Consequences of base modification in hummocks of the barnacle Austromegabalanus psittacus

Abstract

When the barnacle Austromegabalanus psittacus grows in hummocks, the normal structure of the base attached to the substrate, typical of individuals growing in isolation, is modified. In studies undertaken in Metri Bay, southern Chile, cohorts of settled individuals growing in isolation and in groups were evaluated over a period of 14 months, establishing carino‐rostral length as a density‐independent measure of age. One month and six months after larval settlement, size skewness was measured under both growth regimes. Experiments were carried out at two settlement coverages on wooden panels, and growth and mortality were determined. At the same time, shell weight and fecundity of barnacles with a normal and a modified base were evaluated. In monospecific hummocks, the caloric value of tissue was compared between individuals of different sizes, base‐types, and positions within the hummock. No differences were detected in the growth, reproduction and mortality of individuals growing in isolation (normal base) and those growing in groups (modified base). Growth and size skewness were density‐independent during the first 6 months after larval settlement. Fecundity did not vary between individuals of the same size growing in isolation or growing in groups. There were no energetic differences between individuals that grew in isolation with a normal base and those that grew in hummocks with a modified base, or between individuals occupying a central position and those located on the periphery of the hummock. These results suggest that modification of the base in individuals growing at high densities reduces density‐dependent effects, resulting from intraspecific competition for food and substrate. Thus, to interpret intraspecific interactions in barnacles, biological characteristics, such as the capacity to modify base‐types in response to species density must be considered.