It seems we can only go so long before a deep-sea invertebrate sparks another rumour about alien life walking (or swimming) among us. We had the "whale placenta" jellyfish (Deepstaria enigmatica), then the big-fin squid (genus Magnapinna), and now the latest alleged extraterrestrial – Stygiomedusa gigantea, a ghostly jelly informally known as the "guardian of the underworld".


The gelatinous creature can reach 10 metres (33 feet) in length, and thanks to this video from the Monterey Bay Research Institute (MBARI), we get to swim alongside it. 

Despite its size, Stygiomedusa is rarely caught on camera. In the 27 years that MBARI's ROVs have been patrolling the deep-sea, they've observed the strange animal just seven times.

"It's one of the largest invertebrate predators known in the ocean," says the team. "Yet little is understood about its ecology and behaviour."

Those long, draping structures you see in the video aren't stinging tentacles as you might imagine – they're "oral arms", masses of spongy tissue typically used for feeding. It's thought that the jellies use these arms to envelop unsuspecting prey (before you swear off the ocean forever, you should know that this behemoth eats plankton and small fish). 


Stygiomedusa's red colouration helps it stay out of sight against the black backdrop of the deep sea. For the same reason the sky appears blue and the grass green, red fish look red at the surface because sunlight and its full colour bouquet can reach them. The fish absorb the light’s other colours, and reflect the red. But down at the depths the jellies call home (we’re talking up to 2,187 metres below!), red light is far too weak to penetrate. With nothing to reflect, Stygiomedusa simply disappears.

It's a tactic shared by many a deep-sea inhabitant, including my personal favorite: the bloodbelly comb jelly.

Top header image: MBARI