What happens when a shark gets more food than usual? Does it grow faster, reproduce more, or become more resilient to environmental change?
These questions aren’t just fascinating: they’re crucial for conservation.
Our latest research has discovered that elasmobranchs (sharks, skates, and rays) don’t follow fixed life strategies. Instead, they change depending on how much food is available. Dr Isabel Smallegange, Senior Lecturer in Population Biology, explains how this plasticity – an organism’s ability to change its traits in response to different environmental conditions – could change how we predict their responses to climate change and exploitation.
Contents:
- What are elasmobranchs?
- Life history strategies: not as fixed as we thought
- When food increases, reproduction does too
- Predicting population performance: a complex picture
- A new framework for conservation
- Why it matters
What are elasmobranchs?
Elasmobranchs are a subclass of ancient carnivorous fishes that include modern sharks, skates, and rays. Their evolutionary history dates back more than 400 million years.
Elasmobranchs are characterised by skeletons made of cartilage, five to seven pairs of individual gill slits, and placoid scales that give their skin a rough texture. They also lack a swim bladder, instead using a large, oily liver for buoyancy. Male elasmobranchs also possess modified pelvic fins called claspers, used for sperm transfer during mating.
Life history strategies: not as fixed as we thought
We all make a choice about how we partition our time and energy. In fact, all organisms face trade-offs in how energy is used for survival, growth and reproduction. The balance among these processes is known as a life history strategy. Traditionally, these strategies have been treated as fixed, rooted in a species’ evolutionary history. But our new study challenges that view.
We analysed data on how elasmobranchs allocate energy to growth, survival, and reproduction and used it to map a ‘life-history space’, shaped by species energetics and feeding. We identified two key axes that structure elasmobranch life history strategies: reproductive output and generation turnover.
Changes in life-history strategies for 117 elasmobranch species under different feeding levels. Each point represents a species at either low feeding (yellow) or high feeding (blue). For 51 species, arrows connect the two points to show how their position shifts between conditions. As feeding level increases, many species move toward higher reproductive output.
When food increases, reproduction does too
One of our most striking findings was that species shifted their position in life history space when feeding levels increased. In other words, their reproductive output increased when more food was available.
This may sound obvious, but remember that ecologists traditionally assume that life history strategies, including reproductive performance, are treated as fixed. Our findings suggest that elasmobranchs instead adjust their life strategies depending on environmental conditions: a form of plasticity that has rarely been quantified at this scale.
Predicting population performance: a complex picture
We also explored how these life history axes – reproductive output and generation turnover – relate to population performance. Can they predict when a population grows or declines? The axes predict that population growth rates are highest when both reproductive output and generation turnover is high. However, we found that growth rate alone doesn’t necessarily reflect how resilient elasmobranch species are to disturbances, such as over-fishing, habitat degradation, and the accelerating impacts of climate change. A species’ population might grow quickly in number but can still be vulnerable to collapse if its life strategy isn’t robust under stress.
Surprisingly, neither axis predicted a species’ conservation status on the IUCN Red List of Threatened Species – the world’s most comprehensive source on the global extinction risk status of animal, fungus, and plant species. This disconnect highlights the need for more nuanced tools in conservation biology.
A new framework for conservation
Our work integrates demographic data with energy-budget theory to create a more mechanistic framework for understanding life history strategies. Rather than focusing only on population counts or raw survival rates, this approach examines the fundamental energetic trade-offs - how individuals allocate energy to growth, reproduction and survival – and shows how these trade-offs shape demographic rates (e.g., age-specific survival, growth and fecundity).
Because demographic rates determine population size, growth and resilience, linking energetic allocation to demography lets us make more accurate predictions about how species will respond to pressures such as climate change, habitat loss, and human exploitation.
It also serves as a warning.. Life history strategies are shaped by a species' evolutionary past, but individuals can also adjust, growing faster, reproducing earlier, or changing investment in offspring depending on local conditions. That plasticity means that data gathered in one environment won’t always apply neatly to another. Using them without considering those changes risks ineffective or even counterproductive conservation choices.
Our energy-and-demography approach helps because it links how animals use energy to the demographic rates that determine population responses, and so can better account for both fixed differences and flexible responses and thus is a more reliable foundation for protecting biodiversity in a rapidly changing world.
A reef manta ray (Mobula alfredi), one of the focus species of this study. Image credit: Subphoto.
Why it matters
Elasmobranchs are among the most threatened vertebrates on Earth. Many species mature slowly, produce few offspring, and rely on stable ecosystems to survive. These traits make them especially vulnerable to overfishing, habitat degradation, and the accelerating impacts of climate change. Understanding their capacity to adapt to changing environments is essential for effective protection.
Our research highlights the central role of feeding dynamics in shaping life history strategies, and, ultimately, survival. Conservation models need to move beyond simple population counts to incorporate the biological and ecological processes that underlie them. By doing so, we can design strategies that better reflect the realities of elasmobranch life, giving these extraordinary species a stronger chance for the future.
You might also like
- read the paper: Lucas S, Berggren P, Barrowclift E, Smallegange IM. 2025. Changing feeding levels reveal plasticity in elasmobranch life history strategies. Ecology Letters 28: e70201. doi.org/10.1111/ele.70201
- explore for yourself how the life histories of elasmobranchs and many other coldblooded species vary with feeding levels using an interactive app, with instructions here
- learn more about Dr Isabel Smallegange, Senior Lecturer in Population Biology, and her blog
- read more about how big data can predict species responses to climate change in Dr Isabel Smallegange’s earlier blog
- find out more about Dr Sol Lucas, Senior Inshore Fisheries and Conservation Officer at Sussex IFCA and study lead author
- read the press release: Food supply drives reproductive strategies in sharks, skates and rays
- explore our diverse and wide-ranging ecology and conservation research, computational biology research, and our School of Natural and Environmental Sciences
