THE DIET OF PLATYDORIS ARGO (GASTROPODA: NUDIBRANCHIA) AND THE DIETARY SPECIALIZATION OF SPONGE EATING DORIDS.

Abstract

The diet of the dorid nudibranch Platydoris argo was studied in relation to prey availability, and under different environmental conditions to evaluate richness, evenness, and plasticity of its diet. In order to assess the availability of possible prey, the sponge assemblage at two different habitats was characterized. The results showed that P. argo is a relatively specialized benthic carnivore, feeding exclusively on spiculated demosponges. However, it has a relatively polyphagous diet (16 prey-species) and adapts well to contrasting patterns of prey availability at different sites. At the location with lower sponge diversity, the diet of P. argo included more prey categories, was more diverse, and less selective (lower dietary evenness), foraging preferentially on the most frequent resource, Stylopus dujardini. In contrast, at a station with a clearly more diverse and abundant sponge assemblage, the diet was more selective, and Phorbas tenacior was its principal component. Despite its trophic plasticity, P. argo displayed a specialized pattern of resource exploitation in terms of niche breadth within a given habitat. Foraging was focussed on one preferred prey (which changed from one habitat to another) as indicated by the significant positive selection indexes (S. dujardini = +0.29, P. tenacior = +0.74). The revision of quantitative data on the diet of so-called 'sponge eating dorids' indicates the existence of a more specialized guild of 'spiculated demosponge eating dorids'. Most species revised are 'non-stereotyped specialist', which indicates that they have a polyphagous and plastic diet but only exploit one or few main prey species in each habitat. However, obligate specialists, with a monophagous (or rather oligophagous) diet also seem to be present.