Elasmobranchs

Elasmobranchs are an ancient group of predatory fishes, encompassing today’s sharks, skates and rays. Their lineage stretches back over 400 million years. These animals are distinguished by their cartilaginous skeletons, multiple pairs of gill slits, and tough, textured skin covered in placoid scales. Unlike most fish, they lack a swim bladder, relying instead on a large, oil-rich liver for buoyancy. Males also have specialised pelvic fins called claspers, which are used during mating.

Blue shark (Prionace glauca). Source: Wikipedia

Rethinking life history strategies

Every organism faces choices about how to allocate energy, balancing survival, growth and reproduction. This balance forms what biologists call a life history strategy. Traditionally, these strategies have been viewed as largely fixed, shaped by evolutionary history. However, our new findings challenge this assumption.

By analysing how elasmobranchs distribute energy among growth, survival and reproduction, we mapped out a “life-history space” structured by species’ energetics and feeding habits. Two main axes emerged: 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.

 More food, more offspring

One of our key discoveries is that when food becomes more plentiful, elasmobranchs shift their life history strategies: specifically, they increase their reproductive output. While this might seem intuitive, it’s a significant departure from the traditional view that life history traits are static. Our results show that these animals can adjust their strategies in response to environmental changes—a form of plasticity rarely quantified at this scale.

Predicting population dynamics: it’s complicated

We also examined how reproductive output and generation turnover relate to population trends. Do these axes predict whether a population will grow or decline? Our analysis suggests that populations grow fastest when both reproductive output and generation turnover are high. Yet, a high growth rate doesn’t necessarily mean a species is resilient to threats like overfishing, habitat loss, or climate change. A population might expand quickly but still be at risk if its life strategy isn’t robust under stress.

Interestingly, neither axis reliably predicted a species’ conservation status on the IUCN Red List. This gap underscores the need for more nuanced tools in conservation science.

Towards a mechanistic conservation framework

Our research combines demographic data with energy-budget theory to build a more mechanistic understanding of life history strategies. Instead of focusing solely on population numbers or survival rates, this approach looks at the fundamental energetic trade-offs—how individuals allocate energy to growth and reproduction—and how these decisions shape demographic rates like age-specific growth and fecundity.

Because these demographic rates drive population size, growth and resilience, linking energy allocation to demography allows for more accurate predictions of how species will respond to pressures such as climate change, habitat loss, and exploitation.

It’s also a cautionary tale: while evolutionary history shapes life history strategies, individuals can adjust their growth, reproductive timing and investment in offspring, depending on local conditions. This plasticity means that data from one environment may not apply elsewhere, and ignoring these differences could lead to ineffective or even harmful conservation actions.

Our energy-and-demography framework helps bridge this gap, offering a more reliable foundation for protecting biodiversity in a rapidly changing world.

Why this matters

Elasmobranchs are among the most threatened vertebrates on the planet. Many species mature slowly, have low reproductive rates and depend on stable habitats. These characteristics make them especially vulnerable to overfishing, habitat destruction and climate change. Understanding their ability to adapt to changing environments is crucial for effective conservation.

Our study highlights the pivotal role of feeding dynamics in shaping life history strategies and, ultimately, survival. Conservation models must move beyond simple headcounts to incorporate the biological and ecological processes that drive population trends. By doing so, we can design strategies that better reflect the realities of elasmobranch life, giving these remarkable species a better chance for the future.

Useful links

• 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
• read more about how big data can predict species responses to climate change in my 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