ECOLOGICAL STUDIES AT LOUGH INE
227
B. MODE O F A C T I O N
Water movement may affect an organism a t any or all the stages of
its life, and it may act in various ways, beneficial or injurious. It may
dislodge an animal or plant from its substrate; it may prevent settlement of sediment, including organic matter which might otherwise lead
to a deficiency of oxygen; it may prevent the establishment of local
extremes of oxygen or carbon dioxide concentration or of temperature;
it may distribute food, or the organism’s own larvae. It is extremely
difficult to distinguish between these possibilities in any particular case,
although we may speculate with some degree of plausibility in a few
instances.
We have investigated experimentally the preference of t’he gastropod
Gibbula cineraria for water sheltered from strong current and from
waves (Fig. 21) (Ebling et al., 1948). In several tests a number of
G. cineraria were suspended on fronds of laminarian algae both in the
main stream of the Rapids (off Mark 3 on the Rapids Quay) and at
nearby stations sheltered from current. The fronds were placed in position at slack water in order to give the snails the opportunity of adjusting themselves to the current as it increased. They were re-examined
after some hours, and it was found that nearly all the Gibbula had disappeared from the fronds placed in the Rapids, while most remained on
the fronds placed in quiet water. We may reasonably conclude that
GibbuZu cineraria is unable to hold on in the current; nevertheless, we
cannot conclude unequivocally that the absence of G . cineraria from
Laminaria on the Sill is to be explained in this way. At present we can
only surmise that current may prevent the larvae from settling on
algae on the Sill, and that in any case any growing or adult Gibbula
which wander or are carried on t o the Sill will fail to hold on to the
laminarian fronds there. The same explanation may account for the
absence of G . cineraria from laminarian fronds on the open coast.
Inability to hold on is the most likely explanation for the limitation
of many species which inhabit the fronds of Laminaria spp. and Saccorhiza in more sheltered positions, but are scarcer in or missing from the
Rapids. These include the Polyzoa Membranipora membranacea (page
223) and Tubulipora plumosa, and the gastropod Patina pellucida. We
do not yet know whether the adults or the larvae are susceptible to
dislodgement.
We have attempted some preliminary investigation, by means of field
experiments, of the distribution of two species which “prefer” places
where a strong current flows but show no preference between the tops
and the bottoms of boulders.
When rocks bearing Shrtularia operculata were transferred from the
227
B. MODE O F A C T I O N
Water movement may affect an organism a t any or all the stages of
its life, and it may act in various ways, beneficial or injurious. It may
dislodge an animal or plant from its substrate; it may prevent settlement of sediment, including organic matter which might otherwise lead
to a deficiency of oxygen; it may prevent the establishment of local
extremes of oxygen or carbon dioxide concentration or of temperature;
it may distribute food, or the organism’s own larvae. It is extremely
difficult to distinguish between these possibilities in any particular case,
although we may speculate with some degree of plausibility in a few
instances.
We have investigated experimentally the preference of t’he gastropod
Gibbula cineraria for water sheltered from strong current and from
waves (Fig. 21) (Ebling et al., 1948). In several tests a number of
G. cineraria were suspended on fronds of laminarian algae both in the
main stream of the Rapids (off Mark 3 on the Rapids Quay) and at
nearby stations sheltered from current. The fronds were placed in position at slack water in order to give the snails the opportunity of adjusting themselves to the current as it increased. They were re-examined
after some hours, and it was found that nearly all the Gibbula had disappeared from the fronds placed in the Rapids, while most remained on
the fronds placed in quiet water. We may reasonably conclude that
GibbuZu cineraria is unable to hold on in the current; nevertheless, we
cannot conclude unequivocally that the absence of G . cineraria from
Laminaria on the Sill is to be explained in this way. At present we can
only surmise that current may prevent the larvae from settling on
algae on the Sill, and that in any case any growing or adult Gibbula
which wander or are carried on t o the Sill will fail to hold on to the
laminarian fronds there. The same explanation may account for the
absence of G . cineraria from laminarian fronds on the open coast.
Inability to hold on is the most likely explanation for the limitation
of many species which inhabit the fronds of Laminaria spp. and Saccorhiza in more sheltered positions, but are scarcer in or missing from the
Rapids. These include the Polyzoa Membranipora membranacea (page
223) and Tubulipora plumosa, and the gastropod Patina pellucida. We
do not yet know whether the adults or the larvae are susceptible to
dislodgement.
We have attempted some preliminary investigation, by means of field
experiments, of the distribution of two species which “prefer” places
where a strong current flows but show no preference between the tops
and the bottoms of boulders.
When rocks bearing Shrtularia operculata were transferred from the