The Growth and Distribution of Gibbula umbilicalis (Da Costa) on a Rocky Shore in Wales

Abstract

The distribution of Gibbula umbilicalis (da Costa) in the British Isles has been studied by several workers. The most recent surveys are those of Southward & Crisp (1954) and Crisp & Southward (1958). These authors found that it occurred all around the coast of Ireland. In Britain it occurred on the south coast as far east as Bembridge, I.O.W. and was fairly common on the west coast extending northwards to the Isle of Islay. Rendell (1956), however, records it as being occasional in the Orkneys. G. umbilicalis occurs locally throughout Cardigan Bay and its distribution in this region has been studied by Walton (1913) and more recently Evans (1947). At Aberystwyth Evans (1947) recorded that G. umbilicalis prefers flat rather sheltered shores. Though found occasionally on sandy shores and shores with a lot of loose shingle, it is more common on those with large areas of rock and small localized areas of bound boulders and a minimum of sand and shingle. Preference for this type of shore has been noted by other workers including Walton (1913), Colman (1933) and Southward & Crisp (1954). G. umbilicalis is tolerant of a wide variation of exposure though it has a marked preference for more sheltered habitats; it is also found over a wide vertical range. Colman (1933) states that at Wembury the vertical zonation is E.L.W.S.-M.T.L. (Extreme Low Water Spring tides-Mean Tidal Level). Moore (1940) describes the zone of abundance at several localities in the Plymouth area as M.L.W.S.-M.L.W.N. (Mean Low Water Spring tides-Mean Low Water Neap tides). Evans (1947) regards its vertical distribution in Cardigan Bay as being M.L.W.N.-E.H.W.N. (Mean Low Water Neap tides-Extreme High Water Neap tides), though in pools the upper limit may be, 'almost as high as M.H.W.S.' (Mean High Water Spring tides). Southward & Crisp (1954) state that the 'preferred tidal level' is M.T.L. and Moyse & Nelson-Smith (1963) record the vertical zonation at Dale in Pembrokeshire as being M.L.W.S.-M.H.W.N. (Mean Low Water Spring tides-Mean High Water Neap tides). In none of these investigations has a population of G. umbilicalis as a dynamic entity been considered in detail for any length of time. It was decided therefore to select a suitable population and over a period of 2 years study the detailed structure and any variations, seasonal or otherwise, which might occur. At the same time the breeding cycle and zonational pattern would be investigated.