The oceans are undergoing a profound and accelerating transformation, with oxygen levels steadily declining due to climate change. This pervasive deoxygenation poses a severe threat to marine ecosystems, impairing key biological processes and jeopardizing the balance of oceanic food webs. An international team of researchers has now uncovered evidence that the depletion of oxygen in the mesopelagic zone, the twilight layer of the ocean extending from 200 to 1000 meters depth, significantly diminishes populations of lanternfish — a crucial group of deep-sea vertebrates. These findings not only highlight the vulnerability of mesopelagic ecosystems to changing ocean chemistry but also underscore the broader implications for global carbon cycling, fisheries, and biodiversity.
Led by scientists at the Institute of Environmental Science and Technology at the Universitat Autònoma de Barcelona (ICTA-UAB), the study delves into historical episodes of ocean deoxygenation through a meticulous analysis of fossil remains. By investigating ancient otoliths—calcified structures in fish inner ears that serve as reliable indicators of species presence and abundance—researchers have reconstructed past population dynamics of lanternfish in the Eastern Mediterranean Sea. This unique marine setting has historically oscillated between oxygen-rich and anoxic states, providing an unparalleled natural laboratory to observe how marine life responds to fluctuating oxygen levels over millennia.
Lanternfish, belonging to the family Myctophidae, are notable for their bioluminescent capabilities, which they employ for communication and predator avoidance in the perpetual darkness of the mesopelagic zone. Despite their modest individual size, this family collectively represents an immense biomass approximating 600 million tons, potentially making them the most abundant vertebrates on Earth by sheer weight. Their diel vertical migration—from depth during daylight to surface waters at night—positions them as vital conduits for energy and nutrient transfer, effectively linking surface productivity with deep ocean processes. This vertical migration also enhances carbon sequestration by ferrying organic matter into deeper waters, reinforcing their key role in climate regulation.
The paleontological evidence derived from the last 10,000 years reveals stark patterns: periods marked by extreme oxygen depletion saw a dramatic absence of lanternfish, with their numbers plummeting to near extinction in the region. Conversely, their resurgence aligns closely with intervals when oxygen concentrations in the water column recovered, notably around 6,000 years ago. These oscillations reflect the sensitivity of mesopelagic fish communities to oxygen availability and portend what may occur as modern ocean deoxygenation trends continue.
Crucially, the researchers underscore that the loss of lanternfish biomass would ripple through marine ecosystems. As an integral component of mesopelagic food webs, lanternfish serve as prey for a range of species, including commercially important fish, marine mammals, and seabirds. Their disappearance could trigger cascading effects, destabilizing food webs, reducing biodiversity, and compromising the resilience of oceanic ecosystems under stress. Additionally, diminished lanternfish populations could impair the ocean’s natural capacity to sequester carbon, thus exacerbating atmospheric CO2 levels and feeding back into climate change.
The interdisciplinary team brought together expertise from premier institutions including the Scripps Institution of Oceanography, the Woods Hole Oceanographic Institution, the Biodiversity Research Center at Academia Sinica, McGill University, Freie Universität Berlin, and Heidelberg University. By integrating paleontological data with modern analytical approaches, they have provided unprecedented insights into the intricate coupling between oxygen dynamics and mesopelagic life.
The mesopelagic zone, often described as Earth’s largest twilight habitat, plays an outsized role in regulating biogeochemical cycles. Its influence on the global carbon cycle is profound, driven by the biological pump—the process that transfers carbon from surface waters to the deep ocean, effectively locking it away for centuries to millennia. Lanternfish, with their diel migrations, are key agents of this pump. Thus, the oxygenation state of this realm directly influences the efficacy of carbon sequestration, with far-reaching consequences for global climate stability.
Oxygen minimum zones (OMZs), areas of naturally low dissolved oxygen, have been expanding in recent decades as a direct result of warming ocean temperatures, altered circulation, and nutrient influxes. These hypoxic conditions disproportionately affect organisms reliant on well-oxygenated waters, especially those inhabiting the mesopelagic zone. The fossil record unearthed by this study elucidates that elevated deoxygenation events in the past systematically suppressed lanternfish populations, implying that current and future expansions of OMZs may replicate these impacts on a global scale.
Furthermore, the decline of mesopelagic fish undermines not only ecological but also socioeconomic dimensions. Many fisheries depend indirectly on lanternfish as foundational species within the food web, and their reduction threatens fishery yields and, consequently, human food security. The mesopelagic zone’s cryptic biodiversity remains poorly understood, but its significance as a buffer against climate change continues to emerge as a paramount area of concern.
According to Sven Pallacks, the lead author of the study, lanternfish serve as a bellwether for the broader oceanic health under deoxygenation stress. If such an abundant vertebrate group cannot withstand diminishing oxygen environments, the risks posed to other marine fauna — and the entire oceanic system — are formidable. The research thus calls for urgent attention to the patterns of ocean deoxygenation and advocates for mitigation strategies targeting emissions and ocean health preservation.
The implications of this research resonate beyond marine ecology. Understanding how ancient ecosystems responded to oxygen fluctuations gives scientists a predictive model to assess future impacts of anthropogenic climate change. It highlights the urgency of monitoring and managing ocean health to avoid irreversible losses in biodiversity and ecosystem function, critical components underpinning Earth’s climate resilience and human sustenance.
This groundbreaking study, published in the esteemed journal Communications Earth & Environment, charts new territory in marine science by combining paleobiology, oceanography, and climate science. It reveals that the fate of the twilight zone — and by extension the global ocean — hangs precariously on oxygen levels, signaling a clarion call for concerted scientific, policy, and conservation efforts.
As the ocean continues to warm and lose oxygen at an alarming rate, the fate of lanternfish stands as a microcosm of what could unfold beneath the waves worldwide. The mesopelagic realm’s health is a silent but potent indicator of planetary well-being, interlacing marine life, climate regulation, and human prosperity. Protecting this crucial ecosystem is tantamount to securing the stability of life on Earth itself.
Article Title: Ocean deoxygenation linked to ancient mesopelagic fish decline
News Publication Date: 28-Jul-2025
Web References: 10.1038/s43247-025-02568-8
Keywords: Oceanography, Ocean chemistry, Marine life, Marine biology, Marine ecology, Marine conservation, Marine food webs, Pelagic ecosystems
