Until now, scientists assumed that the number of shark and ray species was stable or even increasing. But new long-term data now shows that the number of species of these ancient cartilaginous fish has been declining continuously for 45 million years. The main reasons for this are changes in coastal habitats and climate change. The researchers conclude that the habitats of these sea creatures must be better protected and emissions reduced.
Sharks and rays are truly prehistoric animals: “Cartilaginous fish, which also include the sharks and rays living today, have been on our planet for over 400 million years. They have survived several mass extinction events during this time,” says Manuel Staggl from the University of Vienna. Until now, scientists have assumed that these sea creatures are particularly resilient and that their number of species has been stable or even increasing for millions of years. But is that true? And how does this fit with the fact that today over a third of the species from the group of modern sharks and rays are threatened with extinction?
A team led by Staggl has now investigated these questions. To do this, the paleontologists analyzed thousands of fossils of sharks and rays from 503 genera from the past 100 million years and used them to determine how their populations have developed in the long term. They also compared this data with historical environmental conditions such as water temperature, CO2 content in the atmosphere and the availability of marine habitats.
Marine biodiversity has been declining for a long time
The surprising result: the number of shark and ray species has been declining continuously for 45 million years. Their diversity is therefore by no means stable, but has been decreasing since the Eocene. This now sheds completely new light on the evolutionary history of these sea predators. Sharks and rays survived the extinction of the dinosaurs after the famous asteroid impact 66 million years ago almost without any problems. “These animals proved to be surprisingly resilient and recovered quickly from the disaster,” said Staggl. Until 45 million years ago, these fish continued to evolve and their biodiversity increased. But then came a surprising turnaround and the biodiversity of sharks and rays has been declining ever since. But why?
The data revealed that continents were more fragmented and flooded back then. As a result, there were significantly more shallow, heterogeneous and food-rich coastal waters that served as habitats for sharks and rays. This was a decisive factor in the development of new shark and ray species: “The more different shallow sea areas were available, the more species developed,” reports senior author Jürgen Kriwet from the University of Vienna. “It is precisely these habitats that are threatened today by coastal development, global warming, pollution and unsustainable fishing,” says the paleontologist. The study also reveals a possible lever for protecting today’s shark and ray species: if their habitats near the coast were treated better, their diversity could also be preserved.
CO2 emissions also harm sharks and rays
The analyzes also revealed another factor that affects the biodiversity of sharks and rays: the CO2 content of the atmosphere. Moderate CO2 levels, as they often occurred in the past, had an indirect positive effect on the diversity of marine predators. “To put it simply, CO2 ensures more photosynthesis in the algae and seagrass meadows. This has a positive effect on the rest of the food chain and ultimately on sharks and rays,” explains Kriwet.
However, excessively high CO2 levels, such as those that have prevailed since man-made climate change, cause the oceans to acidify and warm up and are therefore harmful to marine ecosystems – and thus indirectly also to the biodiversity of sharks and rays. The unprecedentedly high pace of climate change is particularly problematic: particularly highly specialized species such as deep-sea sharks, which are adapted to stable, cold environments, can no longer keep up with the current changes in the oceans and are therefore particularly at risk from rapid warming, the team reports. In addition to preserving coastal habitats, it is essential to reduce CO2 emissions in order to preserve these sea creatures. “Our study shows that marine protection doesn’t just mean fishing quotas – we have to keep an eye on the entire habitat and the climate system,” sums up Staggl.
Source: University of Vienna; Specialist articles: Scientific Reports, doi: 10.1038/s41598-025-25653-6