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(Johnston, 1837)

Probably the largest of the British tetrastemmids, attaining a length of 30-60 mm and a width of 2.0-2.5 mm when sexually mature. The general colour is typically yellow or yellowish-green, sometimes more reddish-brown, with a characteristic dark brown or black quadrangular pigment patch on the head. Occasional individuals may be found with minute brownish pigment flecks distributed along the lateral body margins. The mid-dorsal blood vessel arises from the left lateral vessel. Stomach with distinct dorsal infolding and leading directly into the pyloric duct.

Common but rarely abundant. Found intertidally in wet sand, in rock crevices, between laminarian holdfasts and on other fucoid algae. It sublittorally occurs at depths to about 40 m in a wide range of habitats. Sometimes found under conditions of reduced salinity.

Extends from the British Isles to Scandinavia, Madeira, the Canary Islands, the Mediterranean and the coasts of the Blank Sea.

Tetrastemma melanocephalum