Body relatively short, thorax broad, abdomen narrow. Prostomium without glandular ridges and with two eyespots. Gills in two groups close together at middorsum; the three gills of each group arranged in a triangle and connected at the bases.
Sixteen thoracic segments with notopodia with capillary chaetae, the posterior thirteen also with neuropodia with uncini. Abdomen with 13-19 uncinigerous segments. Thoracic uncini with one row of 4-5 teeth, abdominal uncini with two rows of teeth with 4-5 teeth in each row (A. romijni-detail). Pygidium without cirri or papillae.
Up to 5 mm.
A very thin, transparent layer of secretion encrusted with small sand grains and silt in a reddish matrix.
Whitish to brownish, often with brownish transverse lines on the ventral side.
On silt, more or less mixed with sand; eulittoral to a depth of a few meters. A typical brackish water species, living and breeding in polyhaline to oligohaline waters.
The Netherlands to Denmark (Ringkobing fjord), Östfold, Kieler Canal, Baltic Canal, Baltic Sea; Morocco.