Giant clams, among the largest mollusks in the ocean, are a fascinating part of tropical coral reefs.
These enormous creatures can grow up to 4.5 feet long and weigh over 700 pounds.
But what makes them even more extraordinary is their unique way of getting energy—not from eating, but from tiny algae living inside their bodies.
In a recent study published in Communications Biology, scientists from CU Boulder sequenced the genome of Tridacna maxima, the most widespread species of giant clam.
Their research reveals how these clams evolved to live in harmony with algae and how this special relationship may have helped them grow so large.
Giant clams don’t eat other animals or humans, despite what some myths suggest. Instead, they form a partnership with algae.
When the clams are young and swimming through the ocean as larvae, they take in certain algae species.
These algae settle inside special tube-like structures in the clams’ bodies and produce sugar through photosynthesis using sunlight. The clams then use this sugar as their primary source of energy.
“It’s like the algae are seeds, and the clam grows a tree out of its stomach,” explained Jingchun Li, a senior researcher on the study.
In return, the clams provide the algae with protection from the sun and essential nutrients. This mutually beneficial relationship, called photosymbiosis, is key to the clams’ survival.
Not all mollusks rely on algae for energy, so the researchers wanted to understand why giant clams are unique. They compared the genes of T. maxima to closely related species, like cockles, that don’t form symbiotic relationships.
They found that T. maxima has more genes for sensors that help them recognize friendly algae while avoiding harmful bacteria and viruses. At the same time, their immune system is less active, which allows the algae to live inside them long-term. However, this tradeoff means the clams are more vulnerable to viruses, which leave genetic traces in their DNA.
Interestingly, the study also revealed that giant clams have fewer genes related to controlling body weight. This could explain why they grow so much larger than other mollusks.
Giant clams are vital for coral reef ecosystems. They provide shelter for marine animals, help maintain biodiversity, and even serve as food for other organisms. However, like corals, they face significant threats from climate change.
When ocean temperatures rise, giant clams expel their symbiotic algae. Without the algae, the clams can starve. Additionally, overfishing and habitat destruction are putting some species at risk.
Tridacna gigas, the largest giant clam species, is now listed as “critically endangered” by the International Union for Conservation of Nature (IUCN). T. maxima is considered less threatened, but researchers believe we need to study their genetics further to better understand their conservation needs.
“Protecting giant clams is essential for the health of coral reefs and the marine life that depends on them,” said Li.
The researchers hope to sequence the genomes of all 12 known species of giant clams to uncover their diversity and develop better strategies for their conservation.
