6
T. B . REYNOLDSON
This complex situation in the Planariidae raises problems in ecological work as was appreciated by Dahm (1958) who was dealing with
stream-dwelling species which show similar conditions. Obviously, it is
too laborious to examine the cytology of every individual or even a part
of every sample. Nevertheless, it is likely that different karyotypes will
show some contrast in their ecology as already demonstrated by Dahm
(1955) for Dugesia tigrina and by Mirolli (1961) for Dug. Zugubris in
Lake Maggiore. The latter concluded that the triploid races of this species were likely to replace the diploid as seems to have happened in
P. tenuis (Melander, 1963). This is a problem common in ecological work
and further complicated by the fact that the genetic complex of populations is changing all the time a t varying rates depending upon the
species and the habitat. The only practical approach with our present
meagre knowledge of triclad ecology is to ignore intra-specific differences
for the time being until the main outlines of the ecology of the species
themselves are known. Anomalies which have already appeared in
distribution and may well stem from contrasts of this kind, can be
examined more profitably a t a later date.
The life-cycles of these four species are similar in outline and reproduction is only of the sexual or pseudo-sexual type. Although asexual
reproduction by fission has been reported for some races of P. nigra
this has not been observed in British populations to any marked degree.
The young which hatch from cocoons attached to the substratum are
some 2 to 4 mm long and replicas of the adult, except of course in their
immaturity. The breeding seasons of these species overlap but each has
a characteristic peak period, and the effect of temperature upon the
life-cycles differs to a variable degree according to species (Reynoldson
et aZ., 1965; Table V).
111. THE PATTERN O F DISTRIBUTION A N D ABUNDANCE
A. INTRODUCTION A N D FTELD METHODS
This facet of triclad ecology can be treated at several levels. Distribution among the various habitats to be found in most lakes is one
approach which has been studied to some extent. A second is to compare triclad populations in lakes of varying type without regard to specific habitat, while a third method would be to study lakes of contrasted
productivity and restrict the observations to one type of shore. Since
the complexity of factors was likely to be formidable in any case, the
last approach which reduced some of them, seemed most appropriate.
Although lakes may be classified in different ways, one of the most
fundamental is that based on the type of rock in the drainage area.
Hard rocks give rise to chemically poor water and lakes of low
T. B . REYNOLDSON
This complex situation in the Planariidae raises problems in ecological work as was appreciated by Dahm (1958) who was dealing with
stream-dwelling species which show similar conditions. Obviously, it is
too laborious to examine the cytology of every individual or even a part
of every sample. Nevertheless, it is likely that different karyotypes will
show some contrast in their ecology as already demonstrated by Dahm
(1955) for Dugesia tigrina and by Mirolli (1961) for Dug. Zugubris in
Lake Maggiore. The latter concluded that the triploid races of this species were likely to replace the diploid as seems to have happened in
P. tenuis (Melander, 1963). This is a problem common in ecological work
and further complicated by the fact that the genetic complex of populations is changing all the time a t varying rates depending upon the
species and the habitat. The only practical approach with our present
meagre knowledge of triclad ecology is to ignore intra-specific differences
for the time being until the main outlines of the ecology of the species
themselves are known. Anomalies which have already appeared in
distribution and may well stem from contrasts of this kind, can be
examined more profitably a t a later date.
The life-cycles of these four species are similar in outline and reproduction is only of the sexual or pseudo-sexual type. Although asexual
reproduction by fission has been reported for some races of P. nigra
this has not been observed in British populations to any marked degree.
The young which hatch from cocoons attached to the substratum are
some 2 to 4 mm long and replicas of the adult, except of course in their
immaturity. The breeding seasons of these species overlap but each has
a characteristic peak period, and the effect of temperature upon the
life-cycles differs to a variable degree according to species (Reynoldson
et aZ., 1965; Table V).
111. THE PATTERN O F DISTRIBUTION A N D ABUNDANCE
A. INTRODUCTION A N D FTELD METHODS
This facet of triclad ecology can be treated at several levels. Distribution among the various habitats to be found in most lakes is one
approach which has been studied to some extent. A second is to compare triclad populations in lakes of varying type without regard to specific habitat, while a third method would be to study lakes of contrasted
productivity and restrict the observations to one type of shore. Since
the complexity of factors was likely to be formidable in any case, the
last approach which reduced some of them, seemed most appropriate.
Although lakes may be classified in different ways, one of the most
fundamental is that based on the type of rock in the drainage area.
Hard rocks give rise to chemically poor water and lakes of low