Factors involved in herbivore food preference

Herbivore food preference can be measured either in terms of attractiveness or of edibility: these quantities are not necessarily correlated. The isopod Idotea baltica (Pallas) is attracted to large, tough, branched algae (perennials) while the amphipod Ampithoe valida (Smith) is attracted to softer, filamentous or bladed algae (ephemerals). Attractiveness for Ampithoe is related to the nutritive value of the algae; this is not so for Idotea, which responds more to algal morphology and availability. Idotea's mobility, possible susceptibility to fish predation, and preference for moderate wave exposures may select for a primary response to algae as habitat rather than as food source.