Not all nudibranchs are carnivorous: trophic ecology of Polycerella emertoni in the Ebro Delta

Abstract

Nudibranchs have always been regarded as a group of specialized predators. In Alfacs Bay (Ebro Delta, Spain), the cryptic nudibranch Polycerella emertoni is found associated with the pseudo-indigenous bryozoan Amathia verticillata, which has been thought to be its target prey. Here, we assessed the trophic ecology of P. emertoni using a combination of methods, including stable isotope analyses coupled with Bayesian mixing models, examination of stomach contents, and video recording of feeding activities. A morphological analysis of the radula was also conducted to explore a possible relationship with feeding behavior. Finally, we investigated the seasonal abundance of both species throughout an entire year in order to assess the nature and strength of their association. Contrary to current belief, our results show that P. emertoni is a micro-herbivore that feeds on the periphyton covering A. verticillata (up to 99% of the diet, according to mixing models). In particular, a diverse diatom community was observed on A. verticillata, and several of these taxa were found within the stomach contents of P. emertoni. Our results evidence a distinctive radular formula and morphology compared to other species and suggest the presence of multiple feeding strategies in nudibranchs. The high seasonal association observed between P. emertoni and A. verticillata may be attributed to mimicking habitat features as a mechanism to avoid predation. The possible effects of periphyton removal in the survival of detached fragments of A. verticillata may have implications for the dispersal of fouling species and functioning of ecosystems.