Selectivity of Idothea chelipes (Crustacea: Isopoda) grazing on benthic microalgae

Periphyton grazing by the marine isopod Idothea chelipes was studied by exposing periphyton grown on glass slides to a gradient of grazer densities. An analysis of the algal growth rates and their relationships to grazer density revealed two groups of algae. The unicellular diatoms Licmophora ehrenbergii, Fragilaria tabulata, Navicula spp., Cocconeis costata, and the green alga Ulothrix implexa had high maximal growth rates (0.90–1.47 d−1) and suffered high grazing losses (0.41–0.68 d−1 per grazer ind.). The tube dwelling diatom Amphipleura rutilans and the cyanobacteria Lyngbya confervoides and Spirulina subsalsa had low maximal growth rates (0.38–0.81 d−1) and suffered only moderate grazing losses (0.10–0.27 d−1 per grazer ind.). The species of the first group seemed to be less strongly resource limited than did the species of the second group. Grazing by I. chelipes has the potential to drive succession from the well‐edible to the less edible periphyton species.