Turbellarian taxonomic database

Abstract / Notes

Ponds and lakes were examined for triclads in 6 areas: mid-Scotland, Isle of Islay (W. Scotland), Anglesay (N.
Wales), Snowdonia area of Caernarvonshire (N. Wales), north Ireland, and the English Lake District, of these
79 lakes were investigated. Lakes were chosen in such a manner that in each district their productivity
varied widely. Large lakes, and smaller bodies of water were avoided, and as nearly as possible similar
collection places were chosen in each lake, namely gently sloping stoney shores distant from inlets and
neither too exposed to wave action nor too sheltered. Collection was standardized to determine the number of
triclads collected per hour of searching, in order that comparisons among the lakes would be feasible.
Certain chemical determinations also were made. Productivity of each lake was estimated on the basis of its
Ca content and its content of dissolved matter. The number of triclad species, and the number of individuals,
increased with increasing lake productivity. The most common species was Polycelis nigra, which occurred in
76% of the lakes, and in all types from the least to the most productive. P. tenuis and P. hepta were next
most common, but did not occur in the unproductive lakes. Dugesia lugubris and Dendrocoelum lacteum were
encountered less frequently, and only in the most productive lakes. Phagocata vitta occurred only in a few of
the least productive lakes at relatively high altitudes. Competition and other relationships among the
species are discussed. There are 12-13 species of fresh-water triclads in Britain. Their taxonomy is
discussed, and drawings are given to aid in their identification.

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