HABITAT SELECTION BY AQUA!CIC INVERTEBRATES
336
water. No descriptive scheme has yet been evolved to fit this three
dimensional movement (which incidentally is also shown by flying
animals) although Douwes (1968) has attempted to do so after studying
the flight pattern of the geometrid moth Cidaria a Z ~ Z a ~ a
L., and Keegan
and Meadows (unpublished observations) have in a similar way
attempted to document the three dimensional behaviour of Corophium
as it alternately swims and alights on sediments.
Little attention has been paid to the way animals find their habitats.
They may, for example, be led to the right habitat by aseries of different
stimuli that bring them closer by stages. We might envisage smell,
sight and hearing as leading animals to the general area of a suitable
habitat (coarse selection), and then touch, taste and contact chemical
senses being used to define the exact habitat (fine selection). On the
other hand, stimuli from the physical and chemical environment (light,
heat, salinity) might act as coarse indicators, and biotic stimuli
(gregariousness, detection of plants as food) as fine indicators. Not a
great deal is known of these sequences although there is no doubt that
they take place. The larvae of benthic marine invertebrates approaching
metamorphosis, for example, change from being photopositive to photonegative and this will lead them to the general area of the bottom,
after which the more localized stimuli of gregariousness and algal
preferences can take over. Even after contact with the correct
environment, sequential stimuli may play a part, and Hansel1 (1968)
has demonstrated the way in which a series of ordered assessments of
particles are used by caddis fly larvae before they incorporate sand
grains into their cases.
It is possible that certain species may utilize indirect clues during
the selection of a preferred habitat (Lack, 1949, p. 299-300; Moynihan,
1968). Consider, for example, a benthic invertebrate living in and
preferring a sediment of a certain particle size; it might select the
sediment by particle size, but find it suitable to live in not because of
its particle size per se but because its food-a certain species of microorganism-is found only on sand grains of that size. This hypothetical
example may suggest how similar problems could be approached
experimentally, and might lead to a re-examination of already
published work where the results have presented some unexpected and
perhaps contradictory conclusions (cf. Meadows, 1964a, p. 508).
The appreciation of chemicals at a distance from their source
enables various aquatic invertebrates to locate their habitats, and the
distinction between this and contact chemical senses has already been
made when considering larval settlement. The contrast could also be
regarded as one between coarse and h e selection-contact chemical
336
water. No descriptive scheme has yet been evolved to fit this three
dimensional movement (which incidentally is also shown by flying
animals) although Douwes (1968) has attempted to do so after studying
the flight pattern of the geometrid moth Cidaria a Z ~ Z a ~ a
L., and Keegan
and Meadows (unpublished observations) have in a similar way
attempted to document the three dimensional behaviour of Corophium
as it alternately swims and alights on sediments.
Little attention has been paid to the way animals find their habitats.
They may, for example, be led to the right habitat by aseries of different
stimuli that bring them closer by stages. We might envisage smell,
sight and hearing as leading animals to the general area of a suitable
habitat (coarse selection), and then touch, taste and contact chemical
senses being used to define the exact habitat (fine selection). On the
other hand, stimuli from the physical and chemical environment (light,
heat, salinity) might act as coarse indicators, and biotic stimuli
(gregariousness, detection of plants as food) as fine indicators. Not a
great deal is known of these sequences although there is no doubt that
they take place. The larvae of benthic marine invertebrates approaching
metamorphosis, for example, change from being photopositive to photonegative and this will lead them to the general area of the bottom,
after which the more localized stimuli of gregariousness and algal
preferences can take over. Even after contact with the correct
environment, sequential stimuli may play a part, and Hansel1 (1968)
has demonstrated the way in which a series of ordered assessments of
particles are used by caddis fly larvae before they incorporate sand
grains into their cases.
It is possible that certain species may utilize indirect clues during
the selection of a preferred habitat (Lack, 1949, p. 299-300; Moynihan,
1968). Consider, for example, a benthic invertebrate living in and
preferring a sediment of a certain particle size; it might select the
sediment by particle size, but find it suitable to live in not because of
its particle size per se but because its food-a certain species of microorganism-is found only on sand grains of that size. This hypothetical
example may suggest how similar problems could be approached
experimentally, and might lead to a re-examination of already
published work where the results have presented some unexpected and
perhaps contradictory conclusions (cf. Meadows, 1964a, p. 508).
The appreciation of chemicals at a distance from their source
enables various aquatic invertebrates to locate their habitats, and the
distinction between this and contact chemical senses has already been
made when considering larval settlement. The contrast could also be
regarded as one between coarse and h e selection-contact chemical