Tritonia hombergi

Cuvier, 1803

This is the largest British nudibranch and may reach a length of 200 mm. The body ranges in colour from white to dark purplish brown (generally darker with age), lighter ventrally. The mantle bears numerous soft tubercles of various sizes. The dorso-lateral pallial gills are much subdivided and appear arborescent in adults. The five or six largest gills on each side are pedunculate and reflected towards the midline; the smaller gills project out laterally. The number of gills increases markedly with age. In juveniles, formerly regarded as a separate and distinct species (Tritonia alba ), white pigment forms patches mesial to each pallial gill. The oral veil is bilobed, even in the juveniles, and each lobe may bear up to 40 finger-like processes.
This is the only British opisthobranch known to be harmful to man, its secretions causes blisters on the skin.

This tritoniid is always found in association with Alcyonium digitatum , down to 80 m. Its diet in the Mediterranean Sea, where A. digitatum is absent, is a matter for conjecture.

It is known from the Faeroes, Norway (Grøtøy), the British Isles and Brittany, down to Portugal and the Mediterranean coasts of Spain and France (Distr. T. hombergi).