Bean clam: Meet the small but mighty mollusca found on California beaches

Visitors and locals flock to La Jolla’s beaches in the summer, but another group of beach-goers also is making its way to the shore by the thousands.

This month is a good time for those who walk La Jolla’s coastline to find small bivalves called bean clams, according to Charlotte Seid, manager of the Scripps Institution of Oceanography’s Benthic Invertebrate Collection in La Jolla.

Bean clams, found in the intertidal zone and on the beach, live in boom-and-bust cycles “with huge variations from year to year,” Seid said.

“The biggest they get is about one inch … very small but mighty in number,” she said.

Scripps Oceanography, which has studied bean clams since 1955, has been important in the long-term study of the species, especially under Scripps Pier, Seid said. “So there is a lot of La Jolla pride with the bean clam.”

“The biggest concentration recorded was 20,000 per square meter during a boom year,” Seid said. “After that, there were only a handful, so it is hit or miss.”

The bean clam’s size and shape tend to be consistent, but the color can vary to include small stripes or shades of purple and red on a yellowish shell, which Seid said “makes them beautiful and interesting.”

Bean clams start their lives, which last three years at most, as larvae floating in the current. Spawning season tends to be April to November, and each female can produce 50,000 eggs in that time.

“Most won’t survive, but that’s how we get these incredible numbers,” Seid said.

Starting around July, they make their way to the shore to settle.

Because bean clams do not migrate as some other species do, “if you see them, that is where they want to be,” Seid said.

Once they find their homes, bean clams serve several roles in the ecosystem.

They are filter feeders and help clean the water in their immediate area. Additionally, they are food for other marine creatures such as fish, gulls and possibly sharks.

For scientists, the presence of the clams can be an indicator of beach health.

“They prefer a certain type of sand — not too fine or coarse. So they are markers of a quality beach,” Seid said.

“They also make fine microscopic lines every day, like the rings of a tree, that capture [signs of] small changes in the environment.”

In some cases, bean clams have a symbiotic relationship with a creature called a hydroid (an early life stage of a class of small predators related to jellyfish) that can give the bean clams a “fuzzy appearance,” Seid said.

Some believe — though it has not been confirmed — that the hydroid helps the bean clam ward off predators.

Historically, bean clams have been eaten by humans. “Indigenous populations harvested them, and we know that because we have found loads of shells at sites [of Indigenous significance] in San Diego,” Seid said. “So they are edible. But they can harbor parasites. They are not harmful to humans, but I would think twice before eating them.”

Instead, she encourages people to take photos and post them on citizen science apps or websites because it helps document the clam’s boom-and-bust cycles.

The bean clam can be affected by a natural pathogen that affects other bivalves, Seid said. “If a whole population is exposed to this pathogen, that can be a bust year.”

Seid said she finds it fascinating that bean clams have such a “dramatic variation in boom and bust years and how synched up these populations are to the point that thousands find the same place to settle, whereas other species might be more constant from year to year.”

“We never really know what we are taking for granted,” she added. “Everyone here grows up with them, much like we did with sea stars before their numbers started declining from sea star wasting disease. So don’t take them for granted.”