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P. S, MEADOWS AND J. I. OAMPBELL
two ways. Animals may react to them indirectly, by a change in a
pattern of behaviour, or may respond more directly by an avoidance
reaction. As an example of the former, consider the behaviour of a
photonegative planktonic animal moving down during the day from
the surface waters of a rich eutrophic lake. If it continues to swim
downwards, in the summer at least, it will eventually encounter unfavourable anaerobic conditions. It can avoid these however by
becoming photopositive as soon as it meets them; the animal will
then immediately move upwards again. There is evidence from laboratory investigations that switches in behaviour of this sort can occur.
Carbon dioxide induces normally photonegative Gammurus, Daphnia
and Cyclops, and mayfly nymphs to move towards a light source
(Loeb, 1904, 1906a, b ; Wodsedalek, 191 l), although these animals
may be responding to lowered pH since acids produced similar effects.
Allee (1912) noted a change in the behaviour of Asellus in lowered
oxygen or increased carbon dioxide concentrations ; stream forms
became less rheopositive, and also preferred water of high oxygen
content if they had previously lived in this. Other freshwater crustaceans react directly to anaerobic conditions, showing marked
preferences for water containing low carbon dioxide concentrations :
the sensitivity of four Cambarus species studied by Powers (1914)
mirrored their ecological distribution, the stream species being more
sensitive than the pond or mud dwelling ones. However these
preferences might well be for pH rather than anaerobic conditions as
the crayfish showed similar behaviour in response to acetic and
hydrochloric acids. Analogous behaviour has also been noted by
Allee and Stein (1918) for mayfly nymphs. These problems clearly
need reinvestigation in which the effects of anaerobiosis, pH, and
inorganic ions are distinguished (Costa, 1967a, b). In a well designed
factorial experiment, LaRow (1970) has studied the influence of reduced
oxygen tension, of temperature changes and also of the presence or absence of food on the vertical migration of Chaoborus punctipennis Say
larvae (Diptera : Culicidae). These larvae emerge from sediments shortly
after sunset, migrate into the upper water strata, then descend and
burrow again before sunrise. Under experimental conditions the larvae
only migrate upwards in low oxygen concentrations, although the degree
of migration is influenced quantitatively by temperature and food.
Temperature, light and pressure are environmental variables that
fresh waters and the sea share in common, and many animals in fresh
water are known to respond to them.
Temperature, like anaerobic conditions, might affect habitat
selection indirectly as well as directly through temperature preferences,
P. S, MEADOWS AND J. I. OAMPBELL
two ways. Animals may react to them indirectly, by a change in a
pattern of behaviour, or may respond more directly by an avoidance
reaction. As an example of the former, consider the behaviour of a
photonegative planktonic animal moving down during the day from
the surface waters of a rich eutrophic lake. If it continues to swim
downwards, in the summer at least, it will eventually encounter unfavourable anaerobic conditions. It can avoid these however by
becoming photopositive as soon as it meets them; the animal will
then immediately move upwards again. There is evidence from laboratory investigations that switches in behaviour of this sort can occur.
Carbon dioxide induces normally photonegative Gammurus, Daphnia
and Cyclops, and mayfly nymphs to move towards a light source
(Loeb, 1904, 1906a, b ; Wodsedalek, 191 l), although these animals
may be responding to lowered pH since acids produced similar effects.
Allee (1912) noted a change in the behaviour of Asellus in lowered
oxygen or increased carbon dioxide concentrations ; stream forms
became less rheopositive, and also preferred water of high oxygen
content if they had previously lived in this. Other freshwater crustaceans react directly to anaerobic conditions, showing marked
preferences for water containing low carbon dioxide concentrations :
the sensitivity of four Cambarus species studied by Powers (1914)
mirrored their ecological distribution, the stream species being more
sensitive than the pond or mud dwelling ones. However these
preferences might well be for pH rather than anaerobic conditions as
the crayfish showed similar behaviour in response to acetic and
hydrochloric acids. Analogous behaviour has also been noted by
Allee and Stein (1918) for mayfly nymphs. These problems clearly
need reinvestigation in which the effects of anaerobiosis, pH, and
inorganic ions are distinguished (Costa, 1967a, b). In a well designed
factorial experiment, LaRow (1970) has studied the influence of reduced
oxygen tension, of temperature changes and also of the presence or absence of food on the vertical migration of Chaoborus punctipennis Say
larvae (Diptera : Culicidae). These larvae emerge from sediments shortly
after sunset, migrate into the upper water strata, then descend and
burrow again before sunrise. Under experimental conditions the larvae
only migrate upwards in low oxygen concentrations, although the degree
of migration is influenced quantitatively by temperature and food.
Temperature, light and pressure are environmental variables that
fresh waters and the sea share in common, and many animals in fresh
water are known to respond to them.
Temperature, like anaerobic conditions, might affect habitat
selection indirectly as well as directly through temperature preferences,