Helmet jellyfish

About Helmet jellyfish

If you’ve never seen a helmet jellyfish in real life, you’re in good company: it’s one of the few jellyfish (to be accurate, cnidaria) that spends most of its life in the ocean twilight zone. Due to their photo-sensitive red pigment, helmet jellies avoid sunlight like the plague, preferring the frigid depths to the sun’s damaging rays. That red pigment is nonetheless useful for warning would-be predators— and also disguising the bioluminescent prey in their bellies. Like their Coronate cousins, the Atolla jellyfish, helmet jellies display bluish-green lights along a prominently grooved “crown”, a further warning to fish and sea turtles to stay away.

Unlike other jellyfish, these red-helmet types hatch straight from the egg to juvenile stage. They hang out in all oceans except for the Arctic, but could be shifting their range northward. In recent years, the species has proliferated in Norwegian fjords as far north as Svalbard, causing some alarm amongst fishermen who feared that these glowing nightly apparitions on the surface would eat up all the juvenile cod and haddock. This unwelcome guest has nonetheless provided scientists with an opportunity to study helmet jellyfish’s habits, which previously could only take place at depth.

Helmet jellyfish lack brains and eyes, but make use of a simple sensory “bulb” that detects changes in light. When the sun comes out, that’s the helmet jellyfish’s cue to retreat to the murky safety of the twilight zone. However, scientists have been puzzled by the lack of day-night pattern that most other diel vertical migrators follow. They point to the gradual “darkening” of coastal waters due to nutrient runoff as one reason the helmet jelly might be able to survive the light of day.