IIABITAT SELEUTXON BY AQUATIU INVERTEBRATES
277
in any way unpleasant to Arenicola stopped it from burrowing deeper.
Experiments designed to test whether other animals can select particular sediment depths are in principle fairly straightforward, but
apart from Reid’s work have only been attempted by Meadows
(1964b). Corophium volutator does not occur on muddy beaches if the
mud is shallower than about 1 cm, and in multi-choice experiments it
avoids sediments that are as shallow as this. It does not, however,
distinguish between 2, 5 and 9 cm deep muds and this agrees with its
field distribution. Meadows calculated that Corophium probably
avoids shallow sediments because it finds difficulty in constructing or
maintaining its U-shaped burrow in them.
Thixotropic sands become liquid on agitation and firm again on
standing, while dilatant sands firm on pressure-noticeable as a light
area around one’s footprintand soften again once the pressure
is removed. Both occur on intertidal beaches (Chapman, 1949). The
changes in thixotropy and dilatancy associated with differing grades
of sediment (Webb, 1958b) are likely to affect habitat selection (Craig,
1970), particularly in view of Chapman and Newell’s (1947) observation
that Arenicola utilized the thixotropic properties of the sediment as it
was burrowing. The sand crab Emerita analoqa (Stimpson) responds to
changes in sand fluidity of this sort (Cubit, 1969). The lower and upper
edges of the tide’s wash zone are bounded by bands of sand made fluid
by water movement. Emerita is retained between these two bands
because it burrows out of fluid sands and into f%m sand, and so moves
up and down the beach held by the edges of the advancing and receding
tide. Similar behavioural adaptations may account for the tidal
migration of some species of bivalves (Mori, 1938; Ansell and Trevallion,
1969) and for the intertidal distribution of the isopod Eurydice pulchra
Leach (Jones and Naylor, 1970).
Since beaches slope towards the sea, a gravity sense should enable
animals to locate themselves on the shore. Those species that have been
studied show gravity responses of this sort (Fraenkel, 1927; Gowanloch
and Hayes, 1927; Barnes, 1932; Carriker, 1957; Newell, 1958b),
and it is probably a widespread attribute of intertidal animals. The
changes in response to gravity that occur as animals become immersed,
or move from wet to dry sand, will also help in the maintenance of
position in the intertidal zone. Nassarius obsoletus (Say) is geonegative
under water but geopositive in air (Crisp, 1969), Lepihchitona
cinerea (L.) is indifferent to gravity in water but geopositive in air
(Evans, 1951), and the isopod Tylos punctatus is geonegative on wet
sand and geopositive on dry sand (Hamner et al., 1968)”.
The pressure around an intertidal animal living towards low water
* See note added in proof on page 493.
277
in any way unpleasant to Arenicola stopped it from burrowing deeper.
Experiments designed to test whether other animals can select particular sediment depths are in principle fairly straightforward, but
apart from Reid’s work have only been attempted by Meadows
(1964b). Corophium volutator does not occur on muddy beaches if the
mud is shallower than about 1 cm, and in multi-choice experiments it
avoids sediments that are as shallow as this. It does not, however,
distinguish between 2, 5 and 9 cm deep muds and this agrees with its
field distribution. Meadows calculated that Corophium probably
avoids shallow sediments because it finds difficulty in constructing or
maintaining its U-shaped burrow in them.
Thixotropic sands become liquid on agitation and firm again on
standing, while dilatant sands firm on pressure-noticeable as a light
area around one’s footprintand soften again once the pressure
is removed. Both occur on intertidal beaches (Chapman, 1949). The
changes in thixotropy and dilatancy associated with differing grades
of sediment (Webb, 1958b) are likely to affect habitat selection (Craig,
1970), particularly in view of Chapman and Newell’s (1947) observation
that Arenicola utilized the thixotropic properties of the sediment as it
was burrowing. The sand crab Emerita analoqa (Stimpson) responds to
changes in sand fluidity of this sort (Cubit, 1969). The lower and upper
edges of the tide’s wash zone are bounded by bands of sand made fluid
by water movement. Emerita is retained between these two bands
because it burrows out of fluid sands and into f%m sand, and so moves
up and down the beach held by the edges of the advancing and receding
tide. Similar behavioural adaptations may account for the tidal
migration of some species of bivalves (Mori, 1938; Ansell and Trevallion,
1969) and for the intertidal distribution of the isopod Eurydice pulchra
Leach (Jones and Naylor, 1970).
Since beaches slope towards the sea, a gravity sense should enable
animals to locate themselves on the shore. Those species that have been
studied show gravity responses of this sort (Fraenkel, 1927; Gowanloch
and Hayes, 1927; Barnes, 1932; Carriker, 1957; Newell, 1958b),
and it is probably a widespread attribute of intertidal animals. The
changes in response to gravity that occur as animals become immersed,
or move from wet to dry sand, will also help in the maintenance of
position in the intertidal zone. Nassarius obsoletus (Say) is geonegative
under water but geopositive in air (Crisp, 1969), Lepihchitona
cinerea (L.) is indifferent to gravity in water but geopositive in air
(Evans, 1951), and the isopod Tylos punctatus is geonegative on wet
sand and geopositive on dry sand (Hamner et al., 1968)”.
The pressure around an intertidal animal living towards low water
* See note added in proof on page 493.