Southern Ocean Sea Butterflies at Risk as Ocean Warming and Acidification Emerge Due to Climate Change

Pteropods are a group of tiny sea snails that serve as calcifying zooplanktons in the marine ecosystem. Also known as sea butterflies, these mollusks are not well-studied especially in terms of their life cycle and population dynamics. Despite their relatively small size, pteropods play a significant role in different ocean ecosystems.

A group of marine researchers conducted a study regarding the life cycles, abundance, and variability of these pelagic gastropods in a region in the Southern Ocean that experiences some of the fastest climate change.

Delving Into the Depths

In a study published in the National Library of Medicine, a team of scientists led by Dr. Clara Manno from the British Antarctic Survey investigated the potential impact of exposure to rising ocean temperature on the early life stage and shell morphology of pteropods.

Manno and her team gathered sea butterflies in a sediment trap from the region of north-east Scotia Sea 400 meters deep. Since it is impossible to monitor the full life cycle of pteropods in the laboratory, the researchers used sediment trap samples whose life cycle were reconstructed by putting together information about their spawning and population structure.

Two dominant species were gathered, Limacina rangii and Limacina retroversa, and their contrasting life cycles were observed. As a polar species, L. rangii is present as adult and juvenile during the winter months. Meanwhile, L. retroversa is a subpolar species and can be found only as adults during this season.

The researchers found out that sea butterflies exposed to perturbed conditions showed 39% larval mortality. Rise in water temperature and ocean acidification (OA) also resulted in severe shell malformation and dissolution which can lead to increased shell fragility. There is also a decrease in shell growth depending on treatments and exposure time. It demonstrates that short-term exposure of pteropods through OA hotspots poses a serious risk and long-term population viability.

A similar study published by National Oceanic and Atmospheric Administration reveals that carbon dioxide emissions due to human activity results to dissolving of shells of pteropods that thrive in waters off the U.S. Pacific coast.

During the winter season, the water in the ocean is more acidic than during the other times of the year because the low temperatures increase the dissolution of carbon dioxide in the ocean. As a result, the life cycle of microscopic marine sea butterflies that exist at this season is more exposed and more vulnerable to the threats of rising levels of ocean acidification.

Our Oceans Are at Risk

According to the United Nations, the Earth's oceans absorbs about 25% of all carbon dioxide emissions and captures 90% of the excess heat produced by these emissions, making them a very important component in mitigating the effects of greenhouse gases. During the process of absorption, carbon dioxide reacts with seawater resulting to lowered ocean pH levels and reduced carbonate ion concentrations. Studies reveal that the pH level of the surface ocean waters in our planet has dropped by 0.1 pH units since the beginning of the Industrial Revolution in the 18^th century.

Marine organisms use carbonate ion in building and sustaining their shells. Aside from sea butterflies, other organisms such as oysters and corals rely heavily on the combination of calcium and carbonate ion.

Meanwhile, non-calcifying organisms are severely affected by ocean acidification as well. Some fish such as clownfish decrease their ability to detect predators in highly acidic environments. With lower pH levels, larval clownfish also lose their ability to locate suitable habitat. When these organisms are threatened, the entire marine food web and our planet in general might also be at risk.