HABITAT SELEOTION BY AQUATIO INVERTEBRATES
275
water, and, since it is geonegative, also moves upwards. As it emerges,
or as the tide falls, it will be trapped by its photonegative responses in
the fist crevice that it encounters. Similar observations for three other
species of Littorina have been recorded by Gowanloch and Hayes (1 927).
The amphipod Talitrus saltator (Montagu) lives during the day in
burrows at about high tide mark. At night it moves out over the
sand’s surface as the tide falls, sometimes to below mid-tide level (c.f.
Holmes, 1901). From his observations on its behaviour Williamson
(1961b) felt that form vision of sand dunes or hillocks might account
for the species movements, and in subsequent experiments he demonstrated how Talitrus moved towards the angle formed by a dark object
on a flat surface. He suggested that other intertidal amphipods might
react in the same way. A related amphipod, Orchestia agilis S . I.
Smith, has equally well defined light responses (Holmes, 1901). During
daylight when the tide is down it hides under seaweed. If removed it
is at first photonegative, but soon becomes photopositive; under water
it is strongly photonegative. These responses can be repeated under
laboratory conditions. The interpretation of Holmes’ results is,
however, difficult. Perhaps animals disturbed from their seaweed hide
are at first photonegative in an attempt to return there, but if after a
certain length of time they are unsuccessful they become photopositive,
and so, since the sea is brighter than the land, move towards the
water’s edge. Once in water, being strongly photonegative, they will
swim to the bottom. Corophiunz volutator (Pallas), a burrowing amphipod, also has distinctive light responses. It is photopositive when
swimming, photonegative when walking over a surfaae out of water,
and burrows more readily in the light than in darkness (Meadows and
Reid, 1966; Meadows, 1967; Barnes et al., 1969). These responses
ensure that animals will move towards the water line both down the
shore, and up from the sublittoral zone, and will burrow in the brighter
light of shallower waters. Finally, it should be noted that light appears
to play a significant part in setting the cyclical rhythms of swimming
behaviour that enable certain Crustacea to maintain their position on
the shore (Enright, 1963; Fincham, 1970; Jones and Naylor, 1970).
The particle size of sediments on the shore varies from gravel to
fine mud, often doing so within a few metres, and it is obvious even
from a passing glance that the distribution of a number of species on
the shore is influenced by these substrates. What evidence there is
suggests that this is caused by animals preferring sediments of certain
particle sizes (Wieser, 1956; Teal, 1968; Meadows, 1 9 6 4 ~ ;
Croker, 1967;
Sameoto, 1969; Jones, 1970; Phillips, 1971). Only Wieser has attempted
to explain particle size preferences in terms of their relevance to the
275
water, and, since it is geonegative, also moves upwards. As it emerges,
or as the tide falls, it will be trapped by its photonegative responses in
the fist crevice that it encounters. Similar observations for three other
species of Littorina have been recorded by Gowanloch and Hayes (1 927).
The amphipod Talitrus saltator (Montagu) lives during the day in
burrows at about high tide mark. At night it moves out over the
sand’s surface as the tide falls, sometimes to below mid-tide level (c.f.
Holmes, 1901). From his observations on its behaviour Williamson
(1961b) felt that form vision of sand dunes or hillocks might account
for the species movements, and in subsequent experiments he demonstrated how Talitrus moved towards the angle formed by a dark object
on a flat surface. He suggested that other intertidal amphipods might
react in the same way. A related amphipod, Orchestia agilis S . I.
Smith, has equally well defined light responses (Holmes, 1901). During
daylight when the tide is down it hides under seaweed. If removed it
is at first photonegative, but soon becomes photopositive; under water
it is strongly photonegative. These responses can be repeated under
laboratory conditions. The interpretation of Holmes’ results is,
however, difficult. Perhaps animals disturbed from their seaweed hide
are at first photonegative in an attempt to return there, but if after a
certain length of time they are unsuccessful they become photopositive,
and so, since the sea is brighter than the land, move towards the
water’s edge. Once in water, being strongly photonegative, they will
swim to the bottom. Corophiunz volutator (Pallas), a burrowing amphipod, also has distinctive light responses. It is photopositive when
swimming, photonegative when walking over a surfaae out of water,
and burrows more readily in the light than in darkness (Meadows and
Reid, 1966; Meadows, 1967; Barnes et al., 1969). These responses
ensure that animals will move towards the water line both down the
shore, and up from the sublittoral zone, and will burrow in the brighter
light of shallower waters. Finally, it should be noted that light appears
to play a significant part in setting the cyclical rhythms of swimming
behaviour that enable certain Crustacea to maintain their position on
the shore (Enright, 1963; Fincham, 1970; Jones and Naylor, 1970).
The particle size of sediments on the shore varies from gravel to
fine mud, often doing so within a few metres, and it is obvious even
from a passing glance that the distribution of a number of species on
the shore is influenced by these substrates. What evidence there is
suggests that this is caused by animals preferring sediments of certain
particle sizes (Wieser, 1956; Teal, 1968; Meadows, 1 9 6 4 ~ ;
Croker, 1967;
Sameoto, 1969; Jones, 1970; Phillips, 1971). Only Wieser has attempted
to explain particle size preferences in terms of their relevance to the