Investigating eye lens composition for stable isotope analysis in fishes: a comparison between Chondrichthyes and Actinopterygii

Life history ecology provides a framework for understanding the complex interactions between organisms and their environments but is challenging to resolve for long-lived and migratory species. In fishes, the combination of movement and foraging ecology is predominantly explored with stable isotope analysis (SIA) of accretionary tissues, such as otoliths. An alternative archival tissue validated for SIA in Actinopterygii (i.e., ray-finned fishes) is the proteinaceous eye lens and its growth layers (laminae). Here, we aim to expand the SIA of laminae to include Chondrichthyes (cartilaginous fishes; sharks, skates, rays, sawfish, and chimeras). We observe that urea in the eye lens drives patterns of elemental composition (i.e., C:N ratio) for Chondrichthyes, but the isotopic effect of urea is negligible. Actinopterygian fishes exhibit consistent C:N ratios across eye lenses, while chondrichthyan eye lens laminae C:N ratios decrease from the inner post-apoptotic laminae to outer pre-apoptotic laminae. After confirming the presence of urea in the Leopard Shark (<jats:italic>Triakis semifasciata</jats:italic>) eye lens laminae with attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectroscopy and urea assays, we successfully removed urea from eye lens laminae via three deionized water rinses. Subsequently, the C:N ratios of post-urea extracted Leopard Shark eye lens laminae exhibited similar patterns to actinopterygian fishes. Urea removal from Leopard Shark eye lens laminae increased in δ<jats:sup>15</jats:sup>N values as expected, but the isotopic effect was within analytical error. Our results indicate the utility of chondrichthyan eye lens laminae as chronological tissues for SIA. However, there are isotopic effects dependent on urea concentrations. This study demonstrates the need for inter-taxa comparison when establishing novel methodologies in SIA.