Competitive interactions amongst sessile organisms and their mediation by grazing Echinus esculentus in the infralittoral and circalittoral communities on Whirlpool Cliff, Lough Hyne, County Cork, Ireland were investigated using SCUBA

Lough Hyne is a sheltered sea lough, the sublittoral cliffs of which support a diverse assemblage of encrusting sessile organisms. Space on the subtidal cliffs, particularly Whirlpool Cliff, is limited, with percentage cover often reaching 100 percent or greater and leading to competition for space amongst settled organisms. The sheltered nature of the Lough minimises physical disturbance, hence the major factors responsible in structuring the subtidal communities are biological interactions. Previous studies have suggested both the importance of interference competition amongst settled organisms and grazing by Echinus esculentus in influencing species abundance and distribution on Whirlpool Cliff.

The study aimed to investigate suggestions that factors such as thickness and size are important determinants of competitive ability amongst sessile, encrusting organisms and that the results of such competitive interactions are mediated by the dominant grazers in the community The in situstudy allowed detailed observations to be made over all successional stages in the communities and a number of dissections to be carried out

A total of 426 encounters were observed in the infralittoral and 489 in the circalittoral communities, involving 28 and 32 species respectively Of these, 308 (72%) in the infralittoral zone and 273 (56%) in the circalittoral zone, were overgrowth interactions and the remainder were stand-offs Stand-offs were found to be an important space retaining strategy, especially amongst colonial ascidians and sponges. The Corallimorphanan, Corynactis viridis accounted for the majotity of interactions in both communities C viridis and colonial ascidians were the competitively dominant groups (Figure 1 and 2)

The encrusting algae Lithothamnion sp and Hildenbrandia rubra had the lowest competitive abilities in both communities Generally, solitary organisms had a lower competitive ability than colonial forms The competitive hierarchy of the main encrusting groups in both communities followed the pattern

Colonial ascidians =Corynactis viridis = Sponges > Barnacles and Serpulids > Algae

The ability of Corynactis viridis to monopolise space on the cliff was attributed to the anemones ability to resist overgrowth via its powerful nematocysts. It is suggested that the anemone is able to acquire space via rapid asexual proliferahon once space is made available. During an eight day photographic study of 98 individuals, none moved by pedal locomotion as has been suggested recently in studies carried out on Whirlpool Cliff

There was no significant correlation between species competitive ability (expressed as a ratio of number of wins to losses or no of wins to the total number of wins in the community) and the mean marginal thickness in the infralittoral community (w/l rs = 0 297; w/wtot rs = 0.036,6 d f., p<0.05) and a significantly negative correlation in the circalittoral community (w/l rs = -0 793; w/wtot rs = -0.782,9 d.f., p<0.05

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