HABITAT SELEUTION BY AQUATIU INVERTEBRATES
347
present environment of an animal can often determine to a large
degree the behaviour of a species as it selects suitable habitats. In this
h a 1 section we will consider the marked variation in behaviour shown
by individuals of some species, and then go on to relate this and other
topics to the possible ways in which animals may colonize new habitats
and to the part habitat selection might play in the origin of new species.
Provided the surrounding environment is stable and the physiological state of the animals themselves is constant, then one generally
assumes that within fairly broad limits the behaviour of any individual
is much the same as any other individual of the same species.
However in some cases this is not so, and it will pay to examine these.
It is possible to classify the recorded instances of individual variation
in behaviour into three categories. In the h s t , individuals of a species
can show a range of behaviour rather analogous to a cline (Carter, 1951)
where each individual has a repeatable and characteristic response to a
particular stimulus but where different individuals show different
degrees of response. Records of this type of behaviour amongst
invertebrates living in aquatic environments are rare, although Walter
(1907) measured the rate of movement of 10 Planaria under different
light intensities and found their rates of movement different enough to
allow each one to be identified.
In the second category one or two individuals in a population show
responses which are atypical of the population as a whole, but which are
very characteristic of that particular animal. As an example, the
marine polynoid Acholoe astericola D. Ch. in general shows a strongly
positive response by becoming very active and attempting to attach
when presented with the tube feet of its host the starfish Astropecten
irregularie (Pennant) but only responds slightly when presented with
tube feet of other members of the order Phanerozonia (Davenport,
1953a). One animal however (animal no. 3 in Table la, loc. cit.) gave
positive responses to all the starfish species tested. Several papers refer
to occasional odd behaviour in populations of freshwater animals. One
is on Asellus communis (Allee, 1913a) in which one or two individuals
were as often rheonegative as rheopositive while most of the population
were consistently rheopositive, and Walter (1 906) (Lymnaeus) and
Kanda (1916b) (Physa) record other instances. Staropolska and
Dembowski (1950) attempted to analyse the variability of the case
building behaviour of the larvae of Molanna angustata Curtis by pushing larvae out of their cases and observing how they reconstructed
them. The successive cases of one particular larva were different from
those of the others, being always constructed of smaller than normal
sand grains. Furthermore, when the larvae were offered different sized
347
present environment of an animal can often determine to a large
degree the behaviour of a species as it selects suitable habitats. In this
h a 1 section we will consider the marked variation in behaviour shown
by individuals of some species, and then go on to relate this and other
topics to the possible ways in which animals may colonize new habitats
and to the part habitat selection might play in the origin of new species.
Provided the surrounding environment is stable and the physiological state of the animals themselves is constant, then one generally
assumes that within fairly broad limits the behaviour of any individual
is much the same as any other individual of the same species.
However in some cases this is not so, and it will pay to examine these.
It is possible to classify the recorded instances of individual variation
in behaviour into three categories. In the h s t , individuals of a species
can show a range of behaviour rather analogous to a cline (Carter, 1951)
where each individual has a repeatable and characteristic response to a
particular stimulus but where different individuals show different
degrees of response. Records of this type of behaviour amongst
invertebrates living in aquatic environments are rare, although Walter
(1907) measured the rate of movement of 10 Planaria under different
light intensities and found their rates of movement different enough to
allow each one to be identified.
In the second category one or two individuals in a population show
responses which are atypical of the population as a whole, but which are
very characteristic of that particular animal. As an example, the
marine polynoid Acholoe astericola D. Ch. in general shows a strongly
positive response by becoming very active and attempting to attach
when presented with the tube feet of its host the starfish Astropecten
irregularie (Pennant) but only responds slightly when presented with
tube feet of other members of the order Phanerozonia (Davenport,
1953a). One animal however (animal no. 3 in Table la, loc. cit.) gave
positive responses to all the starfish species tested. Several papers refer
to occasional odd behaviour in populations of freshwater animals. One
is on Asellus communis (Allee, 1913a) in which one or two individuals
were as often rheonegative as rheopositive while most of the population
were consistently rheopositive, and Walter (1 906) (Lymnaeus) and
Kanda (1916b) (Physa) record other instances. Staropolska and
Dembowski (1950) attempted to analyse the variability of the case
building behaviour of the larvae of Molanna angustata Curtis by pushing larvae out of their cases and observing how they reconstructed
them. The successive cases of one particular larva were different from
those of the others, being always constructed of smaller than normal
sand grains. Furthermore, when the larvae were offered different sized