275
J. A. KITCHING AND F. J. EBLING
in which twenty fairly large Paracentrotus ( 5 6 5 5 mm) and twenty
smaller ones (30-35 mm) were deposited in a cleared area off Harbour
Wall, the site was examined every 5 min for 12 h, including the whole
night. Portunus puber destroyed four of the smaller Paracentrotus. A
determined attack on a larger specimen failed, with no more damage to
the Paracentrotus than the loss of some spines - a result in accord with
the experiments on predation in cages.
E. P A R A C E N T R O T U S A N D ALGAE
The most extensive beds of Paracentrotus are along the north shore
of the lough, and on the north side of Castle Island. Most of these areas
are practically free from obvious algal growth, whereas algae are abundant in places where Paracentrotus is less plentiful (Kitching and
Ebling, 1961). During the summer of 1950 the innermost part of Curlew
Bay, near the shore, was well populated by Paracentrotus and practically clear of algae. In the slightly deeper water, however, there was a
very dense covering of algae, mainly Enteromorpha clathrata, but including also some Ceramium spp., Ectocarpus conferroides, Chlyocladia
verticillata, Polysiphonia, and others. The boundary of this dense weed
cover in that year is shown in Fig. 69. The population of Paracentrotus
in relation to the occurrence of algae and the nature of the bottom was
examined in a series of metre squares along each of two sections running out northwards into the bay from shore marks. As will be seen from
Fig. 70, Paracentrotus stops where dense algal cover begins.
On 7 July 1959, an area of Curlew Bay amounting to nearly 300 sq. m
was completely cleared of Paracentrotus (Fig. 69); in all, 1957 sea
urchins were removed. By 23 July, the cleared area was about 10%
covered by algae, by 10 August it had risen to 25%, and by 3 September
to 50%, but the dry weight of alga/sq. m was much less than in the
part of the bay normally dominated by Enteromorpha. Parts of the bay
still occupied by Paracentrotus continued practically free of weed. The
boundary between the Paracentrotus-freed sector and the uncleared
areas on either side of it became irregular, but apart from this the
Paracentrotus-freed sector remained completely clear throughout the
summer. In early July 1960, the area cleared of Paracentrotus in the
previous year was thickly and completely covered with Enteromorpha
and other algae. The area to the west, well populated by Parucentrotus,
remained free of weed, but the area to the east of the cleared area,
previously free of algal cover, carried some large dense patches of algae.
The Paracentrotus collected during clearance of the area were transported to various sites in the lough. On 7 July 1959, 1 3 2 6 Paracentrotus were dropped in an elongated heap on the western section in
J. A. KITCHING AND F. J. EBLING
in which twenty fairly large Paracentrotus ( 5 6 5 5 mm) and twenty
smaller ones (30-35 mm) were deposited in a cleared area off Harbour
Wall, the site was examined every 5 min for 12 h, including the whole
night. Portunus puber destroyed four of the smaller Paracentrotus. A
determined attack on a larger specimen failed, with no more damage to
the Paracentrotus than the loss of some spines - a result in accord with
the experiments on predation in cages.
E. P A R A C E N T R O T U S A N D ALGAE
The most extensive beds of Paracentrotus are along the north shore
of the lough, and on the north side of Castle Island. Most of these areas
are practically free from obvious algal growth, whereas algae are abundant in places where Paracentrotus is less plentiful (Kitching and
Ebling, 1961). During the summer of 1950 the innermost part of Curlew
Bay, near the shore, was well populated by Paracentrotus and practically clear of algae. In the slightly deeper water, however, there was a
very dense covering of algae, mainly Enteromorpha clathrata, but including also some Ceramium spp., Ectocarpus conferroides, Chlyocladia
verticillata, Polysiphonia, and others. The boundary of this dense weed
cover in that year is shown in Fig. 69. The population of Paracentrotus
in relation to the occurrence of algae and the nature of the bottom was
examined in a series of metre squares along each of two sections running out northwards into the bay from shore marks. As will be seen from
Fig. 70, Paracentrotus stops where dense algal cover begins.
On 7 July 1959, an area of Curlew Bay amounting to nearly 300 sq. m
was completely cleared of Paracentrotus (Fig. 69); in all, 1957 sea
urchins were removed. By 23 July, the cleared area was about 10%
covered by algae, by 10 August it had risen to 25%, and by 3 September
to 50%, but the dry weight of alga/sq. m was much less than in the
part of the bay normally dominated by Enteromorpha. Parts of the bay
still occupied by Paracentrotus continued practically free of weed. The
boundary between the Paracentrotus-freed sector and the uncleared
areas on either side of it became irregular, but apart from this the
Paracentrotus-freed sector remained completely clear throughout the
summer. In early July 1960, the area cleared of Paracentrotus in the
previous year was thickly and completely covered with Enteromorpha
and other algae. The area to the west, well populated by Parucentrotus,
remained free of weed, but the area to the east of the cleared area,
previously free of algal cover, carried some large dense patches of algae.
The Paracentrotus collected during clearance of the area were transported to various sites in the lough. On 7 July 1959, 1 3 2 6 Paracentrotus were dropped in an elongated heap on the western section in