In Europe this species may reach 100 mm in length, translucent grey or white, marbled, speckled and streaked with white, orange, red and brown in various proportions (D. frondosus creeping). Specimens from deep coastal water (below 25 m) are usually paler than those from shallower habitats. There are also differences associated with age. Juveniles up to a length of 4 mm are pale (D. frondosus-juvenile), then from 4 to 30 mm the developing patterns may become vivid, forming stripes along the sides and back, with contrasting bands of colour on the cerata. In specimens over 30 mm, the pattern once again becomes drab.
The dorso-lateral pallial rim bears on each side of the body a row of up to nine arborescent ceratal processes. Similar processes protrude from the rhinophore sheaths and from the frontal margin of the head. The number and degree of branching of these processes increases with age. The foot is narrow and the whole body laterally compressed, so that it can be lashed from side to side in a feeble swimming escape reaction.
Juveniles feed upon hydroids such as Sertularia cupressina, Dynamena pumila and Hydrallmania falcata, while the adults (50 mm or more in length) attack Tubularia indivisa and T. larynx.
D. frondosus is common in the whole North Sea, but reaches its southern European limit on the French Atlantic coast near Arcachon (Distr. D. frondosus). It is known from the east Siberian Sea to the Barents Sea, and southwards from Jan Mayen Island to the east Greenland coast; Iceland, the Faeroes and Scandinavia. It is also amphi-Atlantic, having been recorded on the west Greenland coast, around northern Canada and southwards along the eastern American seaboard as far as New Jersey. Pacific Ocean records exist.