HABITAT SELECTION BY AQUATIC INVERTEBRATES
283
has been adapted by Harder (1968) to study the behaviour of marine
plankton towards density discontinuities. After testing a range of
species he concluded that almost all of them aggregated at the interface
between waters of Werent densities, and cites instances of this
occurring in the sea. More recently, localized discontinuities in the
microstructure of temperature, salinity, and velocity profiles in the sea
and of temperature profiles in fresh waters have been described (Simpson and Woods (1970), Woods (1971), for references) and may influence
planktonic animals in the same way.
As far as we are aware little is known of the temperature preferences
or current responses of marine planktonic or benthic invertebrates,
apart from studies on the relation of temperature to light preferences
in three species of planktonic copepod (Lewis, 1959), on the relation of
temperature to the burrowing activity of two Penaeus species (Aldrich
et al., 1968; p. 345 below), and on the rheotropic responses of a shallow
water nudibranch (Chromodoris zebra Heilprin) (Crozier and Arey, 19 19).
Except under unusual conditions such as the aftermath of a plankton
bloom, anaerobic conditions are rare in the sea, and there is no record
of planktonic animals responding to them. On the other hand they
must be common in sublittoral sediments although there are no
studies of their possible significance to benthic invertebrates. Many
benthic invertebrates must also respond positively to touching or being
surrounded by solid objects, but again little appears to be known of
this behaviour or of any responses to the microtopography or
roughness of surfaces (Diebschlag, 1938).
Many mobile bottom-dwelling invertebrates, such as brittle stars,
octopods, crabs, and lobsters, seek shelter on the approach of predators,
or live semi-permanently in crevices or dens. This behaviour, which is
well known to divers, has only been experimentally analysed in two
species of crayfish. Nonaka (1966) and Cobb (1971) have shown that
the number and size of shelters, and their relative dimensions, are likely
to influence the local distribution of Panulirus jarponicus von Siebold
and Homarus arnericunus Milne Edwards respectively.
It is well known that benthic marine invertebrates in mud and
sand burrow to different depths-the various burrowing bivalves are
good examples-but there are no experimental studies that attempt to
link possible depth preferences with distribution.
Pravdi6 (1970) has recently described a new parameter by which
sediments may be classified. He has designed an apparatus to measure
the electrical charge at the surface of sediments, and has investigated
the changes in charge as salinity varies. All sediments were negatively
charged in sea water while most of them were positively charged in fresh
water. Charge reversal occurred between 2 and 6%, salinity. It would be
283
has been adapted by Harder (1968) to study the behaviour of marine
plankton towards density discontinuities. After testing a range of
species he concluded that almost all of them aggregated at the interface
between waters of Werent densities, and cites instances of this
occurring in the sea. More recently, localized discontinuities in the
microstructure of temperature, salinity, and velocity profiles in the sea
and of temperature profiles in fresh waters have been described (Simpson and Woods (1970), Woods (1971), for references) and may influence
planktonic animals in the same way.
As far as we are aware little is known of the temperature preferences
or current responses of marine planktonic or benthic invertebrates,
apart from studies on the relation of temperature to light preferences
in three species of planktonic copepod (Lewis, 1959), on the relation of
temperature to the burrowing activity of two Penaeus species (Aldrich
et al., 1968; p. 345 below), and on the rheotropic responses of a shallow
water nudibranch (Chromodoris zebra Heilprin) (Crozier and Arey, 19 19).
Except under unusual conditions such as the aftermath of a plankton
bloom, anaerobic conditions are rare in the sea, and there is no record
of planktonic animals responding to them. On the other hand they
must be common in sublittoral sediments although there are no
studies of their possible significance to benthic invertebrates. Many
benthic invertebrates must also respond positively to touching or being
surrounded by solid objects, but again little appears to be known of
this behaviour or of any responses to the microtopography or
roughness of surfaces (Diebschlag, 1938).
Many mobile bottom-dwelling invertebrates, such as brittle stars,
octopods, crabs, and lobsters, seek shelter on the approach of predators,
or live semi-permanently in crevices or dens. This behaviour, which is
well known to divers, has only been experimentally analysed in two
species of crayfish. Nonaka (1966) and Cobb (1971) have shown that
the number and size of shelters, and their relative dimensions, are likely
to influence the local distribution of Panulirus jarponicus von Siebold
and Homarus arnericunus Milne Edwards respectively.
It is well known that benthic marine invertebrates in mud and
sand burrow to different depths-the various burrowing bivalves are
good examples-but there are no experimental studies that attempt to
link possible depth preferences with distribution.
Pravdi6 (1970) has recently described a new parameter by which
sediments may be classified. He has designed an apparatus to measure
the electrical charge at the surface of sediments, and has investigated
the changes in charge as salinity varies. All sediments were negatively
charged in sea water while most of them were positively charged in fresh
water. Charge reversal occurred between 2 and 6%, salinity. It would be