The diet of Erpobdella octoculata (Hirudinea: Erpobdellidae) in two Danish lowland streams

The diet of leeches and the effect of prey size on feeding success were examined using gut analyses of Erpobdella octoculata from two Danish lowland streams. Five weight (size) classes of leeches were examined. For each insect prey found in the guts the width of the head capsule was measured. Gut contents were compared to invertebrate abundances by means of kick-samples from the streams. Nine prey groups were common in leech guts. Insect larvae dominated making up 79 to 82% of the total prey number. The mean prey number per leech varied from 2.1 to 2.4. A comparison of gut contents with the occurrence of prey taxa in benthos samples indicated that turbellarians and all groups of chironomids were over-exploited, oligochaetes were preyed in proportion to frequency in the benthos, whilst Baetis rhodani, Hydropsyche angustipennis and Simuliidae appeared partly to avoid predation by E. octoculata. The results demonstrated that E. octoculata feeds mainly on chironomids when available. Prey size alone will cause differences in the diet of E. octoculata of different sizes, chironomids being the dominating prey in small leeches, but less so in large individuals which feed on a greater variety of, and larger prey items. The abundance of the trichopteran Hydropsyche angustipennis equalled 4 to 30 % of the stream benthos, but contributed only 1-3 % in the overall leech diet, escaping predation because of its large size.