The Relationship between the Distribution of Dendrocoelum lacteum (Müll.) and Asellus in Britain and Fennoscandia

Summary

1.	In Britain, Dendrocoelum lacteum and Asellus spp. are more prevalent in waters with a calcium concentration of more than 10 mg/l; in Fennoscandia they inhabit lakes with an extremely low-calcium content.
2.	The penetration by Asellus into these low-calcium lakes is probably related to the fact that, unlike British lakes, they are often surrounded with deciduous trees. The decaying leaves may provide a food source for Asellus. The anomalous occurrence of Asellus in 4 unproductive British lakes also containing Dendrocoelum, can be explained similarly.
3.	Quantitative data for British and Swedish lakes provided good evidence for a positive relationship between the distribution of the two organisms. Exceptional instances in which one or other of them was not found in a lake are discussed.
4.	It is argued that this relationship is demonstrable because Asellus forms a more important food source for Dendrocoelum than it does for the other triclad species living in lakes.
5.	Thus, under conditions of severe inter-specific competition for food, Dendrocoelum depends on Asellus as a ‘food refuge’. It is unable to co-exist with the other triclad species in the absence of Asellus.