The spatial organization of an intertidal Spirorbis community

Abstract

Five species of Spirorbis co-occur amongst the macro-algae and rocks of a semi-tidal basin behind a marine waterfall in Strangford Lough, Northern Ireland. Only Spirorbis borealis Daudin and S. pagenstecheri Quatrefages proved significant competitors, S. corallinae de Silva & Knight-Jones being isolated from these by its use of Corallina as substratum, and S. tridentatus Levinsen and S. vitreus Fabricius occurring only on rocks and only in low numbers. The density and growth rates of S. borealis on its major settlement plant, Fucus serratus L., was limited by intraspecific competition and by the characteristics of the plants themselves. Its use of sites of different environmental characteristics was limited by competition from bryozoans, particularly Flustrellidra hispida (Fabricius). Its niche breadth on Fucus serratus increased under intraspecific competition and in the stable conditions provided by quiet rather than agitated waters. Spirorbis pagenstecheri had a wider niche in respect of substratum than had S. borealis but was normally the superior competitor only when on Ascophyllum. Under certain environmental conditions Spirorbis pagenstecheri displaced S. borealis from the normal niche of the latter on Fucus serratus. Although the two species differed in their responses to almost every variable examined at site, plant, and within plant levels only certain combinations of variables proved significant niche dimensions in the competitive organization of the community.