Fossil otolith archives reveal changes in mesopelagic fish energetics across the Isthmus of Panama over the last 8 million years

Mesopelagic ecosystems are vital to the ocean’s health yet face unprecedented threats due to accelerating climate change. Lanternfish (family Myctophidae) are key sentinels of midwater ecosystem health because they dominate mesopelagic fish biomass and mediate energy, nutrient, and carbon transfer across ocean layers. Despite their ecological importance, very little is known about their potential response to climate change stressors due to logistical challenges when studying mesopelagic ecosystems. We use thousands of fossil fish otoliths (calcium carbonate ear stones) recovered from marine surface sediments to reconstruct lanternfish growth, body size and energetic investment across contrasting oceanographic regimes. We compare assemblages from the highly productive, but oxygen-poor Tropical Eastern Pacific (TEP) and the oligotrophic, well-ventilated Caribbean, to test how oxygen availability and food supply shape mesopelagic fish energetics across the Isthmus of Panama. Otoliths from marine sediments serve as a cost-efficient, powerful archive to overcome methodological barriers and allow us to reconstruct long-term changes in lanternfish dynamics. We quantify energetic changes in lanternfish assemblages by reconstructing lanternfish body size estimates and mean per-capita biomass from otolith measurements and growth trajectories derived from increment biochronologies. Our results reveal anomalously small lanternfish in the TEP today, despite the region’s tendency to host larger fishes relative to the Caribbean, providing the first indication that mesopelagic fish size is potentially related to oxygen limitation. We then extend this approach to fossil otoliths to explore changes before the closure of the Isthmus of Panama using the Late Miocene/Pliocene as an analog system for warmer than modern conditions to test how mesopelagic ecosystems might respond to future climate change. We find that lanternfish dominated Panama’s ancient fish assemblages in the Caribbean but declined in relative abundance toward the Isthmus closure, while their mean per-capita biomass remained stable over the past 8 Ma. These results imply higher lanternfish biomass during periods of prolonged warming and lower biomass under less productive, better-oxygenated Caribbean conditions created by the Isthmus uplift. Yet in contrast to Panama’s geological past, our results suggest that oxygen availability exerts a dominant control on lanternfish energetics and production in the modern TEP. By integrating otolith archives from deep time to the recent past, we mapped shifts in lanternfish energetics in response to major environmental changes, revealing their sensitivity to oxygen availability and indicating that projected ocean deoxygenation might constrain the energetic capacity of the mesopelagic zone in a future ocean.