These tiny swimming snails with wing-like appendages are a important food source for Arctic whales. Their fragile aragonite shells are dissolving due to ocean acidification, making them a living indicator of climate change impacts.
A Canary in the Arctic Ocean
In the frigid waters surrounding Svalbard, a tiny creature named Limacina helicina provides a clear and worrying signal of global environmental change. Commonly known as the sea butterfly, this organism is not an insect but a pelagic snail, typically less than one centimeter long. Its "wings" are a modified version of the muscular foot found on all snails, allowing it to "fly" through the water column. These creatures are a part of the polar food web, sometimes making up more than 50% of the total zooplankton abundance. They are a food source for herring, salmon, seals, and several whale species.
The immediate threat to Limacina helicina is ocean acidification, a direct consequence of the ocean absorbing atmospheric carbon dioxide. Polar regions are acidifying faster than other marine environments because colder water can absorb more CO2. This process reduces the availability of carbonate ions, which these snails need to build their shells. The shells of Limacina helicina are made from aragonite, a form of calcium carbonate that is 1.5 times more soluble than calcite, making them exceptionally vulnerable. Studies have already documented extensive shell dissolution and damage on live pteropods captured in the Arctic.
A Chemical Dissolution
The waters around Svalbard and across the Arctic are becoming increasingly corrosive to aragonite shells. The aragonite saturation state (Ωarag)—a measure of how easily aragonite can form or dissolve—is an indicator. When Ωarag drops below 1, the water is undersaturated and aragonite begins to dissolve. Large areas of the western Arctic Ocean, particularly in the Canada Basin, already experience seasonal undersaturation, and this corrosive zone has more than doubled in area between 1980 and 2013. In fjords near Svalbard, the influx of glacial meltwater can further exacerbate acidification.
Researchers have found that pteropod shells in more acidic waters are significantly thinner. One study off the U.S. West Coast found shells were 37% thinner in upwelled, low-pH waters. While pteropods show some ability to repair damaged shells, this process comes at a high metabolic cost. One specimen from the Fram Strait, between Svalbard and Greenland, had generated repair material four times the thickness of its original shell to maintain integrity after dissolution and mechanical damage. The long-term ability of these snails to cope is uncertain. A decline in their population could have severe consequences, disrupting the flow of energy up through the food chain from plankton to polar bears.
💡Fun Facts
The sea butterfly captures its food, mainly phytoplankton and smaller zooplankton, by secreting a spherical mucus web that can be several times its body size.
When disturbed, *Limacina helicina* will abandon its mucus web and retract into its shell.
The species is a protandrous hermaphrodite, meaning individuals begin life as males and later transition to females.
Dead pteropods sink rapidly, with a measured rate of up to 2,000 meters per day, playing a role in transporting carbon to the deep ocean.
The Arctic Ocean is a natural environment with no opening hours, but access is governed by extreme weather conditions, sea ice, and the polar night. Research and tourist expeditions typically operate during the summer months.
Admission
Access to the open ocean is free but requires specialized transport, such as an ice-strengthened vessel.
Accessibility
The terrain is the Arctic Ocean itself. It is one of the most remote and challenging environments on Earth, accessible only via organized expeditions.