The effects of diet mixing on consumer fitness: macroalgae, epiphytes, and animal matter as food for marine amphipods

Abstract

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.

The role of epiphytic algae and different species of Sargassum in the distribution and feeding of herbivorous amphipods

“…The consumption of species of this genus by ampithoids (Duffy, 1990;Cruz-Rivera & Hay, 2000) and its use for the construction of tubes (Norton & Benson, 1983;Schneider & Mann, 1991;Appadoo & Myers, 2003) has also been reported. This suggests that the load of epiphytes, at least as a food resource, is not the main factor to explain density variation of the ampithoids.…”

Fitness-associated consequences of an omnivorous diet for the mangrove tree crab Aratus pisonii

“…The amphipod species Ampithoe valida and Cymadusa compta have significantly higher fecundity when consuming a mixture of algae and animal material than when consuming a diet of animal material alone. Conversely, the closely related amphipod Gammarus mucronatus has comparable growth and survival on an algal-only and mixed algal and animal diet, as well as equivalent fitness on exclusively algal or animal diets, and on diets that mix the 2 food types (Cruz-Rivera & Hay 2000a).…”

Impact of food type on respiration, fractionation and turnover of carbon and nitrogen stable isotopes in the marine amphipod Gammarus aequicauda (Martynov, 1931)

“…It is thus possible that a diet composed only of P. oceanica litter is not appropriate for Gammarus aequicauda, even if dead leaves constitute a major part (N 50%) of its assimilated carbon in the wild (Lepoint et al, 2006;Michel et al, 2015). It can thus be hypothesized that for G. aequicauda, as for other amphipod species, a mixed diet is important for its survival (Cruz-Rivera and Hay, 2000) and that it cannot cope with a diet exclusively composed of a poorly digestible and unpalatable food source such as P. oceanica dead leaves. However it can also be hypothesized that P. oceanica litter can be consumed in quite large amounts if mixed along with various other vegetal, or even occasionally animal food sources, and still achieve nutritional balance (Bernays et al, 1994;Cruz-Rivera and Hay, 2000;Senior et al, 2015).…”

“…It can thus be hypothesized that for G. aequicauda, as for other amphipod species, a mixed diet is important for its survival (Cruz-Rivera and Hay, 2000) and that it cannot cope with a diet exclusively composed of a poorly digestible and unpalatable food source such as P. oceanica dead leaves. However it can also be hypothesized that P. oceanica litter can be consumed in quite large amounts if mixed along with various other vegetal, or even occasionally animal food sources, and still achieve nutritional balance (Bernays et al, 1994;Cruz-Rivera and Hay, 2000;Senior et al, 2015). Another hypothesis could be that our experimental protocol eliminated epiphytes, a very important and attractive constituent of litter (Kitting et al, 1984;Lepoint et al, 2006).…”