Nature's pollutant filters: How giant clams could help remove microplastics from our oceans

Plastic pollution is no longer a problem limited to floating debris and tangled fishing nets. Today, some of the most insidious threats to marine ecosystems are invisible to the naked eye. Microplastics, plastic particles smaller than 5 mm, have infiltrated virtually every marine habitat on the planet, from the deepest ocean trenches to the shallowest coral lagoons (Barnes et al., 2009). For coral reefs already under siege from warming oceans and acidification, microplastic contamination represents an additional stressor that compounds existing vulnerabilities (John et al., 2022). But in the warm waters of the Indo-Pacific, an unlikely ally in the fight against plastic pollution has been quietly filtering the water for millions of years: the giant clam.

 

 
At the IMARCS Foundation, we have long advocated for the ecological importance of giant clams (Tridacninae) as keystone reef organisms, carbon fixation contributors, and thermal tolerance mediators. Now, emerging research is revealing yet another reason to protect and restore giant clam populations: their capacity to actively remove microplastics from the water column.

Microplastics originate from a range of sources, including the degradation of larger plastic items, synthetic textile fibres released during washing, and industrial microbeads. Once they enter the ocean, their small size and often neutral buoyancy mean they can travel vast distances and persist in marine environments for decades (Cole et al., 2013). On coral reefs, microplastics are particularly problematic. Corals have been shown to ingest them, which can obstruct their digestive systems, reduce feeding efficiency, and even trigger stress responses that exacerbate bleaching (Hall et al., 2015; Reichert et al., 2019). Reef fish, crustaceans, and other invertebrates are similarly affected, with microplastics entering the food web at virtually every trophic level.

What makes the microplastics problem so difficult to address is that conventional cleanup technologies struggle with particles this small. Removing microplastics from open ocean environments at ecologically meaningful scales remains one of the most significant technical challenges in marine conservation. This is where biological solutions, and specifically the filtration capacity of bivalves, may offer a path forward.

Bivalves, the class of mollusks that includes clams, mussels, and oysters, are among the most efficient filter feeders in the ocean. Suspension-feeding bivalves can capture particles as small as four micrometres on their gills, and many species filter water for twelve or more hours per day (Ward, as cited in University of Connecticut, 2021). Giant clams of the genus Tridacna are the largest bivalves on earth, and their filtration capacity is proportionally impressive. As we have discussed previously, giant clams serve multiple ecological functions on coral reefs: they house photosynthetic zooxanthellae, contribute to nutrient cycling, provide habitat structure, and support reef calcification. Their role as water filtration agents, however, has only recently begun to receive the scientific attention it deserves.

In 2019, Arossa and colleagues published the first study to directly assess microplastic removal by giant clams. Working with Tridacna maxima specimens collected from the Red Sea, the research team exposed 24 individuals to polyethylene microbeads ranging from 53 to 500 μm over a 12-day period. Their findings revealed that giant clams remove microplastics through two distinct mechanisms: active ingestion and passive adhesion to their shells (Arossa et al., 2019).

The results were striking. Clams actively retained an average of approximately 7.55 microplastic beads per individual per day through ingestion. However, the shells proved to be an even more significant sink. The rough, calcified surfaces of giant clam shells trapped substantial quantities of microplastics, contributing to the removal of over 66% of the microplastics present in the surrounding water column. Passive adhesion to the shell surface was roughly 35 times more effective than active ingestion in terms of total microplastic removal (Arossa et al., 2019). Importantly, even dead clam shells continued to trap microplastics, though at reduced rates compared to living specimens, suggesting that the physical architecture of the shell itself plays a significant role in removal regardless of biological activity.

At IMARCS, we see giant clam microplastic filtration not as a silver bullet for ocean plastic pollution, but as one important component of a broader argument for why these animals must be protected and restored across their range. Giant clams are already critically endangered, facing threats from overharvesting, habitat destruction, ocean warming, and acidification. The discovery that they also serve as natural microplastic sinks adds another dimension to their ecological value, and another reason why their loss would reverberate through reef ecosystems in ways we are only beginning to understand.

Our current research programs in Micronesia, Vietnam, and Japan are investigating multiple facets of giant clam ecology, from thermal-tolerant zooxanthellae transfer to carbon sequestration potential. The microplastic filtration capacity of giant clams represents another compelling research frontier, one that connects reef restoration directly to water quality improvement. If healthy giant clam populations can measurably reduce microplastic loads on reefs, then restoration efforts deliver not only biodiversity and carbon benefits, but also a tangible water purification service.

The science is clear that we cannot clean microplastics from the ocean with technology alone. The scale is simply too vast, and the particles too small and dispersed. What we can do is protect and restore the biological systems that have been filtering ocean water for millions of years. Giant clams are one such system, and the more we learn about their ecological contributions, the stronger the case becomes for their conservation.

As we continue our work at IMARCS, the story of giant clams and microplastics reminds us of something we encounter again and again in marine science: the organisms we seek to protect are often already performing essential services that we have only just begun to appreciate. Protecting giant clams is not only about preserving a species. It is about maintaining the health of the reef ecosystems upon which so much of marine life depends.