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At first sight, the moon clam may not look like a contender for stardom. Its size is humble, its color pale. Most shocking of all, for a clam, its shells can’t even close.��

But this oddball of the seafloor is now a finalist in an international popularity contest, and scientists say the moon clam’s strange shells may hold clues to how animals evolve.

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Moon clams live tucked inside burrows and cracks built by shrimp and sea sponges. (Credit: Andy Tan and Lisa Kirkendale)

Each year, the Senckenberg Institutes in Germany hosts the “Mollusk of the Year” competition, inviting researchers from around the world to nominate their favorite squishy invertebrates with shells or no shells in the Mollusca group, the second largest animal category on Earth. There are more than 86,000 species of mollusks recognized to date, from the giant squid, which is the largest known invertebrate, to the iconic Giant clam.��

The judging criteria? Whichever creature the public thinks is the coolest.

This year, among the finalists is the moon clam (Ephippodonta lunata), a mighty but unique creature found only in the remote waters of southwestern Australia.��

“The mysterious moon clam proves that not all clams live life closed shut,” wrote��Jingchun Li, associate professor in the Department of Ecology and Evolutionary Biology who nominated the species. “Strange, elegant, and almost unreal, this hidden bivalve challenges everything we think a clam should be.”

This marks the first time a nomination from ���Ҵ�ý�ƽ������ researchers has made it to the final five. The winner will receive not just bragging rights, but a full genome sequence, a powerful tool that Li said could crack the code for the mystery of why this clam evolved shells that never close.��

“Moon clams first caught my eye because they don’t look like clams,” said Li, who’s also a curator at the CU Museum of Natural History .��

Most clams have two-part shells that open and close to protect their soft tissues from predators and the environment. Unlike a typical clam, the moon clams’ shells remain apart at nearly 180°, forming a moon-like shape.��

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Small bumps on the moon clam's mantle may act as sensory organs. (Credit: Andy Tan and Lisa Kirkendale)

“I wasn’t sure how these animals survive in the ocean, which is a very dangerous place with tons of predators,” Li said. “It turns out they live in houses built by other animals.”

Smaller than a fingernail, moon clams live tucked inside burrows and cracks built by shrimp and sea sponges. They hide in these shelters, feeding on plankton that drifts by.

Maintaining a thick hard shell takes a lot of energy, Li said. She suspects that by relying on such a symbiotic relationship with shrimp and sponges, moon clams no longer need to invest as much in building their shells.��

Instead, the clam seems to have redirected that energy to a different task: sensing the world.

Their mantle, the soft flesh that typically lines a clam’s shell, extends outward and folds over the shell surface. There are small bumps and tentacle-like structures all over the mantle, and Li said these may act as sensory organs, helping the animal detect changes in its environment through touch or chemical signals.

While many clams have sensory receptors on their mantles that they extend from their shells occasionally, Li said moon clams take this ability to the extreme by evolving soft tissues that remain permanently exposed.

This unusual evolutionary tweak is exactly why scientists are eager to study them.

For decades, scientists focused on mollusks with large and colorful shells. Moon clams and their relatives were often overlooked, even though they may represent one of the most diverse groups of bivalves.

Li is currently working with researchers from 10 other research institutions around the world to map the biodiversity of this understudied group, known as galeommatoidean bivalves. Sequencing the moon clam’s genome would give scientists a blueprint of its biology, helping them understand how it evolved to be the clam that doesn’t close, and what sets it apart from other related species.��

Make the moon clam 2026 Mollusk of the Year!��

Cast your vote between April 13 and 26. The winner will be announced on April 30.

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