The first in DIVE’s Miscellany of Marine Curios – Periphylla periphylla, the helmet jellyfish
The helmet jellyfish (Periphylla periphylla), is a species of crown jellyfish (Coronatae), and a vertical migrator, ascending from the depths at night to feed on copepods and other crustaceans.
They are found in oceans around the world – with the exception of the Arctic Ocean – at depths ranging from thousands of metres to near the surface.
Its transparent umbrella can be as large as 35cm high and 25cm wide, and its large red or orange stomach – which glows in the dark – occupies the upper part of the umbrella from which there are 16 lappets of the jelly’s body dangling around the margin. It has 12 stiff tentacles that are often held in an upward position.
More marine curiosities:
- Horrible natural histories – the sex lives of sea devils
- Ten weird and scary fish for Halloween!
- Horrible natural histories – the bobbit worm
- The horrible natural history of remora
- Marine curio #25: acorn worms
Periphylla jellyfish have four eyes, but no brain, no heart and – apart from their stomachs – no internal organs, yet they possess highly advanced sensory capabilities, enabling them to live a complex life and interact with other jellyfish, including an interesting behaviour which involves squirting slime at each other.
Helmet jellyfish reproduce sexually, the females having the largest eggs of all known Cnidarians, however, unlike most other jellyfish they have no polyp stage, developing instead directly into medusae which remain dependent on the yolk of their eggs for much of its 2-3 month growth period. They are the only known species of Scyphozoa – ‘true’ jellyfish – which do not go through the polyp phase.
They are adapted for the dark and spend most of their time during daylight hours at depths of 150m or more, rising to near the surface when darkness falls. In recent years there have been inexplicable aggregations of them at night in Norwegian fjords in waters as shallow as 15-20m – in calm seas sometimes even all the way to the surface.
