Stable carbon isotope ratios in Red Sea barnacles (Cirripedia) as an indicator of their food source

The stable isotope ratio ¹²C/¹³C was used to investigate the source of carbon in free-living barnacles and in coral-inhabiting barnacles from the Red Sea. The δ¹³C of most of the barnacles collected on the open shore ranges between −17.5 and −19.7‰, indicating relative enrichment of light carbon originating from the open-sea phytoplankton. Those collected in closed habitats showed heavier isotopic composition. The δ¹³C of the coral-inhabiting barnacles ranges from −14.1 to −16.7‰, suggesting that the carbon contribution of open-sea plankton to these barnacles is less important than it is to free-living barnacles. We hypothesize that coral organic matter and zooxanthellae expelled by the host coral contribute carbon to the barnacle, and that a mixture of this relatively heavy carbon with carbon from other sources is responsible for the high values of δ¹³C in coral barnacles. Received: 28 February 1997 / Accepted: 16 September 1997