ENERGY TRADE-OFFS AND FORAGING MODE CHOICE IN AMERICAN EELS

American eels (Anguilla rostrata) use three different modes when handling food: suction, shaking, and spinning (rotational feeding). We investigated mode selection and mode switching during foraging in terms of relative costs and benefits of different modes. Eels were fed food items that differed in size and consistency, thereby inducing particular modes. To test the hypothesis that American eels forage efficiently by selecting foraging modes that maximize net energy intake and growth, we determined whether modes differed in short-term energy costs and long-term effects on growth rate, and whether eels preferred food types that maximize short- and long-term net benefits. Net energy intake (energy ingested - energy consumption represented by oxygen uptake) was greatest when suction was the chief mode employed, and was reduced during shaking and spinning. Animals using spinning fed the slowest and ate relatively little food. Eels fed a diet that required them to feed by only spinning or shaking lost mass faster than eels allowed to feed by suction. When given a choice of food types, eels preferred energetically efficient types (suction over both shaking and spinning, shaking over spinning). Our results suggest that rotational feeding is an alternative foraging mode that facilitates dismembering of large food items, but this benefit can be compromised by energy and time costs. Spinning is the least preferred mode and is used when other modes are ineffective. Mode switching in this species can be considered optimal in that mode choice and switching appear to maximize net energy intake in a manner that directly affects growth rate and, presumably, fitness.