Role of recruitment and growth in determining the upper limit of distribution of the intertidal barnacle Hexaminius popeiana

Abstract

The barnacle Hexaminius popeiana occurs in a zone up to approximately 3.7 m above chart datum on the trunks of mangrove trees in eastern Australia. Recruitment density of H. popeiana declined sharply with tidal height, and almost no recruitment occurred above the upper limit of distribution of sessile adults. Nevertheless, settlement of H. popeiana above the upper limit could be induced by the presence of adults, and there was little difference in the survival of either transplanted or induced recruits within, compared with above, the natural distribution. Surprisingly, therefore, no change in the upper limit of H. popeiana occurred, despite several episodes of recruitment during a two-year study. Factors that may act to restrict recruitment to substrata within the adult distribution are discussed. Growth rate was reduced and sexual maturity delayed at upper compared with lower tidal heights. An evolutionary cost of settling above the adult distribution appears to be delayed sexual maturity and reduced production of offspring, rather than post-settlement mortality per se.