Habitat and feeding preferences of crustacean mesoherbivores inhabiting the brown seaweed Ascophyllum nodosum (L.) Le Jol. and its epiphytic macroalgae

Habitat and feeding preferences of crustacean mesoherbivores inhabiting the brown alga Ascophyllum nodosum (L.) Le Jol. and its macroepiphytes were examined on the Swedish west coast. After an initial survey of the epifauna, three species of mesoherbivores, the isopods Idotea granulosa Rahtke and Jaera albifrons Leach, and the amphipod Gammarus locusta (L.), were selected for further studies. The role of macroepiphytes in the distribution, abundance and recolonization rates of the mesoherbivores were investigated in observational and manipulative field experiments. The results from these experiments were compared with results from multiple-choice feeding experiments in order to see if the distribution pattern of the mesoherbivores was related to, or independent of, feeding preferences. The epifauna was dominated in numbers by gammarid amphipods, but the most abundant crustacean species in terms of biomass was the isopod I. granulosa. The field experiments showed that macroepiphytes had large effects on the distribution and abundance of the amphipod G. locusta, with 5-8 times higher mean densities on A. nodosum plants with macroepiphytes. The effect of macroepiphytes on the abundance of I. granulosa varied strongly among sites, and for the smaller isopod J. albifrons no effects could be observed. The manipulative field experiments showed that complete recolonization by the mesoherbivores on defaunated A. nodosum plants was rapid, occurring on a time-scale of hours or days. Natural densities on plants with and without macroepiphytes were already found after the first 24 h for G. locusta, implying that the distribution pattern was the result of habitat selection by the amphipod, rather than differential predation. Feeding preferences differed among, as well as within, mesoherbivore species. Large individuals of G. locusta clearly preferred to feed on macroepiphytes, while large individuals of I. granulosa mostly grazed on meristematic apices of A. nodosum. Small individuals of both species grazed on both A. nodosum and its macroepiphytes, with a preference for the epiphytes. For J. albifrons no significant feeding preferences could be detected. The general conclusion of this study was that macroepiphytes are important as habitat and food for some of the crustacean mesoherbivores inhabiting A. nonosum, but that large variations in mesoherbivore-epiphyte interactions occur among and within species. Furthermore, the results supported the conception that many crustacean mesoherbivores display large mobility. (C) 1999 Elsevier Science B.V. All rights reserved.