Description: Gametangial phase grows from a small discoid holdfast producing a short stipe and a small blade of total length 1 cm. Often with very small blades growing from the margins. The tetrasporic phase (Erythrodermis allenii Batters) is a crust of up to 7 mm diameter, the tetrasporangial sori are slightly raised.

Habitat: The tetrasporic grows encrusting on broken earthware, stones and coralline algae in the sublittoral to depths of 11 m. The gametophytic phase is epilithic in shady rock pools of the lower littoral and in the sublittoral to 12 m depth

Distribution: Occasional but widely distributed in the British Isles from the Shetlands to the Channel Islands. Europe: France, the Baltic and the Faroes. Atlantic coast of North America: Canada, Maine, New Hampshire, Massachusetts, Rhode Island, Connecticut and New Jersey.

Key Identification Features:

• Gametamgial phase: small size and small proliferations from the margins.

Distribution Map from NBN: Erythrodermis traillii at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Erythrodermis traillii at iNaturalist World Species Observations database.

WoRMS: Erythrodermis traillii at World Register of Marine Species. Accepted name: Erythrodermis traillii (Holmes ex Batters) Guiry & Garbary, 1990. AphiaID: 145655.

Classification: Biota; Plantae; Biliphyta; Rhodophyta; Eurhodophytina; Florideophyceae; Rhodymeniophycidae; Gigartinales; Phyllophoraceae; Erythrodermis