HOST USE BY AUSTRALASIAN SEAWEED MESOGRAZERS IN RELATION TO FEEDING PREFERENCES OF LARGER GRAZERS

Abstract

Marine mesograzers use individual seaweeds as both habitat and food. Previous studies by M. E. Hay, J. E. Duffy, and others have shown that mesograzers can acquire enemy-free space by inhabiting and consuming seaweeds that are chemically defended against larger, more mobile consumers. However, their studies, carried out in North Carolina and two tropical regions, consider only a small number of mesograzer species and focus primarily on fishes, even though some other large consumers can destroy mesograzers' host seaweeds. We asked whether the previous findings could be generalized to (1) mesograzers at the community level, (2) large consumers other than fishes, and (3) a different ecosystem in another region, namely, diverse temperate rocky reef communities in Australia and New Zealand. Results from both Australasian regions were similar. In multiple-choice feeding assays larger grazers (three fishes, four gastropods, and three sea urchins) varied in breadth of preferences, but most often preferred to eat green algae of the family Ulvales and the red alga Pterocladiella capillacea, all of which were restricted to the intertidal zone. Most of the mesograzers surveyed (seven amphipods, three isopods, a gastropod, and a sea urchin) preferred to eat the seaweed species from which they were collected, many of which were either inaccessible or less palatable to many larger grazers. This difference in mesograzer vs. larger grazer feeding preferences was consistent with the findings of Hay and Duffy et al. However, community-level patterns of mesograzer distribution and abundance were not consistent with the extensions of the theory developed here. Seaweeds that were less palatable to the larger grazers did not tend to support more mesograzer individuals, mesograzer species, or specialist mesograzers (for either the entire mesograzer assemblage or just for sedentary mesograzers), but the evenness of entire mesograzer assemblages was significantly higher on seaweeds that were less palatable to larger grazers. We argue that spatial refuges on Australasian rocky reefs and differences in the biology of both the seaweed and animal species in Australasia vs. previously studied locations enable palatable seaweeds to persist in the face of herbivory by larger grazers, which reduces the selective pressure on mesograzers to inhabit unpalatable species.