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Field observations suggest that the common lugworm Arenicola manna (L.) has 2 forms on British shores although taxonomists have htherto mostly recognised it only as a single species showing some morphological variation. Using gel electrophoresis of enzyme systems in homogenised tissue from specimens collected around Swansea (South Wales, UK), we have shown that the 2 forms do not appear to share the same gene pool. The 2 forms are fixed for different alleles at 3 loci out of the 6 which proved to be consistently resolvable and show little similarity in the 2 variable loci at which alleles are shared. Only 4 alleles were found to be common out of 22 investigated. A high value for Nei's Genetic Distance (1.3032) and a low one for Genetic Identity (0.2717) also indicate that they are separate species. An observed heterozygote deficiency is probably due to the mixing of populations as a result of the extended pelagic dispersal phase of larvae and post-larvae.

A new species of lugworm Arenicola defodiens sp. nov. (common name ‘black lug’ or ‘runnydown’) is described from 148 specimens taken from intertidal sands around South Wales. Comparisons are made with the well-known and generally more common lugworm Arenicola marina (L.). To aid in diagnosis, a table illustrating differences in the genetics, ecology and morphology is also provided.

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