Food Niche and Co-Existence in Lake-Dwelling Triclads

Abstract

This account deals with the four common species of triclad found living together in the shallow littoral zone of lakes in Britain, namely, Polycelis nigra (O. F. Muller), P. tenuis Ijima, Dugesia polychroa (O. Schmidt) and Dendrocoelum lacteum (O. F. Muller). A study of the food of these predators by laboratory and field observations (Reynoldson & Young 1963) suggested that, despite considerable overlap in the type of prey, there were specific contrasts. This led to the development of a hypothesis to explain, in broad terms, the distribution of these triclad species in nature on the basis of interspecific competition for food (Reynoldson 1966). Since partitioning of food resources by these triclads seems to be the key to explain co-existence of potentially competing species, more data on their natural food were desirable. Because triclads ingest mainly semi-fluids and tissues by the pumping action of a muscular pharynx, direct examination of the gut contents of large numbers for identifiable remains of their prey was unprofitable. The earlier work showed that out of 1030 examinations only 248 (24% o) contained recognizable items. There was also the danger of biased information with such a method, as shown by Young, Morris & Reynoldson (1964). A serological technique is the obvious answer although it also has limitations (Davies 1969). This approach has enabled the first comparative study of the natural food of lake-dwelling triclads to be carried out and it is directed to the following questions. How much overlap occurs between the species in their food requirements in nature in terms of the relative proportions of the various component organisms taken as food, and what degree of difference is necessary to allow mixed species populations of triclads to persist? This information is used to elaborate the hypothesis that interspecific competition for food determines the distribution and, in part, the abundance of these triclads. In this paper 'niche' is used in the sense described by Hutchinson (1957) and the terms food niche, food regime and diet are synonymous. It is necessary to explain the re-introduction of the species Dugesia polychroa (O. Schmidt) into the British triclad fauna. Recent work has shown that two species occur in Britain under the name D. lugubris. Five of the six Dugesia populations studied here are more properly named D. polychroa (Reynoldson & Bellamy 1970). The Dugesia populations of Cole Mere, The Mere and White Mere were predominantly polychroa but contained some lugubris. Table 10 shows that any D. lugubris which may have been included under D. polychroa for these lakes have not noticeably influenced the data.