For a long time has been confused with H. norvegicus. The main differences may be found in the collar chaetae and operculum.
11-16 (19) pairs of radioli, arranged in semicircles, without basal membrane. Pinnules of almost equal length. Terminal filament of radioli of variable length (1-10 times as long as pinnule). Neither eyes, nor stylodes along radiole.
Operculum about 1 mm long, 1 mm wide, with a proximal serrated cup and a distal crown; centre of the crown with a small projection and spines fused to each other for about half of their length.
Peduncle is smooth, circular in cross section; inserted just below first and second normal radiole. Pseudoperculum present.
Collar is low, with entire edge; continuous with thoracic membranes, which form an apron across anterior abdominal chaetigers.
Collar chaetae with a denticulate zone at the knee, and a hairy fin.
Thorax with 6 uncinigerous chaetigers. No Apomatus-chaetae. Thoracic uncini with 7 (6-8) curved teeth.
44-65 (80) abdominal chaetigers. Chaetae are flat trumpet-shaped. Capillary chaetae present on about 8 posterior chaetigers.
Very young specimens without operculum.
Up to 29 mm long, usually about 20 mm. Width of thorax 0.9-1.6 mm.
Thin-walled, white, smooth or surface with weakly segmented rings. About 5 cm long and 1.4 (up to 1.8) mm wide. Subtrapezoidal to almost circular in cross section, with many fine, sinuous growth lines on the sides. Two low longitudinal keels may be present.
Body reddish, radiole with white and red bands. Operculum predominantly white, with red band over funnel.
On hard substrata; a typical harbour fouling species.
Originally from tropical and subtropical seas. Also found in the artificially heated waters of Shoreham Harbour Canal (English Channel), Bristol Channel (Swansea) and Kanaal door Walcheren (Vlissingen).