A translucent sea butterfly with a yellow swirling shell against a black background.

A sea butterfly with a swirling shell. Image © Katja Peijnenburg, Erica Goetze, Deborah Wall-Palmer and Lisette Mekkes.

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Tiny sea angels survived Earth's last period of climate change

Some of the world's most delicate marine creatures have already faced a major period of rapidly increasing temperatures, a new study has found.

Sea angels and sea butterflies are tiny marine slugs and snails. They have both survived dramatic global climate change before, and they also faced down Earth's most recent mass extinction event 66 million years ago.

Researchers hope they will also be resilient in the face of man-made climate change and the threats it will bring.

Sea butterflies and sea angels make up a group of organisms called pteropods, a type of plankton that sometimes create a calcareous shell around their soft bodies.

They can tell us a lot about the effects of climate change on the world's oceans, because they are very sensitive to carbon dioxide levels.

Researchers have discovered that these organisms originated during the Cretaceous period (130 million years ago). It means they must have survived some difficult periods in Earth's history, including the Palaeocene-Eocene Thermal maximum, a period of dramatic global warming that happened 56 million years ago.

A new paper is published in the journal Proceedings of the National Academy of Sciences.

Dr Jon Todd, senior curator of fossil molluscs at the Museum and a co-author on the paper, says, 'We were astonished to find that these tiny animals were likely swimming in the global ocean 130 million years ago.

'Though we haven't found fossils this old it gives palaeontologists something new to hunt for. This unexpectedly long history shows that the group has survived previous catastrophic episodes of climatic change.' 

A translucent sea butterfly with typical 'wing' structures.

This translucent sea butterfly has typical 'wing' structures that let it move around the water column. Image © Katja Peijnenburg, Erica Goetze, Deborah Wall-Palmer and Lisette Mekkes.

Supporting healthy oceans

Pteropods are marine gastropods that spend their entire life in the open water column. They have thin shells and a wing-like snail foot that that allows them to 'fly' through the water.

Sea butterflies live in their billions all over the world, eating microscopic algae. They are near the bottom of the oceanic food web, which means they are eaten by many other animals, including shrimp, fish and whales.

They are small but mighty when it comes to fighting climate change: a third of the carbon dioxide produced by humans is taken up by the oceans and locked down in the carbonate skeletons of marine organisms. Globally, up to 40% of that calcium carbonate is deposited in the shells of pteropods. Upon their death pteropods quickly sink to the ocean floor and are buried in deep seafloor sediments, taking the carbon with them.

Shelled pteropods can tell us a lot about the effects of climate change because their shells are very thin, and they start to dissolve in acidic seawater. They are already suffering in some parts of the world, and they will be seriously threatened if carbon dioxide levels continue to rise. 

A point-shaped sea butterfly.

Sea butterflies like these can be found all over the world. Image © Katja Peijnenburg, Erica Goetze, Deborah Wall-Palmer and Lisette Mekkes.

Sea angels of the past

Little is known about the evolutionary history of pteropods. Few of their fossils have survived in older rocks, and most are only known from the Eocene period onwards (from 56 million years ago), which is just after a very rapid period of global warming. This led to extinctions and a change in the composition of oceanic plankton. Given this, researchers previously assumed the pteropods only evolved after this warming event.

However, knowing whether they were swimming in the oceans before this period may reveal how pteropods were affected over their long history by other major oceanic events caused by climate change and may even help us predict their future.  

This new study sampled 21 pteropod species from the Atlantic Ocean and collected information about their genetic relationships and their history.

The researchers compared pteropod genes to those of closely related snails called sea hares. Studying fossil sea hare shells helped the researchers date the origin of the pteropods. They found that the two major groups of pteropods, sea butterflies and sea angels, diverged in the early Cretaceous (about 139 million years ago). It means both groups are much older than previously thought and must have survived previous episodes of widespread ocean acidification, such as at the end of the Cretaceous (66 million years ago) and at the Paleocene-Eocene Thermal Maximum (56 million years ago).

It's not all good news: although the study suggests that sea butterflies and angels have been more resilient to past ocean acidification than currently thought, it is unlikely that pteropods have experienced global change of the current magnitude and speed during their entire evolutionary history.

Lead author of the study, Dr Katja Peijnenburg from Naturalis Biodiversity Center in the Netherlands, says, 'Current rates of carbon release are at least an order of magnitude higher than we have seen for the past 66 million years…past performance is no guarantee of future results.'

A rounded, orange coloured sea angel.

Sea angels are also known as sea slugs. Image © Katja Peijnenburg, Erica Goetze, Deborah Wall-Palmer and Lisette Mekkes.

Climate change and the ocean's role

The ocean plays a major role in regulating the climate and supporting life on our planet. The open ocean is the largest habitat on Earth, microscopic algae produce half of the world's oxygen, and the ocean has already absorbed more than 90% of excess heat and 30% of excess carbon dioxide from the atmosphere since global industrialization. However, this is not without consequences.

In the past, changes in ocean processes were responsible for fluctuations in atmospheric carbon dioxide. Now, for the first time in millions of years, the role of the ocean and atmosphere has been reversed: today it is the atmosphere that controls ocean chemistry leading to an increasingly acidified ocean. It is still an open question whether marine organisms, particularly those that calcify, have the evolutionary resilience to adapt fast enough to these changes.