- Kingdom: Animalia
- Phylum: Cnidaria
- Class: Scyphozoa
- Order: Coronatae
- Family: Periphyllidae
- Scientific name: Periphylla periphylla
- Norwegian: kronemanet
The umbrella is usually higher than wide. It may reach a height of 35 cm and a diameter of 25cm. The tip is pointed or dome-shaped. The large, red or orange stomach occupies the upper part of the umbrella. The 12 thick, orange tentacles can be more than 50 cm long. There is no information about a polypoid stage.
This is a common deep sea species. At locations dominated by strong tidal currents, forcing water from 200 meters or more up towards the surface, they can be encountered at any depth. At some locations this phenomenon is highly seasonal. An example is Lurefjorden (Hordaland) where water from 200 meters depth is forced into the fjord through a shallow (20 meters) and narrow (200 meters) sill entrance. In april every year, a large number of helmet jellies comes with the tide. At at other locations, like Skarnsundet, in the Trondheim Fjord, you may find them at any time of the year. Here the entrance is more than 200 meters deep and much wider than in Lurefjorden, so most of the helmet jellies pass below the common scuba diver. The maximum recorded depth is 2700 meters.
The helmet jelly is a cosmopolitan and widespread in the Atlantic Ocean, except in the Arctic. The Trondheim Fjord seems to be north of the "official" distribution area, but the helmet jelly does not know that.