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 ex amined on the Swedish west coast. After an initial survey of the epifauna, three species of mesoherbivores, the isopods Idotea granulosa Rahtke and Ja era albifrons Leach, and the amphipod Gammarus locusta (L.), were selected for further studies. The role of macroepiphytes in the distribution, abunda nce and recolonization rates of the mesoherbivores were investigated in obs ervational and manipulative field experiments. The results from these exper iments were compared with results from multiple-choice feeding experiments in order to see if the distribution pattern of the mesoherbivores was relat ed to, or independent of, feeding preferences. The epifauna was dominated i n numbers by gammarid amphipods, but the most abundant crustacean species i n terms of biomass was the isopod I. granulosa. The field experiments showe d that macroepiphytes had large effects on the distribution and abundance o f the amphipod G. locusta, with 5-8 times higher mean densities on A. nodos um plants with macroepiphytes. The effect of macroepiphytes on the abundanc e of I. granulosa varied strongly among sites, and for the smaller isopod J . albifrons no effects could be observed. The manipulative field experiment s showed that complete recolonization by the mesoherbivores on defaunated A . nodosum plants was rapid, occurring on a time-scale of hours or days. Nat ural densities on plants with and without macroepiphytes were already found after the first 24 h for G. locusta, implying that the distribution patter n was the result of habitat selection by the amphipod, rather than differen tial predation. Feeding preferences differed among, as well as within, meso herbivore species. Large individuals of G. locusta clearly preferred to fee d on macroepiphytes, while large individuals of I. granulosa mostly grazed on meristematic apices of A. nodosum. Small individuals of both species gra zed on both A. nodosum and its macroepiphytes, with a preference for the ep iphytes. For J. albifrons no significant feeding preferences could be detec ted. The general conclusion of this study was that macroepiphytes are impor tant as habitat and food for some of the crustacean mesoherbivores inhabiti ng A. nonosum, but that large variations in mesoherbivore-epiphyte interact ions 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.