THE REACTION-TIME OF LEECH AND TRICLAD SPECIES TO CRUSHED PREY AND THE SIGNIFICANCE OF THIS FOR THEIR COEXISTENCE IN BRITISH LAKES

1. Three species of leeches, Erpobdella octoculata, Glossiphonia compl anata and Helobdella stagnalis, and four species of triclads, Polyceli s nigra, P. tenuis, Dugesia polychroa and Dendrocoelum lacteum, common ly coexist on stony shores in productive British lakes. All species ar e food limited and there is much overlap in their diet. For both leech and triclad communities, coexistence of species is through the occurr ence of food refuges. Leeches are more successful than triclads at cap turing live prey, whereas both groups feed on damaged prey, comprising incapacitated, live or dead animals that are leaking body fluids. If triclads are better than leeches at exploiting damaged prey, this coul d be a mechanism for their coexistence. 2. Laboratory experiments inve stigated the comparative speeds at which leeches and triclads responde d to crushed prey. Young and adult predators were offered a crushed sp ecimen of the oligochaete Tubifex tubifex, the snail Lymnaea peregra, the crustacean Asellus aquaticus or the chironomid Chironomus sp., and their reaction times recorded. These four prey groups constitute the main diet of the predators in the field. Only D. polychroa and D. lact eum showed a significantly different reaction time between young and a dults to crushed prey, and the reason for this is unclear. All predato rs, except H. stagnalis and D. polychroa, showed a difference in react ion time to the four types of prey, presumably a consequence of differ ences in both the 'quality' and 'concentration' of the different prey fluids, and there were some differences between predators in their spe ed of reaction to the same prey type. The following sequence, from fas test to slowest, in general reaction time to prey was obtained: E. oct oculata, D. polychroa, P. tenuis, D. lacteum, P. nigra, H. stagnalis a nd G. complanata. 3. The location of the damaged food by the predators can be explained partly in terms of their foraging behaviour, with E. octoculata, D. polychroa and P. tenuis exhibiting a more seek-out str ategy than other species which have a more sit-and-wait behaviour, and partly on the level of sophistication of their chemosensory system us ed to detect leaked prey fluids. This system is highly developed in tr iclad species but poorly developed in leeches. 4. In a second type of experiment in which prey, L. peregra, A. aquaticus or Chironomus sp., were offered at different time intervals after crushing to H. stagnali s and P. tenuis, few predators fed on food crushed for 24 h or longer, although a few leeches fed on Chironomus crushed for up to 72 h. 5. I t is concluded that coexistence of leech and triclad species on stony shores in lakes is assisted by partitioning of food on a damaged or li ve basis.