Japanese researchers take video of largest deep-sea bony fish in the world

TOKYO -- Japanese researchers have succeeded in taking a video on the ocean floor of a deep-sea bony fish that is over 2.5 meters in length. Named the "yokozuna slickhead," the creature is known as the largest bony fish species in the world among deep-sea fish that live in waters deeper than 2 kilometers, except for cartilage fish such as sharks and rays.

So far, six yokozuna slickhead fish have been caught in a deep-sea area in Suruga Bay of Shizuoka Prefecture in central Japan, with the largest one measuring 1.38 meters long. The fish was reported as a new species in 2021. The Japan Agency for Marine-Earth Science and Technology (JAMSTEC) was surveying the large deep-sea fish, and took the video on the ocean floor south of the bay.

The research team decoded genes by extracting DNA from the fish's scales and particles of dung contained in the deep-sea water as part of an ecological survey in a marine reserve south of Japan. They detected the yokozuna slickhead's genome sequence in samples collected at three locations about 400 to 600 kilometers south of the bay.

The team lowered a camera with a bait cage containing pieces of mackerel to the 2.09-kilometer-deep seabed approximately 400 kilometers south of Suruga Bay in October 2021, and the camera caught the image of a very large yokozuna slickhead. The footage showed the fish with its characteristic indigo blue eyes threatening and chasing away Pacific grenadiers that had gathered near the bait. The yokozuna slickhead was 2.53 meters in length, which was estimated from the size of the cage.

Before the finding of the yokozuna slickhead, the giant rattail had been the largest bony fish known to permanently live in the deep sea, with one caught measuring 2.1 meters long.

Yoshihiro Fujiwara, a senior JAMSTEC researcher, commented, "We measured (the yokozuna slickhead) many times because it was so big. There may be many other large new species in the deep sea."

The researchers' finding was published in a Swiss online scientific journal dated July 1.

The yokozuna slickhead is the largest species of the slickhead family. By eating other deep-water fish, they are at the pinnacle of the ecosystem in the deep sea; hence in 2021 they were named "yokozuna," which means sumo grand champion.

(Japanese original by Tomohiro Ikeda, Science & Environment News Department)