44
T. B. REYNOLDSON
predation. In the present programme Taylor (1960) confirmed that
dragonfly and damselfly larvae fed extensively on Polycelis nigra and
there was some indication that Erpobdella sp. ate a few. These experimeiits were taken further by Young and Reynoldson (1965) who tested a
series of potential predators against young and adult P. nigra, P. tenuis,
Dugesia lugubris and Dendrocoelum lacteum. The results (Table XIII)
show that of nineteen invertebrates selected over a range of phyla,
only adults of Dytiscus marginalis and the Odonata larvae took triclads
in any numbers, and in these cases young rather than adult triclads
were eaten. Fish, especially rudd (Scardinius erythrophthalmus) ate them
to some extent. All these predators took approximately equal numbers
of the four triclad species but the experimental regime did not offer a
choice. There are several reasons for suggesting that such restricted
predation is likely to be even less in the stony, littoral zone of lakes.
First, those invertebrates which attacked the triclads occur most commonly in heavily weeded habitats with a muddy substratum. Second, it
was evident that apart from rudd, only one or two individual fish fed
upon the triclads, despite their evident unpleasant taste. This would
account for Taylor’s ( 1 960) conclusion that minnows did not eat P. nigra.
In nature, with alternative food it is unlikely that many triclads are
eaten. Third, rudd are so sparsely distributed in Britain that they cannot
influence triclad distribution; furthermore, fish with similar feeding
habits are found precisely in those lakes where triclads are so numerous
and diversified. Finally, triclads are so slow-moving or immobile that
they will not attrart fish to the same degree as when both are confined
in a dish with limited cover. Indirect evidence from the study of the
population dynamics of the individual triclad species (p. 40) also suggested that predation was insignificant. However, before this factor can
be dismissed altogether it will be necessary to examine feeding on triclads
by some method applicable to field conditions such as the serological
technique used successfully by Dempster (1960).
Parasites of triolads can be dismissed quickly, since of the several
thousands exanlined by squash techniques (Reynoldson and Young,
twenty populations were infected by a species of Sporozoa. In the latter
case, such populations were usually found in polluted regions of lakes.
There is no evidence of the extent to which the triclads (all species) were
harmed, if at all, by the sporozoan but the populations were not obviously reduced.
2. Competitors
The two main resources for which inter-specific competition usually
occurs are adequate living space and food (Nicholson, 1954; Milne,
1963) jess than ten were found to harbour a trypanosome and less than
T. B. REYNOLDSON
predation. In the present programme Taylor (1960) confirmed that
dragonfly and damselfly larvae fed extensively on Polycelis nigra and
there was some indication that Erpobdella sp. ate a few. These experimeiits were taken further by Young and Reynoldson (1965) who tested a
series of potential predators against young and adult P. nigra, P. tenuis,
Dugesia lugubris and Dendrocoelum lacteum. The results (Table XIII)
show that of nineteen invertebrates selected over a range of phyla,
only adults of Dytiscus marginalis and the Odonata larvae took triclads
in any numbers, and in these cases young rather than adult triclads
were eaten. Fish, especially rudd (Scardinius erythrophthalmus) ate them
to some extent. All these predators took approximately equal numbers
of the four triclad species but the experimental regime did not offer a
choice. There are several reasons for suggesting that such restricted
predation is likely to be even less in the stony, littoral zone of lakes.
First, those invertebrates which attacked the triclads occur most commonly in heavily weeded habitats with a muddy substratum. Second, it
was evident that apart from rudd, only one or two individual fish fed
upon the triclads, despite their evident unpleasant taste. This would
account for Taylor’s ( 1 960) conclusion that minnows did not eat P. nigra.
In nature, with alternative food it is unlikely that many triclads are
eaten. Third, rudd are so sparsely distributed in Britain that they cannot
influence triclad distribution; furthermore, fish with similar feeding
habits are found precisely in those lakes where triclads are so numerous
and diversified. Finally, triclads are so slow-moving or immobile that
they will not attrart fish to the same degree as when both are confined
in a dish with limited cover. Indirect evidence from the study of the
population dynamics of the individual triclad species (p. 40) also suggested that predation was insignificant. However, before this factor can
be dismissed altogether it will be necessary to examine feeding on triclads
by some method applicable to field conditions such as the serological
technique used successfully by Dempster (1960).
Parasites of triolads can be dismissed quickly, since of the several
thousands exanlined by squash techniques (Reynoldson and Young,
twenty populations were infected by a species of Sporozoa. In the latter
case, such populations were usually found in polluted regions of lakes.
There is no evidence of the extent to which the triclads (all species) were
harmed, if at all, by the sporozoan but the populations were not obviously reduced.
2. Competitors
The two main resources for which inter-specific competition usually
occurs are adequate living space and food (Nicholson, 1954; Milne,
1963) jess than ten were found to harbour a trypanosome and less than