Herbivores are thought to achieve adequate nutrition by consuming numerous species of plants or by occasionally consuming
animal tissue. Although active selection of diverse foods is common in nature, the relationship between diet mixing and consumer
fitness is poorly understood, especially in marine environments. We studied the fitness-based consequences of dietary mixing
in the sympatric amphipods
Ampithoe marcuzzii,
A. valida,
Cymadusa compta, and
Gammarus mucronatus by measuring survivorship, growth, and fecundity of these amphipods when they were offered single species of algae, a single
animal food, a mixture of algal species, or a combination of algae and animal matter. For the more sedentary, tube-building
amphipods
A. marcuzzii,
A. valida, and
C. compta, fitness on mixed algal diets was matched by fitness on at least one of the monospecific algal diets, suggesting that they
could benefit from preferential feeding on those algae in the field. The more mobile amphipod,
G. mucronatus, survived and grew similarly on the mixed diets and on the filamentous brown alga
Ectocarpus siliculosus. However, its fecundity was significantly higher when feeding on the algal and animal mixture than on
Ectocarpus alone. Additionally, for
G. mucronatus, fitness on mixed algae, mixed algae plus animal matter, and animal matter alone was equivalent, although female growth (but
not gonad production) was slightly lower on animal matter alone than on the mixed algae combined with animal food. Thus the
more mobile amphipod,
G. mucronatus, was the only species able to perform well on animal food alone. In contrast,
A. valida and
C. compta experienced large negative effects when limited to consuming animal matter alone. For these two species, combining algae
and animal matter did not enhance fitness over combining only algae. Fitness effects of specific algal diets showed some general
similarities, but also considerable variance among the amphipods. For example,
E. siliculosus was generally better food than other algae for all four amphipods, whereas
Sargassum filipendula was generally poor. However,
A. marcuzzii did not suffer negative effects of consuming only
Sargassum. The red alga
Polysiphonia sp. and the green alga
Enteromorpha flexuosa decreased fitness in
A. marcuzzii,
C. compta, and
G. mucronatus, but not
A. valida, and the negative effects of
Polysiphonia were considerably larger for
A. marcuzzii than for the other amphipods. Our data show that nutritional requirements, even among related species (e.g.,
A. marcuzzii and
A. valida), can be dramatically different. Diet mixing may benefit more mobile consumers like
Gammarus that are better able to search for different foods, and may be less important for more sedentary herbivores like
Ampithoe and
Cymadusa that consume, and live in close association with, individual host plants.
Received: 14 September 1998 / Accepted: 22 October 1999
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