No abstract

Relationships among food choice, compensatory feeding, and the consequences for consumer fitness rarely have been quantified. We created foods of varying nutritional quality and evaluated the consequences of compensatory feeding for three sympatric species of amphipods by analyzing food choices, feeding rates, and long‐term effects on fitness. Nutritional quality was manipulated by creating low‐quality diets from algae (low in protein, nitrogen, and total organic carbon), high‐quality diets from commercial fish food (high in protein, nitrogen, and total organic carbon), and intermediate‐quality diets from mixtures of those two foods. When high‐ and low‐quality diets were simultaneously offered, the more mobile, non‐tube‐building amphipods, Gammarus mucronatus and Elasmopus levis, both fed preferentially on the high‐quality diet. The more sedentary, tube‐building amphipod Ampithoe longimana did not discriminate between these foods. When confined to a single food type, all three species exhibited compensatory feeding on the low‐quality diet. Despite compensatory feeding, when Elasmopus levis were cultured on the low‐quality food, they experienced reduced survivorship, growth, and fecundity during two successive ovulations, compared to individuals feeding on more nutrient‐rich foods. Low‐nutrient foods caused similar declines in growth and female gonad size for Gammarus mucronatus. In contrast, the survivorship, growth, and fecundity of Ampithoe longimana was not affected by any of the diets tested. Thus, compensatory feeding allowed the more sedentary species, Ampithoe longimana, to completely circumvent the effects of low nutritional quality, but the same behavior was ineffective for both of the more mobile species, Gammarus mucronatus and Elasmopus levis. The ability of A. longimana to achieve equal fitness by substituting food quantity for food quality may allow this sedentary species to form longer associations with individual host plants, minimize movement among hosts, and thus lower its risk of being detected by predators.

Selective feeding, compensatory feeding, and diet mixing have all been proposed as adaptive strategies allowing herbivores to enhance nutrient intake from low quality plant and algal foods. However, little is known about the relative importance of these alternative feeding strategies for consumer fitness or about how these strategies are affected by prey nutritional traits. To address this, we studied the effects of algal nutritional value and toughness on feeding choices, feeding rates, and survival, growth and fecundity of the amphipod Ampithoe longimana. To assess the value of diet mixing, we compared fitness of amphipods cultured on each of 15 algal species or on 4 different mixtures of algae. We also quantified how sequentially switching between algae that supported higher and lower fitness affected fitness compared to monospecific diets of these algae and to a constant mixture of the algae. Protein, nitrogen, organic content, or toughness of algae did not correlate with food choice by A. longimana. In contrast, we found a strong inverse correlation between feeding rates and algal organic content [ash free dry mass/wet mass (AFDM:WM) and total organic carbon (TOC)], and, to a lesser extent, protein (but not nitrogen). Thus, when confined with algae having lower nutritional value, A. longimana used quantity to compensate for quality. This compensatory feeding was confirmed by feeding amphipods on artificial diets that varied only in their amount of AFDM:WM. Despite broad differences in algal nutrient content or other traits, compensatory feeding allowed this amphipod to maintain high fitness when cultured on most, but not all, algae. Access to algal mixtures did not enhance fitness compared to feeding on several algae offered alone, suggesting that A. longimana need not rely on a mixed diet. Even when significant differences in survivorship or growth occurred between a monospecific diet and a mixed diet, fecundity or size of eggs produced by egg-bearing survivors were generally unaffected. Furthermore, when amphipods were switched sequentially (every 24 h) between 2 different quality algae, only growth (but not survivorship, fecundity, or egg size) was affected, with growth determined primarily by the higher quality alga offered. Amphipods confined with the green alga Codium fragile ovulated significantly later than conspecifics on one mixed diet, but this effect was observed only in 1 of 2 long-term assays. Thus, dietary mixing offered only a moderate benefit to this amphipod under very restrictive conditions. For A. longimana, food selection is relatively unresponsive to algal nutritional quality, apparently because compensatory feeding allows this amphipod to successfully exploit a variety of algal foods. Compensatory feeding also may reduce the need to move among host algae in order to mix diets, thus decreasing the risk of movement-associated detection by predators.

Numerous studies have assessed the individual effects of prey nutritional quality or chemical defenses on consumer feeding behavior. However, little is known about how these traits interact to affect consumer feeding and performance. We tested the separate and interactive effects of prey chemical defenses and nutritional quality on the feeding behavior and fitness of six sympatric crustacean mesograzers. Natural concentrations of diterpene alcohols (dictyols) from the brown alga Dictyota menstrualis were incorporated, or not incorporated, into lower quality and higher quality foods to create artificial diets mimicking prey of variable value and defense. Five amphipods (Ampithoe longimana, A. valida, Cymadusa compta, Gammarus mucronatus, and Elasmopus levis) and one isopod (Paracerceis caudata), representing a continuum of closely to distantly related organisms, were fed intact algae or lower and higher quality diets containing or lacking dictyols. All six mesograzers preferred the green alga Enteromorpha intestinalis to the dictyol producing alga Dictyota menstrualis. In assays allowing consumers to choose between simultaneously available foods, dictyols deterred feeding by all five amphipods, but not the isopod; this occurred for both lower and higher quality foods. In no-choice assays, where consumers were confined with only one of our four treatment diets, effects on feeding became more complex. Nutritional quality alone affected feeding by five of the six species. These grazers compensated for lower quality by increasing consumption. Dictyols suppressed feeding for four of the six species. More interestingly, there were significant dictyol ϫ quality interactions for three species. Dictyols decreased feeding more when placed in lower quality foods than higher quality foods. Two amphipods deterred by dictyols in the choice assays readily consumed dictyolcontaining foods in no-choice situations and suffered few negative effects of doing so. Although all amphipods were deterred by dictyols in choice assays, dictyols decreased fitness (survivorship, growth, or reproduction) for only four of the five species. These effects included large and immediate decreases in survivorship, dramatic effects on reproduction, and modest effects on female growth. Dictyols enhanced survivorship of the isopod. Thus, the effects of secondary metabolites on feeding in choice situations vs. fitness in long-term assays were inconsistent. For three amphipods, certain effects of food quality, dictyols, or their interaction were detected only for females. In general, negative effects of dictyols on fitness were greater in lower than in higher quality foods, suggesting that prey nutritional value may counteract the effects of defensive metabolites. For example, when G. mucronatus consumed dictyols in lower quality foods, mortality was Ͼ80% by day 5; for dictyols in higher quality foods, 80% mortality took 28 days to develop. Lower quality foods alone significantly decreased growth for the isopod, three of the amphipods, and the females of a...

No 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.

No abstract

This study investigates the influence of mesograzer prior exposure to toxic metabolites on palatability of the marine cyanobacterium, Lyngbya majuscula. We examined the palatability of L. majuscula crude extract obtained from a bloom in Moreton Bay, South East Queensland, Australia, containing lyngbyatoxin-a (LTA) and debromoaplysiatoxin (DAT), to two groups: (1) mesograzers of L. majuscula from Guam where LTA and DAT production is rare; and (2) macro-and mesograzers found feeding on L. majuscula blooms in Moreton Bay where LTA and DAT are often prevalent secondary metabolites. Pair-wise feeding assays using artificial diets consisting of Viva clathrata suspended in agar (control) or coated with Moreton Bay L. majuscula crude extracts (treatment) were used to determine palatability to a variety of consumers. In Guam, the amphipods, Parhyale hawaiensis and Cymadusa imbroglio; the majid crab Menaethius monoceros; and the urchin Echinometra mathaei were significantly deterred by the Moreton Bay crude extract. The sea hares, Stylocheilus striatus, from Guam were stimulated to feed by treatment food whereas S. striatus collected from Moreton Bay showed no discrimination between food types. In Moreton Bay, the cephalaspidean Diniatys dentifer and wild caught rabbitfish Siganus fuscescens were significantly deterred by the crude extract. However, captive-bred 5. fuscescens with no known experience with L. majuscula did not clearly discriminate between food choices. Lyngbya majuscula crude extract deters feeding by most mesograzers regardless of prior contact or association with blooms.