What lives around 1,000m (approx 3,200ft) underwater, is the size of a chickpea and looks like the plump pink rear end of a pig? This question had scientists puzzled for a long time.
Teams from the Monterey Bay Aquarium Research Institute (MBARI) occasionally spotted these odd creatures while surveying the Monterey Canyon, off the coast of California, with remote-controlled submersibles.
But they couldn’t figure out what kind of animal it was.
undefined
It wasn’t until the submersible pilots managed to carefully collect a few specimens of these creatures that MBARI scientists had their first chance to study these strange animals up close.
When they did, they realised that they’re actually a kind of bristle worm, or polychaete, a huge group of marine worms that live throughout the ocean, but mostly on the seafloor. They also realised these floating worms are highly unusual.
Sequencing the animals’ DNA revealed them to be members of the Chaetopterus genus.
Unlike their relatives, which have classically ‘wormy,’ elongated bodies divided into segments, in this species one of the middle segments is inflated like a balloon – or a pig’s bum.
The worms’ other body segments are all squashed up at the front and back.
The scientists who solved the riddle of these peculiar creatures not only assigned them the common name of pigbutt worm, but included it in their choice of scientific name Chaetopterus pugaporcinus: puga from the Latin root word meaning ‘rump’; porcus meaning ‘pig’; and inus meaning ‘having the likeness of.’ This is officially the worm that looks like a pig’s rump.
Why it looks like this is less of a mystery. The inflated segment increases the worm’s buoyancy and helps it float.
Most other Chaetopterus worm species spend their lives crawling around on the seabed, but these ones seem to have said farewell to the bottom of the sea and drifted off to begin life bobbing about on the currents.
Pigbutt worms live mostly between 900–1,200m (2,950–3,930ft) underwater, which is where they find most of their food.
Scientists watching them in the wild have often seen them surrounded by a small cloud of goo.
They think this mucus acts like a net and catches falling particles of organic matter, known as marine snow, which is the pigbutt worms’ staple diet.
Live specimens in the lab have also shown they share a common ability with many deep-sea species.
Pigbutt worms glow in the dark. If you gently prod one their body glows blue. And if you watch carefully, you might see them emit a puff of green glowing particles.
These come from what’s technically called the mid-dorsal ciliated groove on the worm’s body – in other words, the part that looks like the crack between its buttocks.
This article was originally featured in the Q&A section of BBC Science Focus Magazine.
To submit your questions, email us at questions@sciencefocus.com, or message our Facebook, X, or Instagram pages (don't forget to include your name and location).
Check out our ultimate fun facts page for more mind-blowing science.