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 gl’owth rates (0.90-1.47 d-l) and suffered high grazing losses (0.41-0.68 d--l per grazer ind.). The tube dwelling diatc,m Amphipleura rutilans and the cyanobacteria Lyngbya confervoides and Spirulina subsalsa had low maximal growth rates (0.38-0.81 d-l) and suffered only moderate grazing losses (0.10-0.27 d-l 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. Grazing on marine benthic microalgae has received much less attention than has grazing on phytoplankton. Most studies have concentrated on bulk effects on biomass, but not on selectivity, between algal species. Most selectivity studies in marine benthic herbivory have concentrated on selectivity between macrophytes and microalgae (Hawkins and Hartnoll 1983; Peduzzi 1987), cleaning of macrophytes from their microalgal epiphytes (Neckles et al. 1993), or selectivity between macrophytes (Schaffelke et al. 1995). Herein, I present results of in situ experiments where periphyton grown on artificial substrates have been exposed to gradients of Idothea chelipes densities. Its congener Idiothea baltica is an omnivore with a broad food spectrum that includes benthic microalgae, filamentous algae, macroalgae, detritus, animals, and even occasional cannibalism. Despite its omnivory, I. baltica is quite selective within food categories. Among macroalgae there is a clear preference of Fucus vesiculosus over Fucus evanescens (Schaffelke et al. 1995); within filamentous algae, germlings of the green alga Enteromorpha spp. are preferred over germlings of the brown alga Pilayella litoralis while adult P. litoralis are preferred over adult Enteromorpha (Schramm et al. 1996). In the western Baltic Sea, Zdiothea spp. are among the most important benthic herbivores.