed shores from the end of 1989 to the end of 1992. Because Chamaesipho varied in size among sites, and because size was not necessarily representative of age, three growth models were proposed to explain this size differential. While rates of growth and periods of growth did not differ, differences in longevity were found to influence size so that larger barnacles survived longer and were more abundant on the more exposed shores. In fact, when barnacles were aged, it was found that a greater proportion of older barnacles (>3 years of age) occupied these exposed areas. It seems that site-specific characteristics influenced longevity in some places so that larger Chamaesipho continued to predominate in these areas.