HABITAT SELECTION BY AQUATIC INVERTEBRATES
345
Temora longicornis (0. F. Muller) and also of the larvae of the archiannelid Polygordius (Loeb, 1893). The latter two animals, and the larvae
of Palaernonetes (Loeb, 1906b), become photonegative if salinity falls,
and will therefore be led away from low salinity river water that flows
out over the more dense sea water in estuaries. Various species living
in fresh water become photopositive as the concentration of CO,
increases (Loeb, 1904; 1906a, b; Wodsedalek, 1911), the larvae of
Palaemonetes behave in the same way to increases in pressure (Bohn,
1912). The responses of stream dwelling animals are also influenced by
their immediate environment. Under experimental conditions Cammarus pulex (L.) are less likely to move upstream at high current speeds
or when food is available but not when more shelter is provided (Hughes,
1970), and Asellus species become less rheopositive as the oxygen
concentration decreases and the carbon dioxide concentration increases
(Allee, 1912) (c.f. Walker, 1906, pp. 26-7). It is interesting to note in
passing that Allee describes a marked decrease on the percentage of
positive responses t o current by Asellus communis during the breeding
season (c.f. Crozier and h e y , 1919, pp. 277-8), which is during the
spring when currents are at their fastest; by this seasonal change of
behaviour, therefore, Asellus cornmunis may be barred from fast flowing streams that it would otherwise have colonized. Finally, three
species of intertidal gastropod of the genus Littorinu investigated by
Gowanloch and Hayes (1 927) and Hayes (1 927) become less geonegative
when desiccated which would lead them away from the upper and
drier regions of the shore. The ecological significance of these switches in
behaviour are usually obvious, but perhaps the clearest example of
behaviour changing with environmental conditions is that of the
burrowing activities of the marine shrimps Penaew axtecus and Penueus
setiferus (Aldrich et al., 1968). I n the laboratory, the postlarvae of
P. aztecus burrow into substrates when the temperature is experimentally lowered from 25°C to 12-17°C and emerge again when the
temperature is raised to 18-21-5"C, whereas, under the same conditions,
the postlarvae of P. setiferw do not. The difference in their
behaviour agrees closely with their distribution in the sea. Both
species spawn at sea and their larvae develop there. In the Gulf of
Mexico the postlarvae then move inshore to estuarine areas such as
Galveston Bay, Texas, where they grow rapidly to subadult size before
migrating offshore again to attain maturity and spawn. There is a
marked difference, however, in the season at which postlarvae of the
two species arrive in Galveston Bay. The postlarvae of P. setiferus
arrive during summer when water temperatures are consistently
warm (25-32OC) whereas most P. aztecus arrive during March and
April when the bay is not only cool (15-25°C) but subject to drastic
345
Temora longicornis (0. F. Muller) and also of the larvae of the archiannelid Polygordius (Loeb, 1893). The latter two animals, and the larvae
of Palaernonetes (Loeb, 1906b), become photonegative if salinity falls,
and will therefore be led away from low salinity river water that flows
out over the more dense sea water in estuaries. Various species living
in fresh water become photopositive as the concentration of CO,
increases (Loeb, 1904; 1906a, b; Wodsedalek, 1911), the larvae of
Palaemonetes behave in the same way to increases in pressure (Bohn,
1912). The responses of stream dwelling animals are also influenced by
their immediate environment. Under experimental conditions Cammarus pulex (L.) are less likely to move upstream at high current speeds
or when food is available but not when more shelter is provided (Hughes,
1970), and Asellus species become less rheopositive as the oxygen
concentration decreases and the carbon dioxide concentration increases
(Allee, 1912) (c.f. Walker, 1906, pp. 26-7). It is interesting to note in
passing that Allee describes a marked decrease on the percentage of
positive responses t o current by Asellus communis during the breeding
season (c.f. Crozier and h e y , 1919, pp. 277-8), which is during the
spring when currents are at their fastest; by this seasonal change of
behaviour, therefore, Asellus cornmunis may be barred from fast flowing streams that it would otherwise have colonized. Finally, three
species of intertidal gastropod of the genus Littorinu investigated by
Gowanloch and Hayes (1 927) and Hayes (1 927) become less geonegative
when desiccated which would lead them away from the upper and
drier regions of the shore. The ecological significance of these switches in
behaviour are usually obvious, but perhaps the clearest example of
behaviour changing with environmental conditions is that of the
burrowing activities of the marine shrimps Penaew axtecus and Penueus
setiferus (Aldrich et al., 1968). I n the laboratory, the postlarvae of
P. aztecus burrow into substrates when the temperature is experimentally lowered from 25°C to 12-17°C and emerge again when the
temperature is raised to 18-21-5"C, whereas, under the same conditions,
the postlarvae of P. setiferw do not. The difference in their
behaviour agrees closely with their distribution in the sea. Both
species spawn at sea and their larvae develop there. In the Gulf of
Mexico the postlarvae then move inshore to estuarine areas such as
Galveston Bay, Texas, where they grow rapidly to subadult size before
migrating offshore again to attain maturity and spawn. There is a
marked difference, however, in the season at which postlarvae of the
two species arrive in Galveston Bay. The postlarvae of P. setiferus
arrive during summer when water temperatures are consistently
warm (25-32OC) whereas most P. aztecus arrive during March and
April when the bay is not only cool (15-25°C) but subject to drastic