Sharks are continuously shedding dermal denticles throughout their lives. These microscopic structures cover the animals’ bodies and perform several functions, including reducing drag as sharks swim, protecting the skin, and influencing how water moves across the body. Unlike ordinary fish scales, dermal denticles are made from a hard, tooth-like material and often possess distinctive shapes that vary among species and ecological types. Once shed, they sink into reef sediments, where their mineralized structure can persist for millions of years. By counting denticles in sediment layers whose ages can be determined, researchers can estimate how frequently sharks occupied the surrounding reef environment at different points in the past. The method functions as a biological archive, preserving evidence of shark communities from periods long before systematic fishing records existed.

The research team applied this approach to sediments deposited between approximately 7,000 and 3,000 years ago, a period that predates intensive industrial fishing, and compared those ancient layers with sediments deposited during the past century. Across 157 samples, the scientists recovered 3,497 denticles. The number of scales in each sample was adjusted using models that accounted for factors such as sediment age, sampling effort, and variation in deposition. This allowed the researchers to compare denticle accumulation rates rather than simply counting raw fragments. A higher accumulation rate indicates that more sharks were likely swimming over or near a reef, shedding denticles that eventually became incorporated into the seafloor. Because the approach captures the combined signal of many animals over time, it can reveal broad population baselines that are impossible to reconstruct from scattered historical observations.

The first striking contrast appeared during fieldwork. In Bocas del Toro on Panama’s Caribbean coast, the team found roughly 50 shark denticles in 10 kilograms of reef sediment. A much smaller sample from a Pacific reef contained about 200 denticles in only one kilogram of sediment. As more material was processed and the data were analyzed, the initial impression became statistically robust. Before humans began fishing at large scales, reefs in the Gulf of Panama supported roughly 20 times more sharks than comparable Caribbean reefs. Over the following centuries, the two coastlines diverged even more. Denticle accumulation on the Caribbean side declined by about 75 percent, while accumulation on the Pacific side remained statistically similar to its prehistoric baseline when averaged across the last hundred years. Today, Pacific reef sediments yield approximately 100 times more denticles than Caribbean sediments.

That result is especially surprising because Panama’s Pacific coast has experienced much heavier fishing pressure. More than 98 percent of the country’s fishing activity occurs there, meaning the Pacific reefs have faced substantially greater direct human exploitation than their Caribbean counterparts. The explanation, according to the researchers, lies in the physical and chemical differences between the two marine environments. The Gulf of Panama is strongly influenced by seasonal upwelling, a process in which winds and ocean circulation bring cold, nutrient-rich water from depth toward the surface. These nutrients stimulate plankton growth and support a productive food web capable of sustaining large predator populations, including sharks. The Caribbean coast of Panama is comparatively nutrient-poor. Its clear tropical waters may resemble the classic image of a coral reef, but lower external productivity means that more of the ecosystem’s energy is stored within reef organisms themselves, leaving shark populations smaller and potentially more vulnerable to fishing.

The ecological distinction changes how scientists should interpret reef recovery. Conservation programs often use the healthiest nearby reef as a reference point, assuming that neighboring ecosystems should support similar numbers of predators or that fishing has affected them in broadly comparable ways. In Panama, that assumption could produce a severe error. If Pacific shark abundance were used as the recovery target for Caribbean reefs, managers might establish goals that are unrealistic for a naturally less productive ecosystem. Conversely, if the lower Caribbean baseline were applied to the Pacific, managers could underestimate the number of sharks that a productive upwelling system can support. The study indicates that ocean productivity must be considered alongside fishing history, habitat quality, climate, and species composition when setting population targets. A reef cannot be evaluated accurately without accounting for the energy available to its food web.

The fossil record does not mean that Pacific sharks are safe. The comparison with ancient sediments may conceal more recent changes because the modern reference samples span roughly a century. Targeted shark fishing in the Gulf of Panama accelerated during the 1980s, so a steep decline in recent decades could be diluted when combined with sediments deposited earlier in the twentieth century. Other research has already shown that shark populations in Pacific Panama are under pressure from intensified fishing. The region may also be entering a period of ecological change: the seasonal upwelling that supplies the Gulf with cold, nutrient-rich water failed for the first time on record last year. If overfishing reduces shark populations while climate-driven changes weaken the productivity that supports them, the Pacific coast could be pushed toward a condition not experienced for thousands of years. Its apparent resilience may therefore represent a capacity for recovery rather than proof that recovery is unnecessary.

Developing the denticle method required more than a decade of work. The project began with the unconventional idea that reef sediments might contain enough shark scales to reconstruct ancient predator communities. Researchers first had to determine which denticles could be reliably identified, how to extract them from complex carbonate sediments, and how differences in denticle shape reflected shark species or ecological lifestyles. False-color scanning electron microscopy helped reveal the fine architecture of the scales, including ridges, crowns, and other microscopic features that can vary among sharks. These characteristics allow researchers to examine not only how many sharks were present but also whether the composition of the shark community changed through time. The work began when lead author Erin Dillon was an intern at STRI in 2014, continued through her doctoral research at the University of California, Santa Barbara, and later developed during her postdoctoral fellowship in Panama.

The study’s most powerful implication is that the ocean floor may preserve a record of ecological loss that conventional monitoring cannot see. Sharks are among the most difficult marine predators to survey because they move across large areas, occur at low densities, and can disappear from heavily fished ecosystems before scientists establish reliable population baselines. Dermal denticles provide a new source of evidence, linking the physical history of sediments with the biological history of predator communities. In Panama, they show that geography alone does not determine shark abundance: the productivity of the surrounding ocean can create radically different natural baselines on nearby reefs. As scientists extend the technique to other regions, microscopic “shark dandruff” could help reveal how many predators ecosystems once supported, distinguish natural differences from human-driven declines, and identify recovery targets grounded in the ecology of each reef rather than in assumptions based on the nearest surviving population.

Subject of Research: Shark abundance baselines, fossil dermal denticles, coral reef ecology, ocean productivity, and conservation targets.

Article Title: Fossil denticles reveal how ocean productivity shapes shark baselines and recovery potential.

Web References: https://doi.org/10.1126/science.aec2144

References: Dillon, E.M., McCauley, D.J., Gonzalez, M., Connolly, S.R., de Gracia, B., Cybulski, J.D., Norris, R.D., Gómez, M.M., Garcia-Pérez, I., Leonard, N.D., Zhao, J., García-Méndez, K., O’Dea, A. 2026. “Fossil denticles reveal how ocean productivity shapes shark baselines and recovery potential.” Science.

Image Credits: Erin Dillon, Jorge Ceballos, Aaron O’Dea, and Ashley Diedenhofen.

Keywords: Sharks, shark denticles, shark dandruff, coral reefs, fossil sediments, dermal denticles, Panama, Gulf of Panama, Caribbean Sea, ocean productivity, seasonal upwelling, overfishing, marine conservation, reef ecosystems, Science.

The material at the center of the study is known technically as a dermal denticle. These tiny, tooth-like structures cover a shark’s body, reducing drag as the animal moves through water and helping protect its skin. Unlike the scales of most bony fish, shark denticles are composed largely of dentine and enamel-like tissue, making them exceptionally resistant to decay. Sharks continuously shed and replace them throughout their lives. Once released, the denticles settle onto the seafloor and can become trapped in layers of reef sediment, where they may survive for thousands or even millions of years. Because denticle shape varies according to its location on the body and its functional role, researchers can often identify the type of shark that produced a fossil scale, even when no skeleton remains.

By counting these preserved denticles in carefully dated sediment samples, scientists can estimate the relative abundance of sharks that once swam above a reef. The method does not count individual animals directly; instead, it measures the rate at which shark remains accumulated in the sediment. When combined with information about sediment deposition, reef area and the age of each layer, denticle concentrations become a powerful proxy for historical shark populations. The approach effectively transforms the seafloor into an ecological archive. It can reveal not only whether shark numbers declined, but also whether the original ecosystem was naturally characterized by a small, moderate or exceptionally large predator population—information that modern surveys alone cannot provide.

The researchers compared sediment deposited between approximately 7,000 and 3,000 years ago with material accumulated during the past century. The older layers predated the era of industrial fishing and therefore provided a window into reef ecosystems before humans began removing large numbers of marine predators. Across 157 samples, the team collected 3,497 shark denticles. The contrast between Panama’s two coasts appeared almost immediately. In Bocas del Toro on the Caribbean coast, researchers recovered roughly 50 denticles from 10 kilograms of reef sediment. On the Pacific coast, their first small sample produced about 200 denticles from only 1 kilogram. As the dataset expanded and statistical models accounted for differences in sampling effort, sediment age and accumulation rates, the pattern remained strikingly consistent.

The fossil record showed that reefs in the Gulf of Panama once supported shark populations roughly 20 times denser than those on the Caribbean coast. The finding is surprising because the two regions are geographically close and share many of the same shark species. Their modern environments, however, function very differently. The Gulf of Panama experiences seasonal upwelling, a process in which winds push surface water away from the coast and draw cold, nutrient-rich water upward from the deep ocean. Those nutrients stimulate phytoplankton growth, which supports zooplankton, fish and increasingly large predators. In effect, the Pacific coast receives a seasonal pulse of energy capable of sustaining a high biomass of sharks. The Caribbean coast is comparatively nutrient-poor, with clearer, more glass-like water but less energy circulating through the food web. Its reefs naturally supported fewer sharks, leaving their populations more vulnerable to additional disturbance.

The modern comparison adds another layer to the story. Since intensive human activity began, shark denticle accumulation on the Caribbean coast has fallen by approximately 75 percent. On the Pacific coast, denticle accumulation during the last century is statistically similar to the prehistoric baseline, despite the fact that more than 98 percent of Panama’s fishing activity occurs on that side of the country. At first glance, the result appears to contradict expectations: the more heavily fished coast seems to have retained more of its historical shark signal. The researchers argue that environmental productivity helps explain the apparent paradox. A highly productive ecosystem may be capable of supporting larger shark populations and may also possess greater capacity for recovery after losses. A naturally low-productivity ecosystem, by contrast, may suffer severe ecological damage even when fishing pressure is lower, because its predator populations begin from a much smaller starting point.

The authors caution that the Pacific coast should not be declared safe. A sediment layer representing the average of the past 100 years may conceal recent and rapid declines. Targeted shark fishing in the Gulf of Panama intensified during the 1980s, meaning that the latest losses could be diluted when combined with earlier decades of higher abundance. Independent research has already documented pressure on Pacific shark populations, and the region may be approaching a critical threshold. The Gulf’s natural resilience is also being challenged by climate change. The seasonal upwelling that supplies the ecosystem with cold, nutrient-rich water failed for the first time on record in 2021, raising concerns that changes in ocean temperature, wind patterns and productivity could weaken the food base supporting large predators.

The study has immediate implications for marine conservation because managers often use the healthiest nearby reef as a reference for restoration. That strategy assumes neighboring ecosystems once supported comparable numbers of sharks and experienced similar degrees of human impact. Panama’s fossil record shows why that assumption can fail. If the Pacific coast were used as the benchmark for Caribbean reefs, managers could set recovery goals an order of magnitude—or more—above what the Caribbean environment can naturally sustain. Conversely, treating the Caribbean’s depleted modern abundance as a realistic target for the Pacific could normalize a profound loss. The researchers say conservation plans must incorporate ocean productivity, habitat quality and historical ecological differences alongside fishing history. A reef’s appropriate shark target cannot be calculated from geography alone.

The broader significance of the work extends beyond Panama. Shark populations have declined worldwide, yet scientists have struggled to determine what “healthy” abundance looked like before commercial fishing, habitat destruction and climate change transformed marine ecosystems. Conventional surveys are too recent to capture long-term variability, while historical records are often incomplete or biased toward areas where people were already fishing. Fossil denticles provide an independent archive that can reach back thousands of years and distinguish broad changes in both shark abundance and community composition. By combining paleontology with ecology and conservation science, the research offers a new way to define realistic recovery goals. It also delivers a vivid warning: the number of sharks an ocean should contain depends not only on how much exploitation it has endured, but on how much energy the ecosystem can supply. In a changing climate, even regions that once supported abundant sharks may be pushed toward ecological conditions unseen for millennia.

Subject of Research: Fossil shark denticles, historical shark abundance, ocean productivity and reef ecosystem recovery potential.

Article Title: Fossil denticles reveal how ocean productivity shapes shark baselines and recovery potential

Web References: University of California, Santa Barbara reference; Science article

References: Science, DOI: 10.1126/science.aec2144

Image Credits: Erin Dillon, Aaron O’Dea and Jorge Ceballos

Keywords

Sharks, shark denticles, fossil scales, coral reefs, marine ecosystems, ocean productivity, upwelling, shark conservation, paleoecology, population ecology, Panama, climate change