In the cold waters off European coastlines, a pale, unassuming creature has quietly been doing something remarkable: protecting fish from a potent neurotoxin. New research reveals that the common marine sponge Halichondria panicea and its relatives may reduce methylmercury contamination in bottom-dwelling fish by more than 50%—simply by eating.
The discovery comes from researchers at Helmholtz-Zentrum Hereon in Germany, who were puzzled by a curious pattern they'd observed for years. Sponges consistently contained low levels of methylmercury—the dangerous form of mercury that accumulates up food chains and reaches harmful concentrations in fish—while showing elevated levels of inorganic mercury. Scientists had long assumed symbiotic bacteria living within the sponges were breaking down the toxin.
But using a sophisticated ecosystem model called GOTM-ECOSMO E2E-MERCY, the research team found that the sponges' own feeding behavior may be entirely responsible. As sponges filter vast amounts of dissolved organic matter from surrounding water, they naturally take up proportionally more inorganic mercury while avoiding methylmercury—particularly species hosting abundant microbial symbionts.
"Our modeling shows that the feeding strategy of sponges itself can provide a plausible explanation," said David Amptmeijer, a researcher at Hereon's Institute of Coastal Systems—Analysis and Modeling and lead author of the study, published in the journal Biogeosciences. "Our findings suggest that we may need to reassess the role of sponges in the cycling of pollutants."
The implications extend well beyond these humble invertebrates. As foundational organisms on seafloors across the world's oceans, sponges sit at the base of marine food webs and influence contaminant exposure throughout their ecosystems. By filtering water through their bodies, they appear to act as natural barriers against methylmercury accumulation—a service with tangible human stakes. Methylmercury contamination costs European economies billions of euros annually in health impacts and lost productivity.
The researchers now plan to expand their one-dimensional model into three dimensions, allowing them to study sponge communities under more realistic conditions across larger marine regions. They also hope their findings will inspire new experimental studies to validate the model's predictions. For now, the team sees their results as an additional argument for protecting marine habitats—and recognizing the quiet services the ocean's smallest residents provide.