Fatty acid profiles of muscle and liver reveal ontogenetic shifts in diet and reproductive investment in an undulate ray from the Bay of Biscay (northeast Atlantic)

Abstract Elasmobranchs, with their slow growth, long lifespans and low reproductive output, are particularly sensitive to fluctuations in nutritional status and energy availability throughout ontogeny. Fatty acids serve as valuable biomarkers for studying trophic ecology, ontogenetic dietary shifts and energy allocation during key life periods such as growth and reproduction. This study examined fatty acid profiles in Raja undulata by analysing muscle and liver tissues to investigate ontogenetic dietary shifts, assess niche breadth and overlap between juveniles and adults, and explore how reproductive development influences fatty acid allocation after sexual maturity. The muscle fatty acid profiles of R. undulata varied across life stages, with adults showing a reduced trophic niche and a diet composed of higher‐trophic‐level prey compared to juveniles. This pattern suggested a dietary shift from a diverse, invertebrate‐based diet in juveniles to more specialized predation on fish and larger crustaceans in adults. The liver fatty acid profiles varied across reproductive stages, with an increasing proportion of omega‐3 (C22:6n‐3, docosahexaenoic acid DHA; 22:5n‐3, docosapentaenoic acid) and omega‐6 polyunsaturated (C20:4n‐6, arachidonic acid ARA) fatty acids, alongside a marked decline in eicosapentaenoic acid (C20:5n‐3), likely reflecting its direct allocation to the ovaries. Additionally, the elevated monounsaturated fatty acids levels in adults and their decline during gametogenesis highlighted their key role as energy sources for oocyte development, supporting previous findings on reproductive energy mobilization in elasmobranchs. While DHA and ARA proportions increased, their stable absolute values suggested a different allocation pattern compared to viviparous species, potentially due to R. undulata 's oviparous strategy. Our findings, in line with previous studies, showed that analysing different tissues is key to understanding how diet and energy use are linked in elasmobranchs.