Giant, colourful clam declared critically endangered after new report

Giant clams (Tridacna gigas), found in the waters around Southeast Asia, Australia and Pacific Islands, have been declared critically endangered according to a new assessment. Its overall population has fallen 84% in the past century.

Another 11 related giant clam species also had their statuses updated.

The evaluation is based on research led by Ruiqi Li, a postdoctoral researcher at the University of Colorado at Boulder’s Museum of Natural History.

Li’s research led to the International Union for Conservation of Nature (IUCN) to update the conservation status of the giant clam from “vulnerable” to “critically endangered” – the highest level before a species becomes extinct in the wild.

The giant clam is known for its size, wavy shell and colourful cape-like mantle – the exposed flesh that lines the shell. They can reach more than 120cm across, 200kg and live on average about 100 years. They live in shallow waters up to about 20m below the surface.

Tridacna gigas is the best-known species of the giant clam.

Li says that overfishing and climate change are the major threats to the marine invertebrates.

Giant clams are harvested around the world for food or their shells.

When waters become too warm, the clams expel the algae which live in their tissues. This is similar to coral bleaching. Without the algae the giant clams can starve.

“Along with coral reefs, these clams provide important habitats for other creatures living in the reefs,” Li says. “They are also important nutrient producers in shallow ocean water, which is generally deprived of food. If the giant clam becomes extinct, it will have significant ecological impacts on tropical oceans.”

The last time T. gigas was assessed was in 1996, when it was listed as “vulnerable” on the IUCN’s Red List of Threatened Species.

Many other giant clam species were previously also not present on the list.

“This is a major issue in the conservation world. People, even scientists, tend to pay more attention to mammals and birds, because they are big, cute and familiar. So there’s a good chance you can find these animals on the Red List. But marine invertebrates are very neglected,” Li says. 

Li collaborated with Neo Mei Lin from the National University of Singapore and the Senckenberg Natural History Museum in Germany. They collected data from previous research and biodiversity databases on the 12 giant clam species.

They found T. gigas had already become locally extinct in many regions including near Taiwan.

“In particular, the populations of Tridacna gigas in Southeast Asia and South Pacific faced the greatest declines,” Mei Lin tells Cosmos via email.

Researchers have warned that Australia’s giant clams are “in trouble”.

“While Australia is one of the few locations that still boast relatively healthy numbers of Tridacna gigas in the wild, Dr Rick Braley’s recent study also found declines in numbers (see his study here). The Australian Tridacna gigas also face the rising challenge of bleaching – see article here – which further threatens the species’ survival,” Mei Lin adds.

And, as recently as 2013, marine biologists have discovered a new species of giant clam, Tridacna crocea, in Ningaloo reef, Western Australia and the waters around the Solomon Islands.

Tridacna mbalavuana, or the tevoro clam, also had its status updated from “vulnerable” to “endangered,” as well as Hippopus hippopus, or horse’s hoof clam, from “least concern” to “vulnerable.”

Mei Lin says that several species previously thought to be extinct were “resurrected”. That is, scientists were able to show that they had been “lumped under” another more dominant species.

“With progress in tools to identify species better – such as DNA barcoding – the scientists working on their taxonomy start to differentiate them as unique,” Mei Lin says. “For the newly resurrected species, because they are only recently known and differentiated, we have limited information to conclude their status or population sizes at the moment.”

“Invertebrates, like insects and molluscs, account for more than 95% of total animal biodiversity in the world, but people are not paying enough attention to them. There’re still a lot of undiscovered or undescribed species out there,” Li adds.