HABITAT SELECTION B Y AQUATIC INVERTEBRATES
281
settlement they become photonegative, except for the larvae of intertidal species which remain photopositive until they stop swimming.
This latter observation would in itself account for the intertidal
settlement of many species. Strong light intensities, increased
temperature and reduced salinity induce some photopositive larvae
to change their response to a photonegative one, which would explain
why few pelagic larvae are found in brackish waters and why larvae
are often not quite a t the water surface (Thorson, 1964). The literature
on invertebrates that are planktonic during the whole of their life is
less massive, presumably because they are more difficult to catch and
keep (Lewis, 1959). Russell (1927, 1936), Spooner (1933) and Cushing
(1951) have reviewed the light responses of planktonic organisms and
it seems that light is a major determinant of vertical migration. In
comparison, not a great deal is known of the light responses of benthic
marine invertebrates (Jennings, 1907; Bauer, 1913; Allee, 1927; Oviatt,
1969; Salazar, 1970).
Benthic and, to a lesser extent, intertidal invertebrates are now
known to undertake horizontal migrations of various magnitudes
(Allen, 1966). The behavioural mechanisms governing these migrations
are not understood, although the persistent rhythmic activity of the
shore crab Carcinus maenas (L.) may well be related to its tidal migrations (Naylor, 1958, 1962). Experimental analysis of these horizontal
migrations is likely to show that they are largely controlled by light,
pressure and temperature, in the same way as light and pressure govern
the vertical migrations of planktonic invertebrates.
Evidence shows that planktonic invertebrates are likely to migrate
vertically and maintain their position in the water column using
pressure as well as light as an environmental clue (Russell, 1927, 1936;
Knight-Jones and Morgan, 1966). Forty-three out of 53 species of a
wide range of adult and larval planktonic invertebrates investigated by
Rice (1964) responded to pressure changes of 1000 millibars or less.
Increased pressure stimulated them to increase their activity and to
move upwards, while decreased pressure had converse effects; the
responses will obviously limit those species that show them to well
defined depths. It is probable that the larvae of many bottom-dwelling
invertebrates, on the approach of settlement may alter their behaviour
to pressure in the same way as they do to light; older larvae of Mytilus
edulis L., for example, are less likely to swim upwards on increased
pressure until at the pediveliger stage (settling stage) they are unaffected
(Bayne, 1963); there appear to be no comparative studies on other
larvae. Little attention has been paid to the way in which pressure
might interact with other environmental variables to influence habitat
281
settlement they become photonegative, except for the larvae of intertidal species which remain photopositive until they stop swimming.
This latter observation would in itself account for the intertidal
settlement of many species. Strong light intensities, increased
temperature and reduced salinity induce some photopositive larvae
to change their response to a photonegative one, which would explain
why few pelagic larvae are found in brackish waters and why larvae
are often not quite a t the water surface (Thorson, 1964). The literature
on invertebrates that are planktonic during the whole of their life is
less massive, presumably because they are more difficult to catch and
keep (Lewis, 1959). Russell (1927, 1936), Spooner (1933) and Cushing
(1951) have reviewed the light responses of planktonic organisms and
it seems that light is a major determinant of vertical migration. In
comparison, not a great deal is known of the light responses of benthic
marine invertebrates (Jennings, 1907; Bauer, 1913; Allee, 1927; Oviatt,
1969; Salazar, 1970).
Benthic and, to a lesser extent, intertidal invertebrates are now
known to undertake horizontal migrations of various magnitudes
(Allen, 1966). The behavioural mechanisms governing these migrations
are not understood, although the persistent rhythmic activity of the
shore crab Carcinus maenas (L.) may well be related to its tidal migrations (Naylor, 1958, 1962). Experimental analysis of these horizontal
migrations is likely to show that they are largely controlled by light,
pressure and temperature, in the same way as light and pressure govern
the vertical migrations of planktonic invertebrates.
Evidence shows that planktonic invertebrates are likely to migrate
vertically and maintain their position in the water column using
pressure as well as light as an environmental clue (Russell, 1927, 1936;
Knight-Jones and Morgan, 1966). Forty-three out of 53 species of a
wide range of adult and larval planktonic invertebrates investigated by
Rice (1964) responded to pressure changes of 1000 millibars or less.
Increased pressure stimulated them to increase their activity and to
move upwards, while decreased pressure had converse effects; the
responses will obviously limit those species that show them to well
defined depths. It is probable that the larvae of many bottom-dwelling
invertebrates, on the approach of settlement may alter their behaviour
to pressure in the same way as they do to light; older larvae of Mytilus
edulis L., for example, are less likely to swim upwards on increased
pressure until at the pediveliger stage (settling stage) they are unaffected
(Bayne, 1963); there appear to be no comparative studies on other
larvae. Little attention has been paid to the way in which pressure
might interact with other environmental variables to influence habitat