40
T. B . R E Y N O L D S O N
on damaged individuals or those behaving abnormally, e.g. struggliug
in mucus, trapped in the water film, or perhaps damaged by other predators. Dendrocoelum is an active hunter of small organisms and uses ail
envelopment technique aided by its anterior sucker to attack prey.
While the role of mucus produced by triclads in ensnaring prey has been
described by Jennings (1 957) for P. felina, we have seen no evidence that
it is important in the lake species of Planariidae; but mucus probably
helps Dendrocoelum to catch prey, a t least under laboratory conditions.
D. ORGANISMS O F THE SAME S P E C I E S
The isolation of the intra-specific events which are considered here is
entirely artificial and their influence upon the distribution and abundance of the several triclad species is indirect. However, this approach
affords background information on the conditions under which the triclads are living and may indicate the important types of interaction
occurring among the species in nature. During the ten years of observation there has been no hint in the field or in the various experiments
that co-operation between individuals, of the type described by Allee
(1938), determines the pattern of distribution of these triclads. It has
already been shown that cannibalism is a rare phenomenon (p. 34). This
leaves intraspecific competition as the main focus (sensu Milne, 1961).
The population dynamics of the four species have been studied in
some detail (Reynoldson, 1960; 1961a; Taylor and Reynoldson, 1962;
Young and Reynoldson, in press), and while they all show basic similarities, there are interesting differences (for example in life-span) between the Planariidae and the Dendrocoelidae which influence events.
Apart from differences in the time of year a t which breeding commences,
which although slight probably influence distribution, the population
biology of the three planariid species is similar. They all have a low
birth rate and recruitment to the population is only about 50% of its
pre-breeding size under natural conditions; in the laboratory with ample
food the birth rates are several times greater. The low recruitment is due
to the gradual development of a condition of food shortage after the
initial hatching of young; this puts a brake upon and finally stops
breeding. A t the same time the individuals decline in size because of food
scarcity, and this is accompanied by some mortality which readjusts
the population to its food supply. This cycle is repeated each year and
represents a self-regulatory system operated by intra-specific competition for food. The more obvious operation of this homeostatic system
compared with most natural populations arises from two properties of
these triclad species. First, their capacity to remain alive under conditions of food shortage for long periods by resorbing their tissues and
shrinking in size. Laboratory observations have shown that an adult
T. B . R E Y N O L D S O N
on damaged individuals or those behaving abnormally, e.g. struggliug
in mucus, trapped in the water film, or perhaps damaged by other predators. Dendrocoelum is an active hunter of small organisms and uses ail
envelopment technique aided by its anterior sucker to attack prey.
While the role of mucus produced by triclads in ensnaring prey has been
described by Jennings (1 957) for P. felina, we have seen no evidence that
it is important in the lake species of Planariidae; but mucus probably
helps Dendrocoelum to catch prey, a t least under laboratory conditions.
D. ORGANISMS O F THE SAME S P E C I E S
The isolation of the intra-specific events which are considered here is
entirely artificial and their influence upon the distribution and abundance of the several triclad species is indirect. However, this approach
affords background information on the conditions under which the triclads are living and may indicate the important types of interaction
occurring among the species in nature. During the ten years of observation there has been no hint in the field or in the various experiments
that co-operation between individuals, of the type described by Allee
(1938), determines the pattern of distribution of these triclads. It has
already been shown that cannibalism is a rare phenomenon (p. 34). This
leaves intraspecific competition as the main focus (sensu Milne, 1961).
The population dynamics of the four species have been studied in
some detail (Reynoldson, 1960; 1961a; Taylor and Reynoldson, 1962;
Young and Reynoldson, in press), and while they all show basic similarities, there are interesting differences (for example in life-span) between the Planariidae and the Dendrocoelidae which influence events.
Apart from differences in the time of year a t which breeding commences,
which although slight probably influence distribution, the population
biology of the three planariid species is similar. They all have a low
birth rate and recruitment to the population is only about 50% of its
pre-breeding size under natural conditions; in the laboratory with ample
food the birth rates are several times greater. The low recruitment is due
to the gradual development of a condition of food shortage after the
initial hatching of young; this puts a brake upon and finally stops
breeding. A t the same time the individuals decline in size because of food
scarcity, and this is accompanied by some mortality which readjusts
the population to its food supply. This cycle is repeated each year and
represents a self-regulatory system operated by intra-specific competition for food. The more obvious operation of this homeostatic system
compared with most natural populations arises from two properties of
these triclad species. First, their capacity to remain alive under conditions of food shortage for long periods by resorbing their tissues and
shrinking in size. Laboratory observations have shown that an adult