New findings show how native algae have changed the DNA of these legendary sea animals, helping them acquire more nutrients.
Giant clams, among the largest mollusks on our planet, have captivated researchers for ages. These remarkable beings can reach lengths of up to 4.5 feet and weigh more than 700 pounds, making them key players in tropical coral reef ecosystems.
Interestingly, these clams do not grow by indulging in high-protein meals. Instead, they substantially depend on the energy provided by algae that inhabit their bodies. A new study from CU Boulder sequenced the genome of the most common giant clam species, Tridacna maxima, shedding light on how these clams have evolved their genome to thrive with algae.
The results, published on January 4 in the journal Communications Biology, offer insights into how this evolutionary process may have played a role in the significant size of giant clams.
“Giant clams are crucial for many marine ecosystems,” explained Jingchun Li, the senior author of the study and a professor in the Department of Ecology and Evolutionary Biology. “Understanding their genetics and ecological role enhances our comprehension of coral reef environments.”
A symbiotic connection
Contrary to popular tales — such as in Disney’s “Moana 2,” where a giant clam consumes humans — these herbivorous mollusks depend on algae within their bodies for energy. When giant clams consume the right types of algae as larvae while floating through the ocean, they form a network of tube-like structures filled with algae inside their bodies. These algae perform photosynthesis, transforming sunlight into sugars that nourish the clams.
“Think of the algae as seeds, and the clam grows a tree inside of it,” Li noted.
Simultaneously, the clams protect the algae from harmful sunlight and provide them with essential nutrients. This mutually beneficial relationship is referred to as photosymbiosis.
“It’s fascinating that many of giant clams’ relatives do not depend on symbiosis, and we want to understand what makes giant clams unique,” Li added.
In partnership with researchers from the University of Guam and the Western Australian Museum, the team compared the genes of T. maxima with related species, like the common cockle, that lack symbiotic relationships. They discovered that T. maxima has developed more genes that detect friendly algae while distinguishing them from harmful bacteria and viruses. Simultaneously, T. maxima has downregulated some of its immune genes, which likely aids the clams in coexisting with the algae in their bodies over long periods, according to Ruiqi Li, the first author and a postdoctoral researcher at the CU Museum of Natural History.
This adjustment to the clam’s immune system has resulted in a surplus of transposable elements in its genome, fragments of DNA left by ancient viruses.
“These characteristics highlight the compromises involved in symbiosis. The host must manage a compromised immune system and possibly increased instances of viral genetic invasions,” explained Ruiqi Li.
The research also found that giant clams possess fewer genes associated with body weight regulation, known as CTRP genes. A reduced number of CTRP genes might have enabled giant clams to grow to larger sizes.
Conservation issues
Last year, Ruiqi Li’s assessment of giant clam populations led the International Union for Conservation of Nature (IUCN) to revise the conservation status of several giant clam species. Tridacna gigas, the largest and best-known species, is now classified as “critically endangered,” the highest risk level before extinction in the wild.
T. maxima, due to its extensive distribution, is currently rated as “least concern.” However, Ruiqi Li cautioned that various species may be grouped together simply because of their similar appearances.
“If people assume these giant clams are all one species, they may underestimate the dangers they face,” Ruiqi Li noted. “Genetic research like this can help differentiate between species and accurately assess their conservation requirements.”
The research team aims to sequence the genomes of all 12 recognized giant clam species to enhance understanding of their diversity.
Similar to corals, giant clams are increasingly threatened by climate change. When ocean temperatures rise too high, the clams expel the symbiotic algae from their tissues. Without this algae, giant clams can face starvation.
“Giant clams play a vital role in marine ecosystem stability and support biodiversity,” said Jingchun Li. She emphasized that many organisms in shallow ecosystems depend on the shells of giant clams for shelter, and they also serve as a food source for various species.
“Conservation of giant clams is essential for the health of coral reefs and the marine life dependent on them.”
