Did Tiny Algae Shape the Evolution of Giant Clams?

The Genetic Secrets Behind Giant Clams and Their Tiny Algal Partners

I had seen them in documentaries and at museums, and we had one that we used to teach students about mollusk diversity at the lab, but nothing prepares you to see one in the ocean.

Giant clams are some of the ocean’s most impressive creatures. These reef dwellers with vibrant colors can grow to more than four feet long and weigh over 700 pounds. Spotting one in the reef leaves everyone speechless, especially if you don’t expect them while doing fieldwork.

However, few know that their massive size isn’t the result of a high-protein diet or an aggressive feeding strategy. Instead, they’ve mastered an unusual trick: partnering with tiny algae to generate energy.

A new study published in Communications Biology reveals just how deeply this relationship has shaped everything in these animal’s anatomy, down to their DNA.

For years, researchers have known that giant clams survive in nutrient-poor tropical waters thanks to their symbiotic relationship with algae. This means that the clams provide a safe home for the algae inside specialized tube-like structures in their tissues. In return, the algae, through photosynthesis, produce sugars that feed the clam. These sugars contribute to the clam’s massive growth and size.

But while many marine creatures rely on symbiosis, the way this process has influenced the evolution of giant clams has remained a mystery… until now.

Let’s look at how the authors managed to crack the Giant Clam Genome.

Scientists sequenced the genome of Tridacna maxima, one of the most widespread giant clam species, to understand how symbiosis influenced giant clam evolution. The idea was simple: they compared its genetic makeup to related mollusks that don’t have symbiotic relationships, looking for differences that might explain how the clam adapted to host algae.

And the most interesting part: What did they find?

Well, the study revealed that T. maxima carries a unique genetic signature shaped by its symbiotic lifestyle. One of the biggest surprises was the sheer number of transposable elements, also known as “jumping genes,” in its genome.

These bits of DNA, often leftovers from ancient viruses, make up about 70% of the clam’s genome — a remarkably high proportion compared to its non-symbiotic relatives.

But what’s behind this genetic quirk? Apparently, the answer likely lies in the clam’s immune system. Here’s the thing: normally, animals have strong immune defenses that prevent foreign organisms from settling in their tissues. But in order to maintain a long-term partnership with algae, T. maxima has tuned down parts of its immune response.

This is a double-edged sword. It makes it easier for the clam to host beneficial algae but also creates an environment where viral DNA elements can stick around in the genome.

“These aspects highlight the tradeoffs of symbiosis. The host has to accommodate a suppressed immune system and potentially more viral genome invasions,” said Dr. Ruiqi Li, the study’s first author and a postdoctoral researcher at the CU Museum of Natural History.

Another key finding was that T. maxima has fewer CTRP genes, which help regulate body weight in other animals while nutrients are scarce. However, since giant clams rely on a nearly unlimited energy source from their algae, they may not need the same genetic controls to regulate growth.

The result? A bivalve that keeps growing as long as conditions allow.

But why does this all matter?

Understanding the giant clam genome isn’t just about deciphering an evolutionary puzzle; it also has implications for conservation. Giant clams are keystone species in coral reef ecosystems, providing shelter and food for many marine organisms.

But climate change and human activity are putting them at risk.

In fact, a conservation assessment led by the study’s researchers recently led to an update in the IUCN Red List, the international guide to endangered species. While T. maxima is still classified as “least concern,” its close relative Tridacna gigas — the largest species of giant clam — is now listed as critically endangered. Yes, critically endangered.

But the study doesn’t end here — the researchers also warn that genetic studies may reveal hidden diversity within T. maxima, meaning some populations could be more at risk than previously thought.

“If you think these giant clams are all the same species, you might underestimate the threat they face,” said Dr. Li.

And there’s more. Just like corals, giant clams rely on their symbiotic algae for survival. This means that when ocean temperatures rise too much, they can “bleach” — expelling their algae and losing their primary food source. If warming trends continue, these once-thriving reef giants could struggle to survive.

But let’s look ahead, what can still be done?

This study provides a fascinating window into how a partnership with microscopic algae can shape an entire species over millions of years. It also highlights the importance of genetic research in conservation, helping scientists better understand which species need protection and why.

For now, the researchers plan to continue their work by sequencing the genomes of all 12 known giant clam species. With each new discovery, they hope to paint a clearer picture of how these iconic reef inhabitants evolved — and what can be done to ensure they remain a part of our oceans for generations to come.

In the meantime, let’s hope that future students and researchers can find them thriving in the reefs through their unique photosynthetic strategy. I can’t wait to see what else we can learn from these fascinating creatures!