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A new study offers the first direct evidence that deep-dwelling mesopelagic fish, which account for up to 94% of global fish biomass, excrete carbonate minerals at rates comparable to shallow-water species. The findings validate previous global models suggesting that marine fish are major contributors to biogenic carbonate production in the ocean.

Scientists at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science studied the blackbelly rosefish (Helicolenus dactylopterus), a deep-sea species living at depths of 350–430 meters (1,148–1,410 feet), to determine whether it forms and excretes intestinal carbonate—known as ichthyocarbonate. This physiological process, common among marine fish, helps maintain internal salt and water balance in saline environments and plays a critical role in marine carbon cycling.

The study, titled "Osmoregulation by the gastro-intestinal tract of marine fish at depth—implications for the global carbon cycle," was on July 15, 2025 in the Journal of Experimental Biology.

"Mesopelagic fish live in deep, cold, high-pressure environments, and until now, it was unclear if they produced carbonate like shallow water fish do—or at what rate," said Martin Grosell, the lead author of the study and chair of the Department of Marine Biology and Ecology at the Rosenstiel School. "This study is the first to confirm that they do and that the mechanisms and characteristics of ichthyocarbonate formation are remarkably consistent across depths."

The blackbelly rosefish was an ideal model. Unlike many mesopelagic species, it lacks a swim bladder and can survive both capture and lab acclimation. Researchers maintained specimens at 6°C, replicating their natural habitat, and found they excreted approximately 5 milligrams of ichthyocarbonate per kilogram per hour, aligning with predictions from thermal and metabolic scaling models.

"This research fills a major gap in our understanding of ocean chemistry and carbon cycling," Amanda Oehlert, co-author and assistant professor in the Department of Marine Geosciences, said. "With mesopelagic fish playing such a significant role, their contribution to carbonate flux—and how it might change with warming oceans—deserves greater attention."

Key findings:

• Deep-sea blackbelly rosefish produce carbonate at rates and compositions comparable to shallower fish, confirming that depth and pressure do not inhibit ichthyocarbonate formation.
• These results strengthen global estimates of fish-derived carbonate production, confirming that mesopelagic fish are substantial contributors to the ocean's carbonate budget.
• Ichthyocarbonate composition is similar regardless of the depth at which it forms, which influences how and where it is stored or dissolved in the ocean.

"These results offer strong support for global models of fish-derived carbonate production, which had assumed—but not verified—that mesopelagic species contribute at similar rates," Grosell said. "Mesopelagic fish aren't just prey; they're chemical engineers of the ocean."

The study underscores the importance of ichthyocarbonate in the ocean carbon cycle, especially given the vast, underexplored biomass of the mesopelagic zone.

The authors say the findings open new avenues for studying deep-sea carbon dynamics and may improve Earth system models, which are sophisticated computer models that incorporate interactions between physical, chemical, and biological processes, such as biological carbon production and export.