Giant clams grow huge without eating much protein

Giant clams, some of the largest mollusks on Earth, have captivated marine biologists for decades. These marine giants can reach lengths of up to 4.5 feet and weigh over 700 pounds, making them key figures in tropical coral reef ecosystems.

Beyond the sheer size of giant clams, their biology holds secrets that researchers are still uncovering. A new study led by CU Boulder provides insight into how giant clams evolved to coexist with algae inside their bodies.

The team sequenced the genome of Tridacna maxima, the most widespread species of giant clam.

The researchers discovered genetic adaptations that enable these creatures to sustain themselves through symbiosis rather than by consuming large amounts of food.

How giant clams grow

Unlike many large marine animals, giant clams do not rely on a high-protein diet to sustain their impressive size. Instead, they derive most of their energy from algae living inside their tissues. These algae harness sunlight through photosynthesis, producing nutrients that the clams absorb.

The recent genomic study highlights how T. maxima has evolved genetic mechanisms that allow it to accommodate these symbiotic algae. By examining the genetic code of T. maxima, researchers found clues about how this partnership contributed to the giant clam’s remarkable size.

Study senior author Jingchun Li is a professor in the Department of Ecology and Evolutionary Biology at CU Boulder.

“Giant clams are keystone species in many marine habitats,” said Li. “Understanding their genetics and ecology helps us better understand the coral reef ecosystem.”

By comparing the genome of T. maxima with those of non-symbiotic mollusks, scientists identified key differences that enable the clam to support algae within its body while maintaining its own health.

Unique symbiotic relationship

Popular myths often depict giant clams as dangerous creatures capable of swallowing unsuspecting divers. In reality, these mollusks are entirely non-predatory and depend entirely on their relationship with algae for survival.

When young giant clams drift through the ocean as larvae, they have the opportunity to ingest algae. If they take in the right species, they begin to develop a specialized internal system of tube-like structures where the algae settle and multiply.

These algae convert sunlight into sugars through photosynthesis, supplying essential nutrients to their host. “It’s like the algae are seeds, and a tree grows out of the clam’s stomach,” Li said.

This relationship benefits both organisms. While the algae provide food, the giant clams offer their symbiotic partners a safe environment within their tissues.

The clams regulate light exposure by expanding and retracting their mantles, ensuring that the algae receive sufficient sunlight without experiencing damage from excessive radiation. The clams also provide essential nutrients that sustain their tiny partners.

This process, known as photosymbiosis, is found in only a handful of marine species, such as corals.

Living with algae

Many other mollusks, even those closely related to giant clams, do not form these symbiotic relationships. Scientists wanted to understand what made giant clams unique in this regard.

To explore this question, the research team collaborated with experts from the University of Guam and the Western Australian Museum. They analyzed the genetic makeup of T. maxima and compared it with species such as common cockles that lack symbiotic partnerships.

The findings revealed that T. maxima possesses a greater number of genes designed to detect and distinguish beneficial algae from harmful bacteria or viruses.

Additionally, the researchers found that the giant clam has evolved a way to suppress parts of its immune system, allowing algae to reside safely within its body for extended periods.

“As a result of the clam’s weakened immune system, its genome contains a large number of transposable elements, which are bits of genetic material left behind by ancient viruses,” said Ruiqi Li, the study’s first author and a postdoctoral researcher at the CU Museum of Natural History.

“These aspects highlight the tradeoffs of symbiosis. The host has to accommodate a suppressed immune system and potentially more viral genome invasions.”

The giant clam’s massive size

Another major finding of the study concerned the genes responsible for regulating body size.

Compared to other mollusks, giant clams have fewer CTRP genes, which control body weight and growth. Scientists believe that this genetic difference may explain how giant clams are able to reach such enormous sizes.

By reducing the genetic restrictions on body growth, these mollusks may have evolved to expand far beyond the size of their closest relatives. Their symbiotic relationship with algae provides the necessary energy to support this growth, compensating for their lack of a high-protein diet.

Understanding these genetic mechanisms not only helps scientists grasp how giant clams evolved but also offers insights into broader ecological patterns. The study demonstrates how organisms can evolve specialized relationships with other species to maximize their survival in challenging environments.

Conservation concerns for giant clams

Last year, a giant clam population assessment led by Ruiqi Li influenced the International Union for Conservation of Nature (IUCN) to revise the conservation status of several giant clam species.

Tridacna gigas, the largest and most iconic species, is now classified as “critically endangered,” meaning it is at extreme risk of extinction in the wild. Conservationists have long warned about the threats facing this species, from overharvesting to habitat destruction.

While T. maxima remains categorized as “least concern” due to its wide distribution, scientists caution that classification may not tell the full story. Ruiqi Li pointed out that different species are sometimes grouped together simply because they look similar, which could lead to an underestimation of conservation risks.

“If you think these giant clams are all the same species, you might underestimate the threat they face,” noted Ruiqi Li. “Genetic studies like this can help us distinguish between species and assess their true conservation needs.”

The research team aims to sequence the genomes of all 12 known giant clam species. By identifying subtle genetic differences, they hope to improve conservation strategies tailored to each species’ needs.

Impact of climate change

Beyond direct human threats, giant clams face increasing dangers from climate change. Similar to corals, these mollusks rely on their symbiotic algae for survival.

When ocean temperatures rise beyond a certain threshold, the clams expel their algae, leading to a condition similar to coral bleaching. Without algae, the clams lose their primary energy source and may not survive.

“The giant clams are very important for the stability of the marine ecosystem and support biodiversity,” said Jingchun Li.

Importance of giant clams

In addition to their role as primary producers, giant clams provide shelter for numerous marine species. Their large shells create microhabitats that protect small fish and invertebrates from predators.

Many marine creatures also depend on giant clams as a food source, further highlighting their importance in coral reef ecosystems.

“Protecting them is essential for the health of coral reefs and the marine life that depends on them,” Li added.

As climate change and habitat destruction continue to threaten marine biodiversity, studies like this offer crucial insights into how species adapt to their environments.

By understanding the genetic basis of symbiosis in giant clams, scientists can contribute to conservation efforts that preserve these remarkable mollusks for future generations.

The study is published in the journal Communications Biology.

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