Extremely rare video of a deep sea predator known as the "Black Seadevil" has emerged after one was spotted by researchers descending into a dark trench in the Pacific Ocean.

The fearsome-looking anglerfish – which is actually only 9cm long – was caught on camera by scientists 600m below the surface in the Monterey Canyon off the coast of California.

It's only the sixth-documented time the mysterious fish, which captures its prey by ambush, has been caught on camera.

Monterey Bay Aquarium researcher Bruce Robinson said he believes the video is the first time a Black Seadevil has been filmed alive and at depth.

What little is known about the elusive anglerfish is truly horrifying.

Females of the species are believed to lure their prey with a luminescent orb dangling from their head and will attack fishes larger than her own size.

Males are tiny in comparison and lack fangs and a proper gut.

To survive, they are forced to latch onto a female's body and live as a parasite sucking nutrients from their host's body.

"If they don't find a female mate, they drown," University of Washington expert Ted Pietsch told the Santa Cruz Sentinel.

"They are not even properly equipped to eat."