34
T. B. REYNOLDSON
chemical properties of water are gradual. Support for this interpretation
is given by the wide distribution of Dendrocoelum lacteum in Feniioscania
(Reynoldson, 1958c) where it had a pattern corresponding to that of
P. nigra in Britain. It seems reasonable to conclude that a biological
property of water per se does not determine the distribution of these
triclads.
c. FOOD
In the present context we are not concerned so much with scarcity
and abundance of food but rather with the kinds taken by the four triclad species as indicating the degree of overlap in their requirements.
Food has been found to be a key resource in deciding the ability of
ecologically similar species to co-exist, and has often been implicated in
determining pattern. This aspect has been studied most thoroughly in
birds (Lack, 1954; MacArthur, 1958); it has been less well studied for
invertebrates, but work on gastropods (Kohn, 1959), and on tsetse flies
(Weitz, 1964) using a serological technique, is an example of what can
be achieved.
No detailed study of the food of triclads occurs in the literature, and
certainly none that would provide the kind of information which is required to determine the extent of overlap among the four species in
nature. Hyman (1951) has stated that triclads do not feed on plants or
decaying organisms (cp. Popham, 1955) and this has been confirmed by
our studies. She also expressed the view that one species may feed upon
another and that cannibalism may be extensive, as it seems to be for
some strains of Dugesia tigrina (Root, 1960; Armstrong, 1964). Both
aspects have been tested in our laboratory in various ways and the
results showed (Reynoldson and Young, 1963) that neither phenomenon
is common in these British species. Only two defbitive observations of
predation have been noted, both by Dugesin lugubris, one upon Polycelis nigra and the other upon P. tenuis; in the latter instance the pharynx
of the P. tenuis “ate” its way through the body wall of Dugesia within
a few minutes of being ingested. No successful attack of one specimen
upon another of the same species has been observed, but a few isolated
examples of this have been reported (e.g. D. Clegg-private communication) again for Dugesia lugubris. It can be concluded that interactions of this sort are not important in nature and play no part in
determining distribution and abundance. The behaviour-patterns which
cause predation and cannibalism among triclads would appear to be
subtle and have not been studied yet.
The literature suggests that triclads as a group feed on small aquatic
annelids, crustaceans and insect larvae, and they also take terrestrial
organisms trapped in water, but the observations are too incidental to
T. B. REYNOLDSON
chemical properties of water are gradual. Support for this interpretation
is given by the wide distribution of Dendrocoelum lacteum in Feniioscania
(Reynoldson, 1958c) where it had a pattern corresponding to that of
P. nigra in Britain. It seems reasonable to conclude that a biological
property of water per se does not determine the distribution of these
triclads.
c. FOOD
In the present context we are not concerned so much with scarcity
and abundance of food but rather with the kinds taken by the four triclad species as indicating the degree of overlap in their requirements.
Food has been found to be a key resource in deciding the ability of
ecologically similar species to co-exist, and has often been implicated in
determining pattern. This aspect has been studied most thoroughly in
birds (Lack, 1954; MacArthur, 1958); it has been less well studied for
invertebrates, but work on gastropods (Kohn, 1959), and on tsetse flies
(Weitz, 1964) using a serological technique, is an example of what can
be achieved.
No detailed study of the food of triclads occurs in the literature, and
certainly none that would provide the kind of information which is required to determine the extent of overlap among the four species in
nature. Hyman (1951) has stated that triclads do not feed on plants or
decaying organisms (cp. Popham, 1955) and this has been confirmed by
our studies. She also expressed the view that one species may feed upon
another and that cannibalism may be extensive, as it seems to be for
some strains of Dugesia tigrina (Root, 1960; Armstrong, 1964). Both
aspects have been tested in our laboratory in various ways and the
results showed (Reynoldson and Young, 1963) that neither phenomenon
is common in these British species. Only two defbitive observations of
predation have been noted, both by Dugesin lugubris, one upon Polycelis nigra and the other upon P. tenuis; in the latter instance the pharynx
of the P. tenuis “ate” its way through the body wall of Dugesia within
a few minutes of being ingested. No successful attack of one specimen
upon another of the same species has been observed, but a few isolated
examples of this have been reported (e.g. D. Clegg-private communication) again for Dugesia lugubris. It can be concluded that interactions of this sort are not important in nature and play no part in
determining distribution and abundance. The behaviour-patterns which
cause predation and cannibalism among triclads would appear to be
subtle and have not been studied yet.
The literature suggests that triclads as a group feed on small aquatic
annelids, crustaceans and insect larvae, and they also take terrestrial
organisms trapped in water, but the observations are too incidental to