Polychaetes, commonly known as bristleworms, are common in the sediments of the northern European seabed. Among ther macrofauna (animals caught by a 0.5 mm mesh sieve) they are generally the dominant group, their numbers usually surpassing those of the molluscs, crustaceans and echinoderms combined. The majority of species are small and short-lived, and feature greatly in the diets of larger, more mobile, predators — including fish. Some species are collected or cultured (e.g. the king ragworm Nereis virens) as bait for fishermen.

Around 1000 species are present in British waters. Although the European polychaete fauna is quite well-known in comparison with certain other areas of the world (e.g., the tropical Indian Ocean), new species still await discovery. The Marine Biodiversity section is actively involved in investigating the polychaetes in the waters around Wales and further afield. Within the Irish Sea alone, we have discovered at least 20 species worthy of further investigation. Many of these are likely to prove new to science.

Apart from describing new species and redescribing poorly known ones, much remains to be discovered concerning relationships within the annelids and between them and other animal groups. New technology, including DNA analysis, is increasingly applied alongside morphological studies to improve our knowledge.