Oysters are so much more than a seafood delicacy. They're ecosystem engineers, capable of building remarkably complex reefs. These structures act as the kidneys of the sea, cleaning the water and keeping the coast healthy, while providing homes for millions of other animals.

Oyster reefs were once thought to be restricted to southern, cooler coastal waters where they're the temperate equivalent of tropical coral reefs. But now, oyster reefs are being found right across Australia's tropical north as well.

These tropical oyster reefs are bigger and more widespread than anyone expected. In fact, they are some of the largest known intertidal oyster reefs (exposed at low tide) left in Australia. And they're everywhere - from the southern limit of the Queensland tropics across to the northern coast of Western Australia - yet we know almost nothing about them.

In our recent research, my colleagues and I completed the first detailed study of Australian tropical oyster reefs. These reefs are so new to science that until now, the species responsible for building them remained a mystery.

Using DNA, we identified the main reef-building oyster species in tropical Australia as "Saccostrea Lineage B", making it a new addition to our national list of known reef-builders.

Lineage B is a close relative of the commercially important Sydney rock oyster (Saccostrea glomerata), but so little is known about this tropical reef-building species that it is yet to be assigned a scientific name.

So why are we only learning about tropical oyster reefs now?

Across the globe, oyster reefs have been decimated by human activity. These reefs declined in most tropical regions long ago, even as far back as 1,000 years ago. Most oyster reefs disappeared without a trace before scientists even knew they were there.

However, Australia's tropical oyster reefs haven't just survived, in some cases they have thrived.

Despite being delicious to many, the species we now know as Lineage B was not very attractive to the aquaculture industry, due to its small size. And while oyster reefs near Brisbane, Sydney and Melbourne were dredged and burned to produce lime for mortar, used in the early construction of roads and buildings, this practice was not widespread in tropical regions. This lack of commercial interest is probably the reason why tropical oyster reefs have persisted unnoticed for so long in northern Australia.

We assessed three tropical oyster reefs in Queensland, Australia. At Wilson Beach, near Proserpine and Turkey Beach, near Gladstone, reefs were surveyed in late winter 2022. The reef at Mapoon in the Gulf of Carpentaria was surveyed in early spring 2023.

Using drone footage, we measured reef area and structure. We then collected oysters for genetic analysis.

Oysters are notoriously difficult to identify, because their shape, size and colour varies so much. Oysters from the same species can look completely different, while oysters from different species can look identical. That's why it's necessary to extract DNA.

We found almost all reef-building oysters across the three locations were Saccostrea Lineage B.

At Gladstone reefs, several other reef-building species were also present, including leaf oysters, pearl oysters and hairy mussels.

In southern Australia, oyster reefs are critically endangered. But we don't really know how threatened their tropical counterparts are, although there is some evidence of decline. Further research is underway.

A new project has begun to map oyster reefs across tropical Australia. Since the project launched in June 2024, more than 60 new reefs have been found across Queensland, the Northern Territory and Western Australia - including some as large as 5 hectares.

These unexpected discoveries provide a beacon of hope in a world currently overwhelmed by habitat decline and ecological collapse. But tropical oyster reefs are not yet protected. It's crucial we include them in assessments of threatened ecosystems, to understand how much trouble they're in and what we can do to protect them into the future.

By locating and understanding these overlooked ecosystems, we can ensure they're not left behind in the global oyster reef restoration movement.

Scientists and others involved in reef restoration are now inviting everyday people across Australia to get involved as citizen scientists in The Great Shellfish Hunt. Anyone can upload tropical oyster reef sightings to this mapping project. It's more important than ever to work together and ensure tropical oyster reefs receive the protection they deserve, so they continue to thrive for generations to come.