HABITAT SELECTION BY AQUATIC INVERTEBRATES
325
somewhat smaller than the beads and its concentration in the animals’
guts was not measured, the ecological significance of Burns’ results is
not clear.
Seaweeds growing in the intertidal and immediately sublittoral
environment are clearly zoned. A number of herbivorous benthic
invertebrates feed on them and may show feeding preferences that
accord with their observed distribution in the field. Van Dongen (1956)
and Bakker (1959) both offered Littorina obtusata (L.) (= L. littoralis)
a range of common algae but have disagreed in the interpretation of
their results. Van Dongen felt that Littorina chose algae which did not
agree with its distribution on the shore, while Bakker concluded that
the vertical distribution of Littorina was mainly determined by its
algal preferences. Other papers of a similar nature are those by Den
Hartog (1959) whose results on nudibranchs are preliminary, by
Frings and Frings (1965) on the chemosensory basis of food-finding and
feeding by the nudibranch Aplysia juliana Pease, and by Sakai (1962)
and Carefoot (1967, 1970) to whose works we will refer in another
section (see section on Physiology and Viability). Leighton (1966) in a
comprehensive paper on the food preferences of eleven species of
herbivorous marine invertebrates, measured the weights of a number of
seaweed species consumed in 24 hours as an indication of preference ;
amongst other observations he noted that the deepest living herbivore
studied, Lytechinus anamesus H. L. Clark (Echinodermata), preferred
the red alga Gigartina armuta, and pointed out the agreement between
this and the supplantation of brown by red algae at greater depths.
There is very little evidence on the plant food preferences of freshwater benthic invertebrates although they no doubt could be demonstrated using suitable techniques. Bovbjerg (1965, 1968) observed four
species of lymnaeid snails feeding for most of the time on algae or
higher plants but occasionally on dead animals. In Y tube experiments,
the snails orientated strongly to chopped crayfish but not to chopped
pond weed, although they stopped moving on contact with the weed
and so in this way could aggregate on it. Bovbjerg (1968) suggested that
animal foods are detected at a distance and plant food after contact ;
but this is not general, for in the marine environment Littorina and
Aplysia can sense fucoids and Ulva respectively at some distance
(Van Dongen, 1956 ; Frings and Frings, 1965).
Carnivores, because they often hunt or forage, are likely to be well
equipped to detect their prey from a distance and to aggregate around
moving or stationary animals. A number of experiments with freshwater and marine invertebrates prove that this is so. The freshwater
leech Clossiphonia heteroclita (L.) eats Lymnaea stagnalis in preference
A.M.B.-lO
13
325
somewhat smaller than the beads and its concentration in the animals’
guts was not measured, the ecological significance of Burns’ results is
not clear.
Seaweeds growing in the intertidal and immediately sublittoral
environment are clearly zoned. A number of herbivorous benthic
invertebrates feed on them and may show feeding preferences that
accord with their observed distribution in the field. Van Dongen (1956)
and Bakker (1959) both offered Littorina obtusata (L.) (= L. littoralis)
a range of common algae but have disagreed in the interpretation of
their results. Van Dongen felt that Littorina chose algae which did not
agree with its distribution on the shore, while Bakker concluded that
the vertical distribution of Littorina was mainly determined by its
algal preferences. Other papers of a similar nature are those by Den
Hartog (1959) whose results on nudibranchs are preliminary, by
Frings and Frings (1965) on the chemosensory basis of food-finding and
feeding by the nudibranch Aplysia juliana Pease, and by Sakai (1962)
and Carefoot (1967, 1970) to whose works we will refer in another
section (see section on Physiology and Viability). Leighton (1966) in a
comprehensive paper on the food preferences of eleven species of
herbivorous marine invertebrates, measured the weights of a number of
seaweed species consumed in 24 hours as an indication of preference ;
amongst other observations he noted that the deepest living herbivore
studied, Lytechinus anamesus H. L. Clark (Echinodermata), preferred
the red alga Gigartina armuta, and pointed out the agreement between
this and the supplantation of brown by red algae at greater depths.
There is very little evidence on the plant food preferences of freshwater benthic invertebrates although they no doubt could be demonstrated using suitable techniques. Bovbjerg (1965, 1968) observed four
species of lymnaeid snails feeding for most of the time on algae or
higher plants but occasionally on dead animals. In Y tube experiments,
the snails orientated strongly to chopped crayfish but not to chopped
pond weed, although they stopped moving on contact with the weed
and so in this way could aggregate on it. Bovbjerg (1968) suggested that
animal foods are detected at a distance and plant food after contact ;
but this is not general, for in the marine environment Littorina and
Aplysia can sense fucoids and Ulva respectively at some distance
(Van Dongen, 1956 ; Frings and Frings, 1965).
Carnivores, because they often hunt or forage, are likely to be well
equipped to detect their prey from a distance and to aggregate around
moving or stationary animals. A number of experiments with freshwater and marine invertebrates prove that this is so. The freshwater
leech Clossiphonia heteroclita (L.) eats Lymnaea stagnalis in preference
A.M.B.-lO
13