HABITAT SELEUTION BY AQUATIC INVERTEBRATES
361
on fucoids on rocky shores, olivacea in general living in the upper half
and citrina in the lower half of the tidal zone, and both subspecies
feed on the fucoids on which they are found. Van Dongen (1956)
offered the two subspecies four species of Fucus that were zoned down
the shore in the hope that the snails would choose species of Fwcus in
keeping with their observed distribution. Unfortunately this was not
the case, but in the present context the results are interesting for the
two subspecies did show slightly Merent preferences. Studies on other
subspecies such as those in the I o t e a group (Isopoda) and those
in the cfarnmarzcs group (Amphipoda) would be worth while.
Differences in habitat preferences, then, can occur between closely
related species living in the same environment, can extend to a
subspecific level, and can sometimes be demonstrated between individuals within one population of a species. Two related topics now
merit attention before we consider how habitat selection might
be involved in speciation. These are the mechanisms by which
populations of a species living in contrasting habitats select their
appropriate habitat, and the mechanisms by which new environments
are colonized.
Most species live in one easily recognizable habitat but on
occasion a species is encountered living in two habitats that contrast
strongly. Of course one explanation might be our inability to appreciate
h e distinctions between two species (cf. Simon, 1968b), but putting
this aside the problem is an interesting one. Perttunen (1961) discovered an isolated population of the intertidal isopod Ligia italica
living in brackish water. Individuals from the brackish water population died when immersed in sea water while animals collected from the
shore survived for many hours. Although the difference is physiological
rather than behavioural, it does illustrate a contrast between animals
from the two habitats. Corophium volutator commonly occurs on
muddy intertidal shores, and in the laboratory has a rhythm of
swimming activity with maxima during the early ebb tide. Morgan
(1966) was able to coIIect a population from a non-tidal brackish pool
environment which showed no such rhythm. The remaining examples
are of larval behaviour, of a commensal relationship, and one of the
behaviour of the freshwater isopod Asellus. The larvae of the marine
polychaete Cirriformia tentaculata Montagu are usually accepted as
being planktonic, but George (1 963) discovered a localized population
in Southampton Water, England, whose larvae were all demersal,
while larvae from a nearby population at Drakes Island were pelagic.
These interesting observations prompted him to survey other areas in
the neighbourhood to disoover how general the phenomenon was
361
on fucoids on rocky shores, olivacea in general living in the upper half
and citrina in the lower half of the tidal zone, and both subspecies
feed on the fucoids on which they are found. Van Dongen (1956)
offered the two subspecies four species of Fucus that were zoned down
the shore in the hope that the snails would choose species of Fwcus in
keeping with their observed distribution. Unfortunately this was not
the case, but in the present context the results are interesting for the
two subspecies did show slightly Merent preferences. Studies on other
subspecies such as those in the I o t e a group (Isopoda) and those
in the cfarnmarzcs group (Amphipoda) would be worth while.
Differences in habitat preferences, then, can occur between closely
related species living in the same environment, can extend to a
subspecific level, and can sometimes be demonstrated between individuals within one population of a species. Two related topics now
merit attention before we consider how habitat selection might
be involved in speciation. These are the mechanisms by which
populations of a species living in contrasting habitats select their
appropriate habitat, and the mechanisms by which new environments
are colonized.
Most species live in one easily recognizable habitat but on
occasion a species is encountered living in two habitats that contrast
strongly. Of course one explanation might be our inability to appreciate
h e distinctions between two species (cf. Simon, 1968b), but putting
this aside the problem is an interesting one. Perttunen (1961) discovered an isolated population of the intertidal isopod Ligia italica
living in brackish water. Individuals from the brackish water population died when immersed in sea water while animals collected from the
shore survived for many hours. Although the difference is physiological
rather than behavioural, it does illustrate a contrast between animals
from the two habitats. Corophium volutator commonly occurs on
muddy intertidal shores, and in the laboratory has a rhythm of
swimming activity with maxima during the early ebb tide. Morgan
(1966) was able to coIIect a population from a non-tidal brackish pool
environment which showed no such rhythm. The remaining examples
are of larval behaviour, of a commensal relationship, and one of the
behaviour of the freshwater isopod Asellus. The larvae of the marine
polychaete Cirriformia tentaculata Montagu are usually accepted as
being planktonic, but George (1 963) discovered a localized population
in Southampton Water, England, whose larvae were all demersal,
while larvae from a nearby population at Drakes Island were pelagic.
These interesting observations prompted him to survey other areas in
the neighbourhood to disoover how general the phenomenon was