Helmet Jellyfish

Periphylla periphylla

Oh, dear. This is a difficult one. Do I want the shiny, grey helmet that's made of metal? It's hard and does what helmets are supposed to do, which seems to be a good attribute for a helmet...

On the other hand, do I want the pink helmet that's made of jelly... and has tentacles? It's soft... but it has tentacles! What a dilemma!

The Helmet Jellyfish is a large, deep sea jellyfish that has been found in every ocean of the world except the Arctic. Having said that, they actually really like the cold. They grow to their biggest size in frigid, Antarctic waters, where they may reach 25 cm (10 in) across and 35 cm (14 in) tall.

They belong to the order Coronatae, the crown jellyfish, which includes Atolla and numerous other species who look like some form of headgear.

Video: wargwind

In the case of the Helmet Jellyfish, it's a particularly odd kind of headgear. It's a helmet with a tentacle-teepee on top! It also uses bioluminescence to light up. I think that settles it, I want the pink, light-up helmet with tentacles.

Helmet Jellyfish have 12, fairly long and surprisingly rigid tentacles. These aren't the kind of flimsy, dingle-dangle things we normally associate with jellyfish. The Helmet is able to hold them right up so that they point the way as it swims.

Video: Varamaski co

Every night, the Helmet Jellyfish ascends from the murkiest depths and enters its hunting grounds. They seem to be ambush predators of agile, flighty copepods who would keenly flee from any sign of a big bag of hungry stomach heading their way. The pumping bell of a jellyfish provides just such a sign, so the Helmet Jellyfish goes in tentacles-first and uses unusually large stinging cells to catch its prey unawares.

Helmet Jellyfish are also unusual in another way. Most jellyfish begin their lives as tiny, slug-like larvae that settle on the sea floor to metamorphose into a polyp which looks and lives a lot like a tiny sea anemone. This is what eventually turns into not one, but several miniature jellyfish that each grow to adult size.

Helmet Jellyfish are different in that they skip all those early stages. They are one of the few jellyfish who's eggs hatch directly into miniature jellies. They can go on living for another 30 years, which is astonishing! Most jellyfish have lifespans measured in months.

Video: Laurent Miroult

Now, I just know you're asking one thing and one thing alone: What are the chances they'll visit Norway some day? The chances would appear to be quite high. Over the past few decades, the deep-yet-narrow fjords of western Norway have been getting crammed full of ludicrous numbers of Helmet Jellyfish. Populations in some fjords have been measured at 40,000 tons of jelly-weight!

This is terribly bad news for all the resident fish, of course. Helmet Jellyfish are good hunters and will eat not only the fish's food, but even baby fish when they're small enough. Research is ongoing into what's causing quite so many jellyfish to enter and flourish in these fjords but it also raises another question which I suspect we'll never truly find an answer to:

How would history differ if the Vikings stormed into battle wearing pink, light-up jelly-helmets with tentacles on top?

Just one of life's unknowables.