Jorunna tomentosa is usually sandy brown in colour with a loosely paired series of dark brown blotches down the sides of the body. It grows to about 6 cm in length.The mantle of Jorunna tomentosa is velvety, caused by the dense covering of small, uniform tubercles with an array of protruding spicules (caryophyllidia). Each tubercle has a characteristic retractile central projecting finger. There are up to 17 tripinnate gills, located within a gill pocket. The oral tentacles are short, slender and finger-like. The foot projects posteriorly when crawling. Jorunna tomentosa feeds on encrusting siliceous sponges, especially Halichondria panicea and Haliclona spp. Its colour and texture ensure this sea slug is well camouflaged amongst the sponges.Spawning is recorded February to August (Thompson & Brown, 1984). The spawn is a broad ribbon laid in neat, tight coil. Each spiral spawn mass contains up to 145,000 eggs (Thompson & Brown, 1984).