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J. A. KITCHING AND P. J. EBLINQ
broken shells were found, and there is little doubt that Carcinus was
responsible for this. We do not know whether Carcinus accounted for
all the losses; it is possible that it may have carried off shells which it
could not break open.
7. Conclusions
Our results show that crabs selectively destroy Thais of the opencoast type in cages, and that water currents selectively remove Thais
of the sheltered-coast type under experimental conditions of fast flow.
When Thais of both types are transferred to either natural habitat,
selection occurs in the expected direction with an intensity which would
be sufficient to account for the suppression of the inappropriate type,
should it occur. Moreover, there is evidence to show that the selective
agents operating are, in fact, wave-action and predation. Our interpretation may be somewhat oversimplified in that it does not take
account of possible differences in behaviour of the two types of Thais.
Nevertheless we believe that we have substantially demonstrated the
significance of these two types for survival.
We do not know to what extent the existence of two types of Thais
in the Lough Ine area depends on environmental modification of the
developing organism, and to what extent the differences are genetic.
Abundance of food, easily obtained, might produce a more rapid
growth of the soft tissues in relation to the deposition of shell material.
Nevertheless, the lack of a planktonic larval stage could encourage the
establishment of local genetic types, and studies of chromosomes carried
out by Staiger (1957) lend support to this interpretation. We think that
it is the more likely explanation. We are a t present examining the
region of transition between open coast and shelter, and we are undertaking studies 011 the growth of transferred individuals.
D. P A I t A CEN TI2 0 T U S A N D PREDATORY CRABS
Paracentrotus lividus is common throughout Lough Ine, though it is
more plentiful in areas of the North Basin than in the South Basin.
During July 1962 all the boulders within a small selected area of the
shallow sublittoral were removed and examined at each of seven stations
in the lough (Muntz et al., 1965). The numbers of Paracentrotus per
square metre at each of these stations are shown in Fig. 67. Small
Paracentrotus, up to 10 or 25 cm diameter, were abundant on the bottom
surfaces of upper layer boulders around the margins of the lough.
Large Paracentrotus, up to 55 mm diameter, were found on the open
sea bed only at North Wall and in Curlew Bay (Fig. 69), although
they occurred in large crevices under boulders in the South Basin. One
J. A. KITCHING AND P. J. EBLINQ
broken shells were found, and there is little doubt that Carcinus was
responsible for this. We do not know whether Carcinus accounted for
all the losses; it is possible that it may have carried off shells which it
could not break open.
7. Conclusions
Our results show that crabs selectively destroy Thais of the opencoast type in cages, and that water currents selectively remove Thais
of the sheltered-coast type under experimental conditions of fast flow.
When Thais of both types are transferred to either natural habitat,
selection occurs in the expected direction with an intensity which would
be sufficient to account for the suppression of the inappropriate type,
should it occur. Moreover, there is evidence to show that the selective
agents operating are, in fact, wave-action and predation. Our interpretation may be somewhat oversimplified in that it does not take
account of possible differences in behaviour of the two types of Thais.
Nevertheless we believe that we have substantially demonstrated the
significance of these two types for survival.
We do not know to what extent the existence of two types of Thais
in the Lough Ine area depends on environmental modification of the
developing organism, and to what extent the differences are genetic.
Abundance of food, easily obtained, might produce a more rapid
growth of the soft tissues in relation to the deposition of shell material.
Nevertheless, the lack of a planktonic larval stage could encourage the
establishment of local genetic types, and studies of chromosomes carried
out by Staiger (1957) lend support to this interpretation. We think that
it is the more likely explanation. We are a t present examining the
region of transition between open coast and shelter, and we are undertaking studies 011 the growth of transferred individuals.
D. P A I t A CEN TI2 0 T U S A N D PREDATORY CRABS
Paracentrotus lividus is common throughout Lough Ine, though it is
more plentiful in areas of the North Basin than in the South Basin.
During July 1962 all the boulders within a small selected area of the
shallow sublittoral were removed and examined at each of seven stations
in the lough (Muntz et al., 1965). The numbers of Paracentrotus per
square metre at each of these stations are shown in Fig. 67. Small
Paracentrotus, up to 10 or 25 cm diameter, were abundant on the bottom
surfaces of upper layer boulders around the margins of the lough.
Large Paracentrotus, up to 55 mm diameter, were found on the open
sea bed only at North Wall and in Curlew Bay (Fig. 69), although
they occurred in large crevices under boulders in the South Basin. One