Mainly mid and lower shore sandy mud or mud in lower estuaries, sheltered bays and marine inlets, often subject to variable salinity. The main characterising species are the ragworm Hediste diversicolor, the baltic tellin Macoma balthica, and the oligochaetes Tubificoides benedii and T. pseudogaster. Further polychaetes that are often common or abundant include Pygospio elegans, Streblospio shrubsolii, Tharyx killariensis, Aphelochaeta marioni, Capitella capitata and Manayunkia aestuarina. The oligochaete Heterochaeta costata and the mud shrimp Corophium volutator may be abundant. The spire shell Hydrobia ulvae is often common. Other species which occur in a significant proportion of samples include the polychaetes Eteone longa and Nephtys hombergii, and bivalves such as the cockle Cerastoderma edule and Abra tenuis. The sand gaper Mya arenaria is superabundant in about a quarter of the samples for this biotope. M. arenaria is probably present in a higher proportion of areas of this biotope, but may be missed in core samples due to its size. Situation: This unit may occur on the mid/lower shore of lower estuarine shores, with units A2.243 or A2.241 on the upper shore. A2.313, A2.3221, NhomAph, and A2.3222 may be present on the same shore. Temporal variation: Enteromorpha spp. and Ulva lactuca may form mats on the surface of the mud during the summer months, particularly in areas of nutrient enrichmnent.