Newswise — When carbon dioxide (CO2) enters the oceans, it can make the water more acidic, which is called ocean acidification. This can make it harder for some ocean creatures, like pteropods, to build and keep their shells. Pteropods are important for the ocean environment.

Pteropods are small sea creatures, including shelled sea butterflies. Not much is known about them because of their size, and they don't survive well in captivity. A team of marine scientists studied the life cycles, abundance, and seasonal changes of shelled sea butterflies in the north-east Scotia Sea, where climate change is happening quickly.

Dr Clara Manno, a researcher at the British Antarctic Survey and the corresponding author of the study published in Frontiers in Marine Science, said that the decrease in pteropod populations in the Antarctic Ocean could have significant consequences for the food web and carbon cycle. She also said that understanding the life cycle of this important organism could help to predict the impact of ocean acidification on the Antarctic ecosystem.

Population stability essential for species survival

To study the life cycle, abundance, and seasonal variability of shelled sea butterflies in the north-east Scotia Sea, a team of marine scientists collected sea butterflies in a sediment trap, a sampling device that was anchored at a depth of 400 meters. Since observing the entire life cycle of sea butterflies in a laboratory setting is impossible, the researchers had to gather information about their spawning, growth rate, and population structure. They were able to reconstruct the sea butterflies' life cycle for a year using the samples from the sediment trap.

The scientists found that the two most commonly collected species, Limacina rangii and Limacina retroversa, have different life cycles and therefore have different levels of vulnerability to changes in the ocean. During the winter months, L. rangii, which is a polar species, can be found as both adults and juveniles, while L. retroversa, a subpolar species, is only found as adults during the winter.

Sea butterflies that exist during the coldest season are more vulnerable to increased levels of ocean acidification. This is because ocean water is more acidic during this season than during other times of the year due to cooler temperatures that increase CO2 dissolution in the ocean. The researchers explained that the life stages of sea butterflies during this time are more exposed and vulnerable to the effects of ocean acidification.

L. rangii has a survival advantage over L. retroversa because both adults and juveniles coexist during winter, which reduces the risk of population instability if one cohort is vulnerable. In contrast, if one cohort is removed from L. retroversa, the entire population could be at risk.

Prolonged exposure is a survival challenge

The researchers pointed out that although the two species are affected differently, both are likely to suffer harm if they are exposed to unfavorable conditions for prolonged periods.

The scientists cautioned that the intensifying and prolonged events of ocean acidification overlap with the spring spawning period, posing a threat to the most vulnerable life stage, the larvae. This could endanger the future populations of sea butterflies.

The research team plans to further study the sea butterflies in the Scotia Sea to see how they would be affected by the increase in the intensity and duration of ocean acidification events. They will study multiyear sediment trap samples to identify potential changes in the life cycle that may occur due to environmental changes. Dr Jessie Gardner of the British Antarctic Survey, lead author of the study, said this will be the next step in their research.

Journal Link: Frontiers in Marine Science