Welsh Journals

CHANGES IN THE TRICLAD (Turbellaria, Platyhelminthes) FAUNA OF SOME NORTH WALES LAKES INTRODUCTION The triclads living in the lakes of Snowdonia and Anglesey were recorded during the period 1948 to 1955 (Reynoldson, 1956) as part of a comprehensive study to explain the distribution and abundance of these flatworms in terms of basic ecology. Although several of the lakes have been observed at intervals since then, so little is known about faunal changes in general that a survey after 20 years was considered to be worthwhile. Seven species of triclad are typically lake-dwelling but may be found also in the quiet areas of rivers and streams (Table 1); two species, Polycelis felina and TABLE 1 Characteristic habitats of the triclad species found in Britain. Phagocata vitta is mainly a ground water species. LAKES Polycelis tenuis (Ijima) Dugesia polychroa (Schmidt) D. tigrina (Girard) Dendrocoelum lacteum (0. F. M'uller) Bdellocephala punctata (Pallas) Planaria torva (0. F. Miiller) Crenobia alpina (Figure 1), characterize streams although they may sometimes be found in high altitude or high latitude British lakes (Reynoldson 1953). Polycelis nigra and Phagocata vitta seem to be equally at home in both still and moving water. Polycelis hepta Melander & Reynoldson recorded in the first survey is now recognised as not being distinct from P. tenuis (Lender & Moigne 1960; Moigne 1962; Benazzi 1963). Three species, P. nigra, P. tenuis and Ph. vitta dominate the triclad faunas of North Wales lakes; sparse populations of the large species Bdellocephala punctata were found in Llyn Mymbyr (SH 5771) and Geirionydd (SH 7660) in the earlier survey. A more varied fauna may occur in small artificial ponds on calcareous substrata e.g. Vaynol Ponds near Port Dinorwic (SH 5268) (Reynoldson 1956) with Dugesia polychroa and Dendrocoelum lacteum co-existing with P. nigra and P. tenuis. Faunas are determined by both historical events such as the establishment of land bridges, changes in sea-level and drainage basins which influence the dispersal of species and also by ecological processes including interspecific competition. It has been argued (Reynoldson 1966) that while the triclad fauna of Anglesey is largely dominated by historical events, particularly the island's separation from the mainland by the Menai Straits, the lake faunas of Snowdonia and other mainland lakes in Britain are now maintained by ecological interactions. The majority of lakes in the North of Scotland have not yet been colonised by triclads. DURING THE 20 YEARS UP TO 1973. T. B. REYNOLDSON and R. M. JAQUES STREAMS Polycelis felina (Dalyell) Crenobia alpina (Dana) LAKES AND STREAMS Polycelis nigra (0. F. Muller) Phagocata vitta (Duges)