Macroalgal traits and the feeding and fitness of an herbivorous amphipod: the roles of selectivity, mixing, and compensation

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 lash 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.