Bleaching, not just for corals

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Bleaching occurs when a stressed marine life, most commonly a coral, expels its symbiotic algae and turns a ghostly white, often in response to a warming sea. But bleaching affects more than just coral. Giant clams — giant mollusks that can reach more than 4 feet in diameter and weigh up to 500 pounds — can also bleach. And in more recent research, scientists have learned more about how bleaching disrupts these sedentary giants, affecting everything from their diet to their reproduction.

Giant clams live on coral reefs and are the largest clams on earth. Like coral, giant clams bleach when stressed, often in response to water that is too warm. Like a coral, a bleached giant clam expels the algae that live inside it, called zooxanthellae. These algae live in the soft tissue of the mussel’s mantle and provide energy to the animal through photosynthesis, leaving a bleached mussel with less energy and nutrients. At worst, bleaching can kill giant clams through lack of food.

Scientists have been studying giant clam bleaching for decades. In 1997 and 1998, during a brief period of widespread coral bleaching worldwide, with coral deaths in at least 32 different countries, bleached giant clams were observed from Australia’s Great Barrier Reef to French Polynesia after sea temperatures in the South Pacific had increased significantly. In 2010, similar temperatures in the water off Thailand’s Ko Man Nai island also resulted in numerous deaths.

Of the 12 giant clam species, some are more resilient to heat stress than others. But scientists have found that even if a giant clam survives bleaching, other physiological functions can still be severely compromised.

For example, a recent study of wild mussels in the Philippines found that bleaching can affect their reproduction. Bleaching reduces the number of eggs giant clams produce, and the more severe the bleaching, the fewer eggs they produce. Reproduction “takes a lot of energy. Instead of using that energy to reproduce, they just use it to survive,” says Sherry Lyn Sayco, the study’s lead author and a PhD student at the University of the Ryukyus in Japan.

Mei Lin Neo, a marine ecologist and giant clam expert at the National University of Singapore who was not involved in the study, says the work adds to the story of how climate change can have “impacts on species longevity”.

In general, she says, we know a lot more about how climate change is affecting corals than we do about marine species with similar physiologies. “By understanding how other symbiotic species respond to climate change, each species becomes a unique indicator of how the entire reef ecosystem is faring.”

As it turns out, bleached giant clams are often better at handling bleach than coral. Near the island of Ko Man Nai, 40 percent of the bleached mussels recolored after a few months as zooxanthellae recolonized their tissues as temperatures cooled again. After the 1997–1998 bleaching event, over 95 percent of the 6,300 bleached mussels recovered near Australia’s Orpheus Island.

Giant clams also seem suitable for restocking. In the Philippines, where the largest species, Tridacna gigaswent extinct locally in the 1980s, repopulation has brought it back.

“Mussels aren’t just any organism,” says Sayco. “It’s not that we just conserve them so that they’re there,” she adds, “they have many benefits and ecosystem services, such as [boosting] fishing [and] Tourism.”


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