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(Johnston, 1837)

Description:
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.

Habitat:
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.

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

Tetrastemma melanocephalum