Trophic Position, Estimated using Stable Isotope Ratios, in Co-occurring Barnacle Species Varies with Feeding Appendage Morphology

Barnacles represent important interface feeders in the rocky shore ecotone. We studied the feeding dynamics of three species of barnacle (Tetraclita serrata, Octomeris angulosa and Chthamalus dentatus) co-inhabiting the Eastern Cape coastline in South Africa. We assessed the diets of the barnacles using stable isotope signatures, and compared the trophic information to cirral morphology (i.e. cirral length, setal length, distance between setae and number of setae per mm), determined using light and scanning electron microscopy. We hypothesized that those species with larger cirral measurements feed at higher trophic levels, as their appendages facilitate the capture of larger plankton. Isotopic models showed that zooplankton made larger contributions to the diets of T. serrata (63%) and O. angulosa (72%) than C. dentatus (47%), resulting in C. dentatus feeding at a lower trophic level overall. These diet findings were related to cirral morphological measurements, which showed significant distinctions in C. dentatus (i.e. this species had the shortest cirri and setae, the shortest distance between setae, and the most setae per mm). Our results contribute a mechanism by which co-occurring species can partition the available food in a highly dynamic and competitive interface environment.