ECOLOGICAL STUDIES AT LOUQH INE
277
that it was not possible to reach end-point in the test, either for lack
of time, or, for some tests with Cancer, because the crab destroyed the
largest Paracentrotus available.
Carcinus, up to 6&60 mm carapace width, were able to destroy only
very small Paracentrotus of 1&20 mm diameter, and some crabs failed
to damage any at all. Portunus puber (up to 115 mm) destroyed Paracentrotus mare readily and up to 60 mm in diameter. Cancer of very
moderate size (143 mm) destroyed Paracentrotus of the largest size
available, even two in one night. One large Cancer destroyed eleven
Paracentrotus in seven nights.
The height of Paracentrotus is about one half its horizontal diameter.
An analysis of all the measurements of shell diameter and shell height
in centimetres gave the relation D = 1.93 H + 0.02, the constant
having no significance. From measurements of the maximum gap of
the chelae of all the test crabs it is evident that crabs can destroy Paracentrotus of a height greater than would fit within the tips of either
chela.
The predatory behaviour of five male Porturms puber (78-84 mm)
was observed in running sea water in the Dromadoon Laboratory. On
a number of occasions Paracentrotus small enough to be gripped within
the chelae were quickly crushed and consumed except for spines and
shell fkagments. Paracentrotus greater in height than the gaps of either
chela were attacked on several occasions, and spines were broken off.
In one case the crab (the most aggressive of the five) pulled the Paracentrotus free, broke off a number of spines, and finally broke through
the oral surface by pressing its chelae downwards into the upturned
Paracentrotus. It proceeded to enlarge the hole with its right chela and
was no longer disturbed by torchlight. A small Cancer pagurus (a 56 mm
male) was observed to break up and clean out a Paracentrotus in the
same way.
In several experiments a number of large Paracentrotus were collected
in the North Basin and transported under water to sites in the South
Basin. The transplanted Paracentrotus quickly diminished in numbers,
no doubt partly because they walked away. Nevertheless, small groups
of spines and pieces of broken shells showed that some had fallen
victim to predators.
Finally, attempts were made to identify the predators in two further
experiments. I n one, carried out on 24 August 1963, one hundred large
Paracentrotus were put in Renouf's Bay, one hundred off Harbour Wall,
and eighty off Mermaid Rock, east of Glanna€een Beach. A large Cancer
was seen to eat one Paracentrotus and attack another off "Harbour Wall,
and a Portunus puber was seen on the site at both Renouf's Bay and
Mermaid Rock, though neither was seen to attack. In the final transfer,
277
that it was not possible to reach end-point in the test, either for lack
of time, or, for some tests with Cancer, because the crab destroyed the
largest Paracentrotus available.
Carcinus, up to 6&60 mm carapace width, were able to destroy only
very small Paracentrotus of 1&20 mm diameter, and some crabs failed
to damage any at all. Portunus puber (up to 115 mm) destroyed Paracentrotus mare readily and up to 60 mm in diameter. Cancer of very
moderate size (143 mm) destroyed Paracentrotus of the largest size
available, even two in one night. One large Cancer destroyed eleven
Paracentrotus in seven nights.
The height of Paracentrotus is about one half its horizontal diameter.
An analysis of all the measurements of shell diameter and shell height
in centimetres gave the relation D = 1.93 H + 0.02, the constant
having no significance. From measurements of the maximum gap of
the chelae of all the test crabs it is evident that crabs can destroy Paracentrotus of a height greater than would fit within the tips of either
chela.
The predatory behaviour of five male Porturms puber (78-84 mm)
was observed in running sea water in the Dromadoon Laboratory. On
a number of occasions Paracentrotus small enough to be gripped within
the chelae were quickly crushed and consumed except for spines and
shell fkagments. Paracentrotus greater in height than the gaps of either
chela were attacked on several occasions, and spines were broken off.
In one case the crab (the most aggressive of the five) pulled the Paracentrotus free, broke off a number of spines, and finally broke through
the oral surface by pressing its chelae downwards into the upturned
Paracentrotus. It proceeded to enlarge the hole with its right chela and
was no longer disturbed by torchlight. A small Cancer pagurus (a 56 mm
male) was observed to break up and clean out a Paracentrotus in the
same way.
In several experiments a number of large Paracentrotus were collected
in the North Basin and transported under water to sites in the South
Basin. The transplanted Paracentrotus quickly diminished in numbers,
no doubt partly because they walked away. Nevertheless, small groups
of spines and pieces of broken shells showed that some had fallen
victim to predators.
Finally, attempts were made to identify the predators in two further
experiments. I n one, carried out on 24 August 1963, one hundred large
Paracentrotus were put in Renouf's Bay, one hundred off Harbour Wall,
and eighty off Mermaid Rock, east of Glanna€een Beach. A large Cancer
was seen to eat one Paracentrotus and attack another off "Harbour Wall,
and a Portunus puber was seen on the site at both Renouf's Bay and
Mermaid Rock, though neither was seen to attack. In the final transfer,