There is a diaphanous creature in the deep sea that looks like a jellyfish. The group of animals known as the Pelagothuria natatrix lie around on the sea cucumber like giant rubbery worms.

Pelagothuria: species of sea cucumber spotted in deep sea dive – video

For a long time, this sea cucumber was only known from a few battered specimens brought up in scientific nets. Chris said that they are almost to the point of being intangible. They are very difficult to study.


The ocean is a wild place. Sometimes it seems to border on the absurd, from fish that look up through transparent heads to golden snails with iron armour. We know a lot more about deep space than we do about the deep ocean.

As mining companies push to industrialise the sea floor and global leaders squabble over how to protect the high seas, a new Guardian Seascape series will profile some of the most recently discovered weird, wonderful, majestic, ridiculous, hardcore and mind blowing creatures. They show how little is known about the environment on Earth.

When he looked through a database of deep-sea images, he spotted an umbrella-like Pelagothuria that was mistaken for a Jellyfish. Only a few scientists were aware of the species. People looked out for them during deep-sea surveys. A team of scientists working in the Pacific Ocean got a great view of the gossamer creatures in their natural environment. Video footage of Pelagothuria was beamed up in real time from a deep-diving robot on the research ship. During the course of nine dives between American Samoa and Hawaii, they spotted close to 100 of these swimming sea cucumbers at depths ranging from 196 to 4,450 metres and in areas with very low oxygen in the water. The tactic could be used to avoid predator that are more oxygen-hungry.

Pelagothuria, the swimming sea cucumber that looks like a jellyfish

Pelagothuria natatrix was first named in 1893. Photograph: NOAA/OER

The mystery of how Pelagothuria can survive in these challenging conditions is still being solved. Animals living in the deep sea have bodies made mostly of water with a small amount ofCollagen mixed in. It's ideal for animals living at depths because it requires little energy to make and maintain. Jelly-based animals don't waste precious energy and oxygen swimming vigorously to stay afloat so they can just drift around. Pelagothuria is the only sea cucumber that spends most of its time swimming. The web surrounding it's mouth propels it through the water column. Some sea cucumbers are swimmers. They are able to swim when they want to. A starfish's approach can cause a sea cucumber to move. A short time in the water can be enough to escape.

Photo of a transparent Cystisoma with a black background

The crustacean with eyes for a head was discovered in the deepest part of the ocean.

This could be how Pelagothuria's ancestors started out, evolving to be better and better swimmers until they adopted a full-timeJellyfish lifestyle. There is a case of convergent evolution in which distantly related organisms have solved challenges with a similar outcome. Every creature will have its own story as to how it got there.