'Co-consumption' and 'protective coating': two new proposed effects of epiphytes on their macroalgal hosts in mesograzer-epiphyte-host interactions

Multiple-choice feeding experiments were performed with the isopod Idotea g ranulosa and the amphipod Gammarus locusta as consumers. In a first experim ent, 2 different types of tissues of the brown seaweed Fucus vesiculosus an d its main macroepiphytes, Ulva lactuca and Elachista fucicola, were offere d. I. granulosa rejected apices of F. vesiculosus and preferred E. fucicola , while G. locusta clearly preferred F. vesiculosus tissue, especially the meristematic apices. In a second experiment, F. vesiculosus tissue with and without E. fucicola was offered together. For I. granulosa, the consumptio n of F, vesiculosus was enhanced by the presence of the epiphyte, while for G. locusta there was no difference in consumed E vesiculosus mass. G. locu sta, however, showed behavioural rejection of E. fucicola, and thus, the ep iphyte acted as 'protective coating'. We conclude that host (E vesiculosus) tissue could be 'co-consumed' by mesograzers (I. granulosa) that were attr acted by the presence of epiphytes and that these epiphytes therefore may h ave a 2-fold negative effect on the host (i.e. competion for light, nutrien ts etc. and attraction of consumers). 'Co-consumption' and 'protective coat ing' add 2 more facets to the very variable and case-dependent interrelatio nships of mesograzer-epiphyte-host systems; their relevance in nature, howe ver, remains to be demonstrated.