The study of a number of sandy beaches covering a range of environmental conditions revealed faunistic variation which reflected the different species' tolerance of or preference for the conditions prevailing on particular beaches. Whenever a balanced situation in the environmental conditions existed, especially between shelter and exposure, a typical marine north-temperate water community characterised by the bivalve Tellina tenuis was present. This comunity included a basic faunistic complement consisting of polychaetes (both sessile and errant) on the mid and lower shore, and a few crustaceans on the upper shore. Severe exposure restricts diversity, eliminating sedentary forms, especially bivalve molluscs and, encouraging the numerical dominance of agile swimmers such as haustoriid amphipods and isopods. These species have a short life-span and the fauna is characterised by its flexibility. Extreme shelter favours the establishment of a predominantly sessile community of polychaetes and long-living bivalves.
