The majority of freshwater invertebrates are relatively small, the two exceptions being the decapod crustaceans and some bivalve molluscs. By far the largest mobile invertebrate in the British Isles is the crayfish. After the last ice-age a sinlge species, the White-clawed or Atlantic Stream Crayfish, came to inhabit our waters. Whether it was introduced from mainland Europe (where it is widespread) or derived from relict populations is uncertain.