EXTREME SPATIAL VARIATION IN THE GROWTH AND REPRODUCTIVE OUTPUT OF THE ACORN BARNACLE SEMIBALANUS-BALANOIDES

Geographic variation in secondary production can profoundly influence population and community processes yet is rarely studied in benthic marine organisms. In this paper, we document striking variation within and among locations in the growth and reproductive output of the acorn barnacle Semibalanus balanoides and discuss the potential consequences of our results on understanding recruitment and other population processes. Within Narragansett Bay (Rhode Island, USA) barnacle growth and reproduction is far greater than at nearby open coast locations. At comparable tidal heights, recruits in the Bay grew to nearly twice the basal diameter and had almost 10 times the reproductive output compared to recruits on the open coast. Transplant experiments suggest that the high secondary production of Bay barnacles is largely a phenotypic response to high primary production. Within- and among-site patterns in barnacle secondary production associated with enhanced flow velocities were also conspicuous. Barnacles at low tidal heights, in habitats exposed to large tidal currents and in wave-exposed habitats, had higher growth and reproduction than barnacles at higher tidal heights or habitats with lower velocities. Water column food concentrations and fluid velocities both strongly influence barnacle growth and reproduction through their combined effects on the flux of food to individuals. Our results imply that predictable variation in food supply rates may lead to important and equally predictable consequences on regional larval production and the density-dependent dynamics of adult benthic populations.