How the dinosaur-killing extinction could help save modern bivalves

Bivalves are some of the most diverse marine animals alive today – but even they’re not safe from extinction.

Clams, mussels and oysters are just some of the many different types of these shelled animals that are found across the world. As well as being an important source of food for millions of people, their filter-feeding abilities help to keep streams, rivers and oceans clean.

Though these animals are vital to marine ecosystems, our impact on the planet is pushing bivalves to the brink. Habitat loss, overharvesting and climate change are raising the extinction risk for many species, amid what is often described as a modern mass extinction.

As bivalves have overcome these destructive episodes before, looking back at their history can reveal clues about what might be in store for them. A new study, published in the journal Science Advances, examined how mass extinctions affect the roles that bivalves play in their ecosystems.

Dr Katie Collins, one of our curators of benthic molluscs, was among the co-authors of this paper. They say that while it’s likely molluscs will keep carrying out the same roles they do today, there’s no guarantee which species will survive.

“After the Cretaceous mass extinction, there was a total shuffle in which bivalves were doing what,” Katie explains. “Most of the same taxonomic families survived, but their abundance and which jobs they excel at have shifted.”

“As a result, we can’t predict which species will survive if we continue to change our planet. While we can be relatively sure that bivalves as a whole will continue, there’s no way to know if commonly eaten species will win the survival lottery.”

How did the Cretaceous mass extinction affect the ocean?

The mass extinction at the end of the Cretaceous Period is perhaps the most famous in history. Over 66 million years ago, a 15-kilometre-wide asteroid struck what is now Mexico and changed life on Earth forever.

While anything in the vicinity was immediately vapourised by the force of the impact, shockwaves spread the asteroid’s damage much further around the world. As debris from the impact blocked out the Sun, the growth of plants and other photosynthetic organisms was hit – causing food chains to collapse globally.

As a result of the impact, as much as 78% of all life on Earth went extinct. While we often think about the impact of life on land, such as the extinction of non-avian dinosaurs, the asteroid strike also had a severe impact on life in the sea.

“We know that the asteroid caused major changes in the oceans,” Katie explains. “Temperatures went crazy for a while, while it’s likely that the debris would have affected marine life as well. But there’s no smoking gun that explains exactly what caused so many species to become extinct.”

“However it happened, we know that over 60% of bivalve genera were lost during this mass extinction. We wanted to see what happened in the aftermath – did the bivalves recover and go back to the same roles they had before, or were things completely different?”

Focusing on the past to protect the future

To investigate, the researchers examined nearly 2,000 genera of bivalves from before and after the Cretaceous mass extinction, as well as today. By comparing the shells of living and extinct species, scientists can understand the lifestyle of ancient animals.

“The shell is the tool bivalves use to interact with the world,” Katie explains. “Its shape dictates what a bivalve can or can’t do.”

“For example, we know that birds have to be a certain shape and size if they want to fly through the air, and it’s a similar story for bivalves that want to swim. If they’re not the right shape, they simply can’t do it.”

The team found that rather than the bivalve community recovering, or being reset entirely, there was instead a ‘chaotic rebuilding’ after the mass extinction. While very few roles carried out by bivalves were lost overall, each family was knocked back to just one or two surviving lineages. Subsequently, the families which dominate each lifestyle changed significantly.

This is a finding that could have important implications for bivalve conservation. If it’s impossible to anticipate which species might go extinct, then the researchers suggest that protections should focus on ensuring that the ecosystem roles of bivalves continue.

“If we have to prioritise conservation at a species level, then we want to pick groups that are likely to evolve new functions relatively easily,” Katie says. “True mussels, for example, are amazing at this.”

“We also need to preserve groups with unique ways of life that are unlikely to evolve ever again. For example, the glass scallops both swim and are carnivorous, which is a really unusual combination for bivalves.”

“Such animals will probably never come back if they die out, so we need to do all we can to help them survive.”