Uncovering the lazybones of the shark world

Our ability to reconstruct the biology of organisms from inanimate objects has a magical quality to it – be it their ecology, physiology or behaviour. I think this partially explains the allure of dinosaurs. Palaeontologists use science, along with lifeless fossilized remains, to open windows into the past, shedding light on how these incredible creatures once lived. However, not all the fun and awe are reserved for those studying dinosaurs. Ichthyologists also wield these powerful tools, which give us unprecedented insights into the life of fish. There is, of course, the enviable group that falls in the middle of this Venn diagram who study the fish fossil record (Bell, 2009; Cooper et al., 2023; Maisey et al., 2021; Paillard et al., 2021), including celebrities like the Otodus megalodon (Cooper et al., 2022). Coming back to the present-day and extant (for now) fish species, researchers have utilized an array of hard structures in fish that act as archives to understand more about their study organism. Be that scales to determine age (Horká et al., 2010), otoliths and chemical analysis to determine habitat use (Antunes et al., 2025) or the length of consumed pectoral spines to understand the size of prey (Glade et al., 2025). Perhaps one of the most illuminating methodologies, however, is stable isotope analyses, which, from just an otolith or scale, can be used to infer information on location of origin (Willmes et al., 2024), migration (Mainguy et al., 2023) and feeding patterns of fish (Feeney et al., 2024). Although teleosts by definition have a large number of calcified structures which can be used as stable isotope archives through time, these same principles can also be used on chondrichthyan skeletons, because while they are cartilaginous, vertebrae, jaws and teeth which need extra strength are commonly mineralized. Indeed, shark vertebrae have been successfully used, alongside mass spectrometry, to identify lifetime habitat use across salinity gradients (Grant et al., 2023). In this issue, the value of chondrichthyan calcified cartilage is further explored by Díaz-Delgado et al. (2025) who use stable isotopes to understand metabolic rate in 13 different species of shark, ray and chimaera. What they found is that the light isotope of carbon, which is particularly abundant in metabolic carbon sources, was more strongly included in calcified cartilage from highly active thunniform species compared to less active species. This is the same as seen in teleost otoliths, where higher metabolic rates lead to more metabolic carbon included in the otolith. The stable isotope composition of carbon in shark mineralized cartilage is reflective of metabolic rate. Because measuring metabolic rate is so difficult in large chondrichthyans like these, this methodology can provide valuable proxy, plugging data gaps. What the research also showed is that vertebrae and jaws were the best tissues to use, with teeth providing anomalous results. These new results demonstrate the power of stable isotopes, going beyond identifying where a fish has come from, or where it has been, and instead providing data on key life-history traits. Advances in analytical tools have allowed us to use fossil biomolecules to reconstruct the metabolism of dinosaurs (Wiemann et al., 2022), but using stable isotopes we can now more easily measure the metabolic rate of their far-older, albeit extant, relatives.