32
T. B. REYNOLDSON
were kept in the cages in Lake Ogwen and observed for cocoon production; these triclads were fed regularly. During a period of 30 days
P. nigra produced two, P. tenuis twenty, Dugesia five and Dendrocoelum eleven cocoons. The majority of these proved fertile in the Ogwen
water showing that these species are not only able to tolerate such conditions but are also able to breed under them.
The growth and development of t'he young of each species except
P . nigra was followed in the laboratory under a standard feeding and
temperature regime in Lake Ogwen water, for comparison with similar
experiments in water of higher calcium content (20 nig/litre, see Young,
1963 for details). Progress was similar in both types of water, culminating in the production of fertile cocoons in all species. Dendrocoelum
lucteum took a little longer in the calcium-poor Ogwen water where the
average maturation period was 217 days with limits of 188-232; in
calcium-rich water it averaged 180 days (S.D. 19) at 10" C.
The conclusion to be drawn from these various experiments is that the
chemical nature of lake water plays no direct part in determining the
pattern of distribution; Dugesia and Dendrocoelum are as capable of
living in calcium and T.D.M. poor water as are the Polycetis species, nor
were either of the Polycelis species specially fitted for life under such
conditions.
The occurrence of several freshwater organisms in large lakes normally outside their usual range as measured by edaphic factors has
always been puzzling, and various interpretations have been offered
(e.g. Macan, 1950; Tucker, 1958; Williams, 1962a). In the case of triclads it is clear that the presence of Dugesiu and Dendrocoelum in Lake
Windermere with 5.0 mg/litre calcium and in Loch Lomond with 3-0 mg
need not be explained in terms of physiological races as the Ogwen
experiments showed. Nor is lake size necessarily involved, since both
are found in the small Newton Mere with 5.0 mg calcium, while Dendrocoelum also occurs in the small Llyn Mair with only 2.4 mg/litre. This
ability of the four species to tolerate equally well such a wide range of
calcium and other ions, while of much interest to physiologists, lies outside the context of the present ecological problem.
There are of course many other variables such as pH and dissolved
oxygen which have not been considered, but there was no indication in
the field work that they are relevant to distribution. Apart from some
dystrophic lakes on Islay, low pH values have not been encountered and
it is unlikely that dissolved oxygen becomes limiting in the littoral zone
of stony shores. There was some indication (Reynoldson, 1958a) that in
lakes with comparable water chemistry, those dystrophic in character
supported a smaller population, but more work is needed to substantiate
this view.
T. B. REYNOLDSON
were kept in the cages in Lake Ogwen and observed for cocoon production; these triclads were fed regularly. During a period of 30 days
P. nigra produced two, P. tenuis twenty, Dugesia five and Dendrocoelum eleven cocoons. The majority of these proved fertile in the Ogwen
water showing that these species are not only able to tolerate such conditions but are also able to breed under them.
The growth and development of t'he young of each species except
P . nigra was followed in the laboratory under a standard feeding and
temperature regime in Lake Ogwen water, for comparison with similar
experiments in water of higher calcium content (20 nig/litre, see Young,
1963 for details). Progress was similar in both types of water, culminating in the production of fertile cocoons in all species. Dendrocoelum
lucteum took a little longer in the calcium-poor Ogwen water where the
average maturation period was 217 days with limits of 188-232; in
calcium-rich water it averaged 180 days (S.D. 19) at 10" C.
The conclusion to be drawn from these various experiments is that the
chemical nature of lake water plays no direct part in determining the
pattern of distribution; Dugesia and Dendrocoelum are as capable of
living in calcium and T.D.M. poor water as are the Polycetis species, nor
were either of the Polycelis species specially fitted for life under such
conditions.
The occurrence of several freshwater organisms in large lakes normally outside their usual range as measured by edaphic factors has
always been puzzling, and various interpretations have been offered
(e.g. Macan, 1950; Tucker, 1958; Williams, 1962a). In the case of triclads it is clear that the presence of Dugesiu and Dendrocoelum in Lake
Windermere with 5.0 mg/litre calcium and in Loch Lomond with 3-0 mg
need not be explained in terms of physiological races as the Ogwen
experiments showed. Nor is lake size necessarily involved, since both
are found in the small Newton Mere with 5.0 mg calcium, while Dendrocoelum also occurs in the small Llyn Mair with only 2.4 mg/litre. This
ability of the four species to tolerate equally well such a wide range of
calcium and other ions, while of much interest to physiologists, lies outside the context of the present ecological problem.
There are of course many other variables such as pH and dissolved
oxygen which have not been considered, but there was no indication in
the field work that they are relevant to distribution. Apart from some
dystrophic lakes on Islay, low pH values have not been encountered and
it is unlikely that dissolved oxygen becomes limiting in the littoral zone
of stony shores. There was some indication (Reynoldson, 1958a) that in
lakes with comparable water chemistry, those dystrophic in character
supported a smaller population, but more work is needed to substantiate
this view.