ECOLOUICAL STUDIES AT LOUGH INE
257
the Western Trough, and we have failed to catch crabs below 25 m,
although Carcinus and Portunus arcuatus occur where the bottom is
not so deep. Portunus arcuutus extends down to 20 m over a muddy
bottom apparently comparable in texture with that in the Trough.
When this crab was placed in cages on the bottom below the thermocline, in experiments carried out in 1965 by Miss Jancis Betney, it became inert or died within 24 h, although when placed on the bottom
above the thermocline it appeared unharmed. The most probable explanation is in terms of oxygen concentration, although confirmation is
wanted by means of experiments in the laboratory. So far we have not
found any animals below the thermocline in the Western Trough,
although much more investigation is needed. The polychaete Pectinaria
belgica, the decapod Calocaris macandreae and Priapulus caudatus (all
found by Mr. John Heath and Mr. Colin Irwin while diving) extend to
about that level.
VII. PREDATION AND GRAZINQ
A. INTRODUCTION
The importance of predation in determining the distribution of
marine organisms first became apparent from our studies on the mussel
Mytilus edulis. The distribution of mussels presents a problem, since
they occur plentifully only in what appear to be widely separated ecological sites, for example, on beds in the sheltered mouths of estuaries,
on open rocky coasts exposed to wave-action, and generally on poles
and other vertical surfaces. We were able t o show that mussels rapidly
disappear when transplanted to sites where they are normally scarce,
even though they readily survive when they are protected in cages. In
such sites crabs were seen eating the mussels. The ability of crabs to
break mussels was tested by placing the animals together in cages, and
the feeding of crabs was observed in the aquarium. Finally, careful
surveys by several methods of the occurrence of crabs showed that the
distribution of competent predators was inversely related to that of
mussels.
The sea urchin Parmentrotus lividus also has two habitats in Ireland:
intertidal rock pools along much of the west coast and the shallow sublittoral in certain sheltered localities. In the Lough Ine area it occurs in
small numbers in rock pools around Carrigathorna, and abundantly
within the lough. The rock pool habitat is better exemplified in Connemara. Investigations similar to those described for Mytilus have created
a chain of circumstantial evidence to explain the distribution of Paracentrotus in relation to its predators within Lough Ine. Moreover, it
appeared that areas where Paracentrotus was plentiful were practically
257
the Western Trough, and we have failed to catch crabs below 25 m,
although Carcinus and Portunus arcuatus occur where the bottom is
not so deep. Portunus arcuutus extends down to 20 m over a muddy
bottom apparently comparable in texture with that in the Trough.
When this crab was placed in cages on the bottom below the thermocline, in experiments carried out in 1965 by Miss Jancis Betney, it became inert or died within 24 h, although when placed on the bottom
above the thermocline it appeared unharmed. The most probable explanation is in terms of oxygen concentration, although confirmation is
wanted by means of experiments in the laboratory. So far we have not
found any animals below the thermocline in the Western Trough,
although much more investigation is needed. The polychaete Pectinaria
belgica, the decapod Calocaris macandreae and Priapulus caudatus (all
found by Mr. John Heath and Mr. Colin Irwin while diving) extend to
about that level.
VII. PREDATION AND GRAZINQ
A. INTRODUCTION
The importance of predation in determining the distribution of
marine organisms first became apparent from our studies on the mussel
Mytilus edulis. The distribution of mussels presents a problem, since
they occur plentifully only in what appear to be widely separated ecological sites, for example, on beds in the sheltered mouths of estuaries,
on open rocky coasts exposed to wave-action, and generally on poles
and other vertical surfaces. We were able t o show that mussels rapidly
disappear when transplanted to sites where they are normally scarce,
even though they readily survive when they are protected in cages. In
such sites crabs were seen eating the mussels. The ability of crabs to
break mussels was tested by placing the animals together in cages, and
the feeding of crabs was observed in the aquarium. Finally, careful
surveys by several methods of the occurrence of crabs showed that the
distribution of competent predators was inversely related to that of
mussels.
The sea urchin Parmentrotus lividus also has two habitats in Ireland:
intertidal rock pools along much of the west coast and the shallow sublittoral in certain sheltered localities. In the Lough Ine area it occurs in
small numbers in rock pools around Carrigathorna, and abundantly
within the lough. The rock pool habitat is better exemplified in Connemara. Investigations similar to those described for Mytilus have created
a chain of circumstantial evidence to explain the distribution of Paracentrotus in relation to its predators within Lough Ine. Moreover, it
appeared that areas where Paracentrotus was plentiful were practically