The use of stable isotope chronologies in cephalopod beaks and eye lenses in trophic ecology studies: among-tissue fractionation patterns and among-species isotopic niche overlaps

This thesis explores the trophic ecology of Sepia officinalis in the English Channel using stable isotope analysis (SIA) of δ13C and δ15N to investigate inter-tissue fractionation patterns and potential niche overlap with other marine species. As isotopes of carbon and nitrogen, δ13C reflects the primary sources of carbon in the diet, while δ15N provides insight into an organism's trophic level. Together, these isotopic values allow for feeding habits and habitat use throughout an individual's life to be investigated. S. officinalis, an ecologically and commercially important cephalopod, plays a critical role in the English Channel ecosystem. However, its populations face increasing pressure due to overfishing and environmental changes, therefore a deeper understanding of its trophic ecology is essential to the resilience of this species. Chapter 2 focuses on the isotopic differences between various tissues of S. officinalis, including archival tissues like eye lenses and beaks, alongside metabolically active muscle tissue. Eye lenses effectively track ontogenetic changes in isotope ratios, while beaks display lower δ15N values, likely influenced by chitin and its morphology. Cross-tissue comparisons allowed for the development of correction factors, enabling more accurate and standardized isotopic measurements across tissues. The study also investigates isotopic niche overlap between S. officinalis and co-occurring species (Cancer pagurus and Homarus gammarus). Significant isotopic overlap could suggest shared dietary and habitat resources, with variability also be linked to differences in their ecology such as life cycles and foraging. These findings provide new insights into tissue-specific isotopic variation and ecological interactions, enhancing our understanding of the role of S. officinalis in marine ecosystems and which may help support sustainable fisheries management in the English Channel.