HABITAT SELECTION BY AQUATIC INVERTEBRATES
293
at sunset and assess the light intensity before swimming upwards or
burrowing deeper into the sediment again.
Freshwater animals are likely to meet more saline water if they
move into a river’s tidal reaches, and as well as their positive response
to water currents, an avoidance of salty water would be advantageous
(Costa, 1966a). [In Finland, Lagerspetz and Lehtonen (1961) and
Lagerspetz and Mattila (1961) record Asellus aquaticus from fresh waters
and from the Pucus vesiculosus L. zone on the shores of the Baltic, where
salinity can rise to 8%,. Individuals collected from fresh or brackish
waters could not distinguish between tap water and brackish water from
the Baltic (54-6*0%,), and Lagerspetz and Mattila (1961) conclude from
this that localized variations in salinity caused by, say, melting ice are
not likely to influence the species distribution. However, they obtained
rather different results using sodium chloride. Individuals from fresh
water distinguished between tap water and lo/,,,, NaC1, whereas, although
those from brackish water could not make this distinction, they did
prefer S%, NaCl to tap water. The problem needs further investigation
on this and other species that are likely to encounter brackish waters.
Lagerspetz and Lehtonen (1961) also offered Asellus from fresh water
and brackish water different humidities, and individuals from both
environments preferred the wetter of the two sides of a choice dish.
Other freshwater animals living at the edges of rivers and lakes would
be expected to avoid low humidities but there are no studies.
Occasionally authors have attempted to assess the relative
importance of a range of environmental variables and their results
illustrate how successful the approach can be (Bovbjerg, 1952b; Hughes,
1966; McLachlan, 1969; LaRow, 1970). Bovbjerg’s work is the most
elegant of these studies He firstly considered the ecology of two crayfish species and then showed how much of their local distribution
could be accounted for by their behaviour and by viability limits.
Cambarus fodiens lives in small muddy ponds that are often temporary.
In summer the ponds stagnate, are hot, and often dry out ; they have a
low oxygen content in both summer and winter, and a low CaCO,
content. Orconectes propinquw lives in clear, rock bottomed streams
and in lakes. Its environment is much more stable : temperature, depth
and oxygen content remain fairly constant over the year, there is no
summer stagnation, and there is a high CaCO, content. I n reciprocal
field transplant experiments, both species survived in the other’s
environment. Bovbjerg also studied their burrowing abilities. Both
species were placed on mud in the field at the time of the summer
drying and in the laboratory on mud without a covering of water ; in
both types of experiments Cambarw fodiens burrowed immediately and
A.Y.B.--IO
11
293
at sunset and assess the light intensity before swimming upwards or
burrowing deeper into the sediment again.
Freshwater animals are likely to meet more saline water if they
move into a river’s tidal reaches, and as well as their positive response
to water currents, an avoidance of salty water would be advantageous
(Costa, 1966a). [In Finland, Lagerspetz and Lehtonen (1961) and
Lagerspetz and Mattila (1961) record Asellus aquaticus from fresh waters
and from the Pucus vesiculosus L. zone on the shores of the Baltic, where
salinity can rise to 8%,. Individuals collected from fresh or brackish
waters could not distinguish between tap water and brackish water from
the Baltic (54-6*0%,), and Lagerspetz and Mattila (1961) conclude from
this that localized variations in salinity caused by, say, melting ice are
not likely to influence the species distribution. However, they obtained
rather different results using sodium chloride. Individuals from fresh
water distinguished between tap water and lo/,,,, NaC1, whereas, although
those from brackish water could not make this distinction, they did
prefer S%, NaCl to tap water. The problem needs further investigation
on this and other species that are likely to encounter brackish waters.
Lagerspetz and Lehtonen (1961) also offered Asellus from fresh water
and brackish water different humidities, and individuals from both
environments preferred the wetter of the two sides of a choice dish.
Other freshwater animals living at the edges of rivers and lakes would
be expected to avoid low humidities but there are no studies.
Occasionally authors have attempted to assess the relative
importance of a range of environmental variables and their results
illustrate how successful the approach can be (Bovbjerg, 1952b; Hughes,
1966; McLachlan, 1969; LaRow, 1970). Bovbjerg’s work is the most
elegant of these studies He firstly considered the ecology of two crayfish species and then showed how much of their local distribution
could be accounted for by their behaviour and by viability limits.
Cambarus fodiens lives in small muddy ponds that are often temporary.
In summer the ponds stagnate, are hot, and often dry out ; they have a
low oxygen content in both summer and winter, and a low CaCO,
content. Orconectes propinquw lives in clear, rock bottomed streams
and in lakes. Its environment is much more stable : temperature, depth
and oxygen content remain fairly constant over the year, there is no
summer stagnation, and there is a high CaCO, content. I n reciprocal
field transplant experiments, both species survived in the other’s
environment. Bovbjerg also studied their burrowing abilities. Both
species were placed on mud in the field at the time of the summer
drying and in the laboratory on mud without a covering of water ; in
both types of experiments Cambarw fodiens burrowed immediately and
A.Y.B.--IO
11