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DescriptionTritonia hombergi is the largest British nudibranch growing up to 20 cm in length. The body varies in colour from white to a dark purple brown and is lighter ventrally (colour darkens with age). The rhinophores are branched at the tip with basal sheaths. The mantle is covered with numerous soft tubercles, which release an irritant compound. On the sides of the upper surface there are noticeable gills, of varying size. The larger gills orientate towards the centre of the body while smaller gills flex out to the side. The total number of individual gills increases significantly with age. The front section of the head (the oral veil) is divided into two lobes and, characteristically for members of the genus Tritonia, each lobe is divided into numerous finger-like projections.Tritonia hombergi has annual life cycle and the juveniles were once thought to be a different species known as Tritonia alba.
Tritonia hombergii is one of the few opisthobranch thought to be harmful to man as its secretion has been reported to cause the skin to blister. However there have been no recent recorded evidence of this (Thompson & Brown, 1976).