LAKE DWELLING TRICLADS
41
Polycelis tenuis of 8-9 mm length can live up to 8 months without food,
in the meantime shrinking to 0.5 mm length; similarly Dugesia lugubris
can shrink from 14 mm to 2 mm; the accompanying change in biomass
is of course much greater. Such tenacity has not been observed in the
field but shrinkage to 2-3 mm in both species has been noted. Furthermore, such field specimens are able to grow and breed again when fed,
showing high resistance to senescence. Thus, triclads react initially to
food shortage by a considerable change in biomass; they are more resistant to change in numbers. Secondly, these triclads appear to have no
important predators or parasites in the stony, littoral zone of lakes (p. 44)
and therefore intra-specific competition is able to exert a maximal effect.
This hypothesis has been tested by field experiments in which the
amount of food per individual has been increased artificially both by
adding food and by reducing the triclad population. Also, newly created
habitats have been studied in which the prey was initially numerous
relative to the triclads (Reynoldson, 1964). Such populations showed an
increased birth rate and recruitment, while shrinking was delayed and
its extent reduced (Table XII). It seems evident that natural populations are under some restraint both in growth of the individual and in
fecundity for a considerable period of each year, and that t,hey show
great numerical stability which provides optimal conditions for survival
(Martin, 1959) to which their longevity will also contribute (Hutchinson, 1957). They show to a high degree the population homeostasis which
forms a key part of Wynne-Edwards’ hypothesis (1962), even to the
extent that those individuals which fail to breed in a particular year
may form a strategic breeding reserve. But in these simple organisms
there is no evidence of social organization which could result in selfregulation, and in competing for food they show what Nicholson, (1054)
TABLE XI1
Effect of Increasing the Food Supply upon Total Recruitment and
Proportion of Adults during the Climax Month. (Reproduced with
permission from Reynoldson, 1964.)
Total recruitment,
Percentage adults
Triclad species
Normal Increased food
Normal Increased food
(%I
(%)
Polycelis tenuia
50
350
4-3
55.2
P . tenuis
50
175
4 -3
13.5
(College pond)
(New pond)
Dugesia lugubria
38
137
2.2
36.9
-
-
Polycelia nigra
90
500
41
Polycelis tenuis of 8-9 mm length can live up to 8 months without food,
in the meantime shrinking to 0.5 mm length; similarly Dugesia lugubris
can shrink from 14 mm to 2 mm; the accompanying change in biomass
is of course much greater. Such tenacity has not been observed in the
field but shrinkage to 2-3 mm in both species has been noted. Furthermore, such field specimens are able to grow and breed again when fed,
showing high resistance to senescence. Thus, triclads react initially to
food shortage by a considerable change in biomass; they are more resistant to change in numbers. Secondly, these triclads appear to have no
important predators or parasites in the stony, littoral zone of lakes (p. 44)
and therefore intra-specific competition is able to exert a maximal effect.
This hypothesis has been tested by field experiments in which the
amount of food per individual has been increased artificially both by
adding food and by reducing the triclad population. Also, newly created
habitats have been studied in which the prey was initially numerous
relative to the triclads (Reynoldson, 1964). Such populations showed an
increased birth rate and recruitment, while shrinking was delayed and
its extent reduced (Table XII). It seems evident that natural populations are under some restraint both in growth of the individual and in
fecundity for a considerable period of each year, and that t,hey show
great numerical stability which provides optimal conditions for survival
(Martin, 1959) to which their longevity will also contribute (Hutchinson, 1957). They show to a high degree the population homeostasis which
forms a key part of Wynne-Edwards’ hypothesis (1962), even to the
extent that those individuals which fail to breed in a particular year
may form a strategic breeding reserve. But in these simple organisms
there is no evidence of social organization which could result in selfregulation, and in competing for food they show what Nicholson, (1054)
TABLE XI1
Effect of Increasing the Food Supply upon Total Recruitment and
Proportion of Adults during the Climax Month. (Reproduced with
permission from Reynoldson, 1964.)
Total recruitment,
Percentage adults
Triclad species
Normal Increased food
Normal Increased food
(%I
(%)
Polycelis tenuia
50
350
4-3
55.2
P . tenuis
50
175
4 -3
13.5
(College pond)
(New pond)
Dugesia lugubria
38
137
2.2
36.9
-
-
Polycelia nigra
90
500