Use of red algae as hosts by kelp-associated amphipods

To what extent densities of amphipods associated with red algae are related to food value or habitat form and architecture were investigated. Four epiphytic red algae common on kelp stipes (Laminaria hyperborea) were sampled, and the densities of three species of associated amphipods were analysed. The algae were chosen to represent different structures and levels of architectural complexity. Palmaria palmata and Delesseria sanguinea are leaf-shaped, and Ptilota gunneri and Polysiphonia elongata are branched. The algae were later fed to the common epiphyte-associated amphipods Ampithoe rubricata, Jassa falcata and Caprella septentrionalis in no-choice laboratory experiments. Survival and growth were measured. J. falcata was found at the highest densities on P. gunneri and D. sanguinea, C. septentrionalis was found in highest densities on P. elongata, and P. gunneri and A. rubricata were found at the highest densities on P. gunneri. The survival and growth were highest on P. palmata for all amphipods. This indicates that the form and function of the algal host is more important for the distributions of amphipods than the food value. Interspecific distribution differences between amphipods may have been related to differences in their body form and size, as well as to crypsis.