'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 granulosa and the amphipod Gammarus locusta as consumers. In a first experiment, 2 different types of tissues of the brown seaweed Fucus vesiculosus and its main macroepiphytes, Ulva lactuca and Elachista fucicola, were offered. 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 consumption 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. locusta, however, showed behavioural rejection of E. fucicola, and thus, the epiphyte acted as 'protective coating'. We conclude that host (E vesiculosus) tissue could be 'co-consumed' by mesograzers (I. granulosa) that were attracted by the presence of epiphytes and that these epiphytes therefore may have a 2-fold negative effect on the host (i.e. competion for light, nutrients etc. and attraction of consumers). 'Co-consumption' and 'protective coating' add 2 more facets to the very variable and case-dependent interrelationships of mesograzer-epiphyte-host systems; their relevance in nature, however, remains to be demonstrated.