Beyond beaks: assessing isotopic variations in cephalopod tissues to improve dietary analysis in teuthophagous predators

Cephalopod beaks are commonly used to study the diets of teuthophagous predators because they persist in stomach contents, however, their isotopic composition differs from soft tissues, requiring species-specific corrections to accurate interpretation. We compared δ¹³C and δ¹⁵N values between lower beaks and soft tissues of two ecologically important cephalopod species in the western Mediterranean Sea: Todarodes sagittatus and Eledone cirrhosa, to quantify tissue-specific offsets and assess implications for predator dietary studies. δ¹³C differences were small and inconsistent, while δ¹⁵N values were consistently lower in beaks, with mean differences of 4.6‰ in T. sagittatus and 4.7‰ in E. cirrhosa. These findings provide correction factors for using beak-derived isotopic data in reconstructing teuthophagous predator diets. A marginally significant correlation between body size and δ¹⁵N values in beaks was found in both species. Isotopic niche analyses revealed a broad long-term overlap when using beaks, in contrast to distinct and minimally overlapping niches from soft-tissue, reflecting the different turnover rates of inert beaks versus metabolically active tissues. These findings highlight the importance of integrating multiple cephalopod tissues to capture both short-term and long-term trophic ecology traits.