🧊 Divers in northern Japan have long called this animal "Ikuone," after the photographer whose book cover first made it famous. This summer it got a scientific name of its own, Clione ikuoi. It is the sixth known species of sea angel, and the third to come out of the seas around Japan in 10 years.

A nickname from Shiretoko

Sea angels are small, see-through swimmers of cold seas. In Japan they go by "kurione," from the genus name Clione, and are often called the angels of the drift ice. They swim by flapping two wing-like lobes.

Ikuo Nakamura, an underwater photographer who is now 81, met this one in the 1990s, he told the Asahi Shimbun. He was diving under drift ice off the Shiretoko Peninsula in eastern Hokkaido when he spotted two of them floating together as a pair, about 10 meters down. Their outline was rounder and plumper than that of the common sea angel. His photo went on the cover of his 1995 book Kamui no Umi, published by the Asahi Shimbun.

A Shiretoko dive guide who is also an underwater photographer began calling the animal "Ikuone," a blend of Ikuo and Clione, and others picked it up. Nakamura told the paper he felt uneasy about it at first. The nickname spread among divers anyway, and the formal Japanese name followed it: Ikuo-hadaka-kamegai.

Six arms, in pairs

Scientists had placed Ikuone in a different genus, as Paedoclione doliiformis. Sea angels feed with buccal cones, tentacle-like arms that shoot out from around the mouth. In the genus Clione there are six, arranged in three matching pairs. Paedoclione was defined by cones that are not symmetrical and by a tiny, barrel-shaped body. According to the Asahi, Ikuone was thought to have only four.

A team from the Toyo Institute of Food Technology in Kawanishi, Hyogo, and the Okhotsk Sea Ice Museum of Hokkaido in Mombetsu re-examined living animals, recorded their mating behavior and studied their mouthparts under light and electron microscopes. Ikuone has six buccal cones in three symmetrical pairs. The Asahi reports that genetic analysis pointed the same way. The paper by Tomoyasu Yamazaki and Takashi Kuwahara, published in July 2026 in the Journal of Asia-Pacific Biodiversity, moves the animal into Clione and names it Clione ikuoi in Nakamura's honor.

The paper describes an animal about 25 millimeters long, with a translucent body and an orange to reddish-orange core. The specimens used to describe it came not from Shiretoko but from the Kitami-Yamato Bank in the Sea of Okhotsk off Hokkaido.

The angel is a hunter

Clione limacina, the Arctic sea angel, photographed during a NOAA expedition. This is not the new species.

Kevin Raskoff, NOAA Office of Ocean Exploration (public domain, via Wikimedia Commons)

The angel label hides what these animals do for a living. Sea angels are snail relatives whose adults have no shell, and they are predators. Clione limacina, found in the Arctic Ocean and the cold North Atlantic, eats almost nothing but sea butterflies, tiny swimming snails that still carry shells.

When it strikes, the buccal cones flick out and grip the prey's shell. Small hooks then pull the soft body free, and the sea angel swallows it whole.

Four of the six since 2016

In 2016, the sea angels of both the North Atlantic and the North Pacific were still filed under one name, Clione limacina. That year Yamazaki and Kuwahara described Clione okhotensis from shallow water off eastern Hokkaido (online in 2016, in print in 2017). In the same paper they proposed treating the North Pacific population as a separate species, Clione elegantissima.

In 2025, Yamazaki and two colleagues added Clione japonica, found in deep water in the Sea of Japan and in shallow water in the southern Sea of Okhotsk. Clione ikuoi now makes six.

Of the six species accepted today by the World Register of Marine Species, four were named or proposed as separate species in papers Yamazaki co-authored since 2016. Three of them were described from the seas around Japan.

A shrinking sea of ice

All three of the Japanese species have been recorded in the southern Sea of Okhotsk, the stretch of water off Hokkaido where drift ice arrives each winter. That ice is shrinking. Japan Meteorological Agency data for 1971 to 2026 show the maximum ice extent in the Sea of Okhotsk falling by about 59,000 square kilometers per decade, equal to 3.7% of the sea's area. The agency says warming may play a part but that more data are needed to say for certain.

The Clione ikuoi paper also looked at long-term sea surface temperatures off Mombetsu. It suggests that continued warming may shorten the cold-water season when sea angels are seen near the coast.

Ikuone had been on a book cover for about 30 years before it got a correct name. Is there a creature in the waters near you that everyone recognizes on sight, and that nobody has looked at twice?

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