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.

This study demonstrates that the two species of lugworm, previously classified as Arenicola marina occupy distinct zones within sandy beaches around South Wales. Arenicola marina (sensu stricto) or blow lug, is found higher up the shore, tending to be concentrated at mid-tide level, whilst A. defodiens Cadman & Nelson-smith, 1993 or black lug is most numerous at or below mean low water of spring tides and, unlike A. marina in this area, also apparently extends subtidally. Arenicola marina is found in estuaries, unlike A. defodiens, and seems to inhabit slightly less exposed beaches. Degree of sorting, mean and median particle size do not appear to be a significant factor in determining lugworm distributions. Some possible explanations for these observed differences, including differences in burrow depth and salinity tolerance, are discussed.

Reproduction in the ctenostomate bryozoan Alqonidium mytili Dalyell, 1848, was studied over 2 years, revealing confusion in the literature over the diagnostic characters of the species and incorrect assumptions about the mode of reproduction. The literature is briefly reviewed and an approximate type locality from the Firth of Forth, Scotland, is identified. Alqonidium mytili is comprehensively redescribed from material collected in the Firth of Forth and from 2 years' collections in the Daugleddau estuary, near Pembroke Dock, South Wales. Zooid and polypide dimensions and modal tentacle number (16) have been determined from large samples. Most importantly A. mytili possesses a conspicuous intertentacular organ when reproductive; is oviparous, with eggs discharged through the intertentacular organ developing into cyphonautes larvae; and has a prolonged winter reproductive period. The contrary indications in the literature are a consequence of confusion with a superficially similar but larviparous species.