Photo/Illutration A Clione limacina swims off Hokkaido’s Rausu in February. (Tomoyuki Yamamoto)

Clione, translucent sea slugs dubbed “angels in drift ice,” could be wiped out off eastern Hokkaido because increasingly acidic seawater from carbon dioxide emissions is destroying their sole food source, studies show.

In waters of Japan’s northernmost main island, Clione limacine, a 2-centimeter-long variant also known as the naked sea butterfly, feast on a small sea snail species called Limacina helicina, whose clear, glass-like shells measure 3 to 5 millimeters in diameter.

Katsunori Kimoto, a senior researcher at the Japan Agency for Marine-Earth Science and Technology, said the snails’ shells are dissolving and developing holes in the Arctic Ocean.

Kimoto said higher concentrations of airborne CO2 from human activities, such as burning fossil fuels, have increased the volume of CO2 that dissolves in seawater.

The resultant chemical reactions have decreased the hydrogen-ion concentration measured in pH in oceans.


According to the Japan Meteorological Agency, the pH level for surface seawater around the world is about 8.1, meaning the alkaline level is still strong. But the figure is projected to reach a more acidic 7.7 by the end of this century.

The phenomenon is making it difficult for Limacina helicina to maintain a healthy external structure because its shell is made of calcium carbonate, which is vulnerable to dissolving in high acidity levels.

“Shells with holes can allow in bacteria, causing fatal infectious diseases or make it impossible for the creature to swim in a balanced manner, which makes it easier to being preyed upon by its natural enemies,” Kimoto said.

He stressed that the worsening ocean acidification could lead to the extinction of Limacina helicina, citing experiments that showed acidified seawater eradicating the variant in a tank.

CO2 dissolves more easily in colder seawater like in the Arctic Ocean, so the acidity trend’s effects are expected to be seen in waters off Hokkaido earlier than in other places in Japan.

Tomoyasu Yamazaki, director of the Shellfish Museum of Rankoshi in Hokkaido, said the extinction of Limacina helicina would mean the end of clione.

“Clione has evolved to live exclusively on Limacina helicina,” Yamazaki said.

Ocean acidification has also been reported in other areas of the world, including the North Pacific off the United States.

Damage to Limacina helicina’s shells could have a wide negative impact on the food industry.

According to Kimoto, the snail variety is an important nutrient source for salmon, cod and herring.

Ocean acidification is problematic for other shellfishes.

Abnormalities have been detected with larvae from an edible crab species. And young oysters died in large numbers off the U.S. west coast in the 2000s.

There are concerns that oysters in Japan will face such problems in the future.


In mid-February, a diving tour to see clione was held off Rausu on the Shiretoko Peninsula in eastern Hokkaido.

Participants checked out a “konbu” kelp forest thriving on the seabed in the minus 1 degree water.

Diving instructor Tomoyuki Aoyagi, 53, then spotted a Clione limacina, whose semi-transparent body allowed views of its vivid red internal organs. Its slowly flapping lobes resembled angel wings as it drifted in the water.

Also discovered during the tour was the Limacina helicina.

Clione can be found off Rausu between January and May every year.

In the peak February-March period, divers show up for clione from around Japan and overseas.