Prey selection by the predatory leech Nephelopsis obscura in relation to three alternative models of foraging

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Abstract
Prey selection by a predatory leech, Nephelopsis obscura, was examined in a series of laboratory experiments. Two chironomids, Glyptotendipes paripes and Chironomus riparius, and the tubificid worm, Tubifex tubifex, were used as alternative prey. These three prey are all tube dwellers found in the same microhabitat at the mud-water interface. The quality of each of the three prey types was estimated by their ability to promote the growth of the leeches. The two chironomids were much superior prey than Tubifex. Mixed diets did not improve growth. The selectivity of starved leeches offered equal densities of the two chironomids was not affected by the absolute abundance of prey. After 72 h the selectivity was biased toward Chironomous, the more mobile prey, and this bias increased with prey density. Varying the relative abundance of these two prey did not alter the selectivity of the leeches with a single exception. When Chironomus was the more abundant prey, selectivlty was biased towards it, but disappeared after 72 h. There were no significant effects of absolute abundance, relative abundance, or feeding history on the selectivity of leeches offered a choice between Glyptotendipes and Tubifex. Tubifex was rarely captured in any of the treatments tested. It was concluded that a description of prey choice by Nephelopsis does not require explanations based in optimal diet theory or frequency-dependent foraging. Differences in capture rates of different prey species may be related to their motility or vulnerability rather than any active choice on the part of Nephelopsis.