Helmet Jelly

• It is one of the very few true jellyfish to have done away with the fixed polyp stage: the egg develops straight into a medusa, which lets it live permanently in open water without depending on a seabed.
• It shuns light to an extreme degree, and in some Norwegian fjords with dark, stagnant water it has proliferated into extremely dense populations that have displaced the fish.
• It is intensely bioluminescent and flashes blue when disturbed.
• It is photophobic in the extreme: light damages it, which is why it lives in the permanent darkness of the deep.
• Its bell is a deep reddish brown, a colour that at depth amounts to being invisible, because red light does not reach that far.
• The stomach, also dark red, hides the bioluminescence of the prey it has just swallowed, so as not to give away its position.
• It has invaded several Norwegian fjords, forming populations so dense that they have displaced the fish and disrupted the local fishery, an uncommon case of a jellyfish establishing itself permanently in a coastal ecosystem.
• Unlike almost all other scyphozoans, it has no polyp stage: it develops directly from egg to medusa.
• It can live up to thirty years, an extraordinary lifespan for a jellyfish.

Habitat & environment

Deep oceanic waters and fjords, between 200 and 2,700 m

Sources consulted: Péron y Lesueur (1810); Jarms et al.; Sørnes et al. (2007); Institute of Marine Research (Noruega)