Welsh Journals

THE DISTRIBUTION AND ECOLOGY OF SPHAEROMA (ISOPODA) ON ANGLESEY, NORTH WALES D. J. HEATH INTRODUCTION Four species of the isopod genus Sphaeroma occur in southern Britain and in Europe. S. serratum (Fab.) is a marine species, S. monodi (Bocquet, Hoestlandt and Levi) is found in the middle reaches of estuaries, S. rugicauda (Leach) is found higher up the estuary, whilst S. hookeri (Leach) is found at the uppermost limits of tidal influence (Lejuez, 1966; Harvey, 1969; Harvey, Jones and Naylor, 1973; Naylor 1972.) The distribution of these four species along an estuary is probably governed by their different tolerances to water of low salinity. S. serratum survives least well and S. hookeri survives best (Lejuez, 1966; Kerambrun and Szekielda, 1969; Jansen, 1970; Harris, 1971; Harvey, Jones and Naylor, 1973). In Britain, S. serratum is near the northern limits of its range and is found only along the south and west coasts of England and Wales and as far north as Anglesey (Hoestlandt, 1955a; Harvey 1969). Hoestlandt (1955b) considered that it is probably limited in the north by sea-water temperatures. S. monodi in Britain is also near the northern limits of its range (Harvey, 1969; Naylor, 1972), and has been recorded only in South Wales and eastern Ireland. S. rugicauda is common in Britain (West, 1964), but S. hookeri is patchily distributed (Harvey, 1969), and is found regularly only on the south and east coasts, and also on the coasts of Europe, as far north as the Baltic. The results presented here are from a survey of estuarine localities on the island of Anglesey, North Wales.This area lies midway between the areas of South Wales and eastern Ireland where both S. monodi and S. hookeri occur, although neither species has been recorded on Anglesey. Anglesey also represents the most northerly limit of S. serratum. S. rugicauda has been recorded on Anglesey (West, 1964). The object of the survey was to verify the old records for S. serratum and S. rugicauda and also to determine the distribution of the three estuarine species. The estuarine species were also scored for their colour polymorphism (West, 1964; Lejuez, 1966). METHODS A number of localities on Anglesey were sampled in May and October, 1974. Samples of isopods were collected, either from under stones or with a net from salt-marsh habitats at high tide, or from salt-marsh pools. The number of isopods were counted and a subjective estimate of abundance was also made. Species were identified acording to Naylor (1972). Notes were made on the habitat, including the dominant vegetation and the salinity of the water. The three localities, (Trearddur Bay, Porth Nobla and Porth Cwyfan) in which S. serratum was found by Hoestlandt (1955a) were revisited.