An ecological account is given of the rocky shallow sublittoral of Lough Ine, County Cork, Republic of Ireland, from low water level to about 1 m below this level. With increasing distance from the Rapids mouth a forest of laminarian algae gives way to low algal bush, and still further into the lough the sea urchin
Paracentrotus lividus
destroys all soft upstanding algae. Much of the grazed area becomes overgrown by crustose coralline algae. Patches of the green algae
Codium fragile
ssp.
tomentosoides
and
Enteromorpha clathrata
fringe the
Paracentrotus
graze patches. An account is given of the effects of
Paracentrotus
on the shallow sublittoral community. Algae in the ungrazed areas accommodate large numbers of individuals and many species of invertebrates, especially amphipods, small gastropods and small polychaetes. In grazing,
Paracentrotus
destroys this population and its habitat. However, the removal of these algae, and of the sediment that they trap, provides a new habitat, suitable for plant and animal species that can resist
Paracentrotus
. Crustose coralline algae cover much of the rock, and are burrowed into or enclose the tubes of a characteristic and entirely different polychaete fauna. On the surface of the rocks are found saddle oysters (
Anomia ephippium
), limpets (
Patella aspera
) and other hard- shelled animal species. The relations between
Codium
and
Paracentrotus
have been investigated by transfer experiments in the ‘field’ and by observations with an aquarium tank.
Paracentrotus
readily eats
Codium
, especially when the urchins are at a high population density; but
Codium
benefits from the clearance of other algae, and is a quick recolonist, so that on balance it benefits from the presence of the urchin.
Paracentrotus
feeds mainly by day, and on a steep shore some wander up into the littoral region as the tide rises and destroy
Fucus serratus
. This accounts for the almost complete absence of
F. serratus
from the North Basin.
Paracentrotus
does not move upwards by night. The hard-shelled animal species
Anomia, Patella, Chlamys varia
, adult
Gibbula cineraria
and
Pomatoceros
, found plentifully on graze patches in the North Basin, diminish in abundance southwards even within grazed areas, while the numbers of the starfish
Marthasterias glacialis
under the rocks increase. Experiments show that
Marthasterias
readily eats
Anomia
. These
Marthasterias
are quite small. As they grow bigger they move away onto muddy areas and extend their diet to include large buried lamellibranchs. Small
Paracentrotus
are usually found in larger numbers underneath boulders than above, while larger ones tend to come up by day onto the tops, where they form graze patches. From a study of growth lines in the interambulacral plates and from growth in cages we conclude that a horizontal diameter of 30-40 mm (with considerable variation) is reached in 3-4 years from settlement. The peak in numbers of
Paracentrotus
visible in the South Basin in 1979 might perhaps be ascribed to a good settlement in the warm summers of 1975 and 1976. Temperature of the shallow marginal water of the lough is subject to diurnal fluctuation, as in a tide pool, and can reach high levels in summer. This might favour
Paracentrotus
. However, numbers of
Paracentrotus
are probably severely reduced by predators in the South Basin. Crabs have already been implicated. It is possible that the small or half grown
Marthasterias
under shallow sublittoral rocks might destroy newly settled
Paracentrotus
, although this still has to be demonstrated.
Water column stability as an important factor controlling nitrite-dependent anaerobic methane oxidation in stratified lake basins
