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

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Abstract
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 δ 13 C and δ 15 N 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. δ 13 C differences were small and inconsistent, while δ 15 N 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 δ 15 N 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.
Funding Information
  • Universitat de Barcelona

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