Observations on the comparative ecology of lake-dwelling triclads in southern Sweden, Finland and northern Britain

Abstract

There are more lake-dwelling species of triclad in northerm Britain (10) than in southern Sweden (7) and the distribution of some differ in the two countries over approximately comparable types of lake. Dendrocoelum lacteum (Muller) occurs in a wide range of lake types in s. Sweden but is restricted to more productive lakes in n. Britain. Polycelis nigra (Muller) (absent from Sweden) has a similar, wide occurrence in Britain. Planaria torva (Muller) is more common in s. Sweden in lakes of all types but particularly in the less productive lakes. The Polycelis species P. tenuis (Ijima) and P. hepta E. H. and Y. Melander, also the Dugesia lugubris species complex show much the same distribution in both areas. Polycelis felina (Dalyell) does not occur in Sweden. The small nuber of Finnish lakes sampled confirm the data for s. Sweden. There is significant correlation between the total abundance of triclads and both the hardness and dissolved matter contents of the Swedish lake waters. Quantitatively the s. Swedish triclad populations compare most closely with those of Mid-Scotland in the n. Britain area.