This article was originally posted on Hakai magazine, online publication about science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.
Bleaching occurs when a stressed sea creature, most commonly a coral, expels its symbiotic algae and turns ghostly white, often in response to a warming sea. But bleaching doesn’t just affect corals. Giant clams – massive clams that can reach over 1.2 meters in diameter and weigh up to 225 kilograms – can also become discolored. And in a recent study, scientists have learned more about how bleaching destroys these sessile giants, affecting everything from their nutrition to their reproduction.
Giant clams live on coral reefs and are the largest bivalves on Earth. Like corals, giant clams bleach when they are stressed, often in response to excessively warm water. As with coral, a bleached giant clam expels algae called zooxanthellae that live inside it. These algae live in the soft tissues of the clam’s mantle and provide the animal with energy through photosynthesis, leaving the discolored clam with less energy and nutrients. In the worst case, bleaching can kill giant clams due to a lack of food.
Scientists have been studying the bleaching of giant clams for decades. In 1997 and 1998, during a short period of widespread coral bleaching worldwide, corals died at least 32 disparate countries, bleached giant clams have been observed from the Great Barrier Reef in Australia to French Polynesia following a significant rise in water temperatures in the South Pacific. In 2010, similar water temperatures off Koh Man Nai in Thailand also killed dozens of people.
Of the 12 species of giant clams, some are more resistant to heat stress than others. But even if the giant mollusk survives the bleaching, other physiological functions can be severely affected, the scientists found.
Recent Research in the Philippines, wild mollusks, for example, have found that bleaching can prevent them from reproducing. Bleaching reduces the number of eggs that giant clams produce, and the stronger the bleach, the fewer eggs they lay. Playback “requires a lot of energy. So instead of using that energy to reproduce, they just use it for their own survival,” says Sherry Lin Saiko, lead author of the study and a PhD student at Ryukyu University in Japan.
Mei Ling Neo, a marine ecologist and giant clams expert at the National University of Singapore who was not involved in the study, says the work contributes to the story of how climate change may have “impacts on the lifespan of species.”
Overall, she says, we know much more about how climate change is affecting corals than we do about marine species with similar physiologies. “By understanding how other symbiotic species are responding to climate change, each species becomes a unique indicator of how the entire reef ecosystem works.”
It turns out that bleached giant clams are often better at dealing with bleaching than corals. Near Koh Man Nai, 40 percent of the discolored clams changed color after a few months as zooxanthellae repopulated their tissues when temperatures dropped again. Since the 1997-1998 bleaching, more than 95 percent of the 6,300 bleached clams off Australia’s Orpheus Island have recovered.
Giant clams also seem to lend themselves to replenishment. In the Philippines, where the largest species live, Tridacna gigasbecame extinct locally in the 1980s, replenishment brought it back.
“Shellfish are not just an organism,” says Saiko. “It’s not that we just keep them there to be there,” she adds, “they have a lot of benefits and ecosystem services like [boosting] fishing [and] tourism”.