ECOLOGICAL STUDIES AT LOUGH INE
283
Cancer p q u r u s L. is distributed much as Portunus puber but is apparently less common. It is fairly easily found in the south-east corner of
the South Basin, and is seen sporadically in the lough, but not at North
Wall or in the inner part of Curlew Bay or in the Goleen.
Portunus arcuatus Leach is common all around the lough, but is found
mainly on or near a muddy bottom. Portunus corrugatus, also abundant
in Barloge Creek, has been found a little way offshore in Curlew Bay
and North Wall, on a muddy bottom or on weed over mud (but was
very local). Portunus depurator has not been seen in the shallow sublittoral, although it has been trapped in the middle of the South Basin.
Maia squinado has been seen in the North Basin and a t the north end
of the Rapide, but is not a t all common. Xantho incisus is widely distributed but not common; Xantho hydrophilus is even less common.
Homarus vulgaris has been seen and captured occasionally, mainly in
the Rapids area.
The most important decapod predators in the shallow rocky sublittoral, which are powerful enough to break open prey protected by
shells or tests, are thus probably Cancer pagurus, Portunus puber and
Carcinus m e n a s . The eastern part of the South Basin, i.e. that part
which is washed by the great vortex set up by the incoming tide, differs
from the rest of the lough in the more frequent occurrence of the large
crabs Cancer pagurus and Portunus puber.
G. CONCLIJSIONS
It is extremely difficult to be sure that the absence of an organism
from any site is due to the activity of any particular predator. The fact
that a predator can damage an animal under experimental conditions in
cages or that it can be seen to do so in the laboratory does not establish
that this is a normal event in the sea, where the chances of encounter
may be very much less. Moreover, such an experiment takes no account
of possible behavioural interactions between different predators, or between predators and other species which are not directly involved in
the predator-prey relationship. Similarly the demise of an animal after
transplantation is a contrived event, although there is a closer approach
to natural conditions in such an experiment. A normal absence from
che site in question, coupled with destruction on transplantation to it,
does not in itself show that the predator concerned is the normal cause
of the failure of the prey to establish itself there. This failure could be
due to a variety of causes, biotic or otherwise, acting upon the younger
stages. Equally, the survival of a mature sedentary animal after transplantation does not prove that it could have developed in situ.
Nevertheless, if evidence from experiments on predation in cages and
283
Cancer p q u r u s L. is distributed much as Portunus puber but is apparently less common. It is fairly easily found in the south-east corner of
the South Basin, and is seen sporadically in the lough, but not at North
Wall or in the inner part of Curlew Bay or in the Goleen.
Portunus arcuatus Leach is common all around the lough, but is found
mainly on or near a muddy bottom. Portunus corrugatus, also abundant
in Barloge Creek, has been found a little way offshore in Curlew Bay
and North Wall, on a muddy bottom or on weed over mud (but was
very local). Portunus depurator has not been seen in the shallow sublittoral, although it has been trapped in the middle of the South Basin.
Maia squinado has been seen in the North Basin and a t the north end
of the Rapide, but is not a t all common. Xantho incisus is widely distributed but not common; Xantho hydrophilus is even less common.
Homarus vulgaris has been seen and captured occasionally, mainly in
the Rapids area.
The most important decapod predators in the shallow rocky sublittoral, which are powerful enough to break open prey protected by
shells or tests, are thus probably Cancer pagurus, Portunus puber and
Carcinus m e n a s . The eastern part of the South Basin, i.e. that part
which is washed by the great vortex set up by the incoming tide, differs
from the rest of the lough in the more frequent occurrence of the large
crabs Cancer pagurus and Portunus puber.
G. CONCLIJSIONS
It is extremely difficult to be sure that the absence of an organism
from any site is due to the activity of any particular predator. The fact
that a predator can damage an animal under experimental conditions in
cages or that it can be seen to do so in the laboratory does not establish
that this is a normal event in the sea, where the chances of encounter
may be very much less. Moreover, such an experiment takes no account
of possible behavioural interactions between different predators, or between predators and other species which are not directly involved in
the predator-prey relationship. Similarly the demise of an animal after
transplantation is a contrived event, although there is a closer approach
to natural conditions in such an experiment. A normal absence from
che site in question, coupled with destruction on transplantation to it,
does not in itself show that the predator concerned is the normal cause
of the failure of the prey to establish itself there. This failure could be
due to a variety of causes, biotic or otherwise, acting upon the younger
stages. Equally, the survival of a mature sedentary animal after transplantation does not prove that it could have developed in situ.
Nevertheless, if evidence from experiments on predation in cages and