The Incidence and Intensity of Predation on Lake-Dwelling Triclads in the Field

Abstract

It is the purpose of this paper to evaluate the incidence and intensity of predation on each of the four species of triclad in habitats ranging from stony to weedy shores with particular reference to two questions. Firstly, can predation on either a stony or weedy shore reduce populations of triclads below the level atwhichcompetition for food operates; secondly, are some species of triclads preyed on more heavily than others? A direct study of predation by the examination of the gut contents of predators or their faecal pellets cannot be used successfully for soft-bodied prey such as triclads. For the detection of such non-visually identifiable remains of triclads consumed by predators, a serological approach was used. In essence, four specific antisera, one against each species of triclad were produced which reacted with the remains of ingested triclads but not against the predator itself or other prey organisms. Initial cross reactions were absorbed out and specific rabbit antisera produced against Polycelis nigra, and Dendrocoelum lacteum. Loss of sensitivity after absorption prevented the preparation of a specific anti-Polycelis tenuis serum and therefore an antiserum which retained sensitivity but cross-reacted with P. nigra to a slight degree was used. Until recently (Reynoldson & Davies, in press) distinction between Dugesia polychroa and D. lugubris was not made, the two species being recorded under the name D. lugubris. The anti-D. polychroa serum reacted with D. lugubris but this is unlikely to have influenced the data since most habitats contained only D. polychroa. In the three habitats which supported both species D. lugubris was very much in the minority. For all serological tests Morris's (1964) version of the modified Oakley-Fulthorpe method (Preer 1956) was employed with the macerate