Enhancing our understanding of stable isotope analysis in the eye lenses of fishes

Background/Question/Methods

: Ecological studies in fishes often use stable isotope analysis (SIA) of accretionary tissues to resolve detailed movement, trophic patterns, and natal origin; however, the majority of these studies use accretionary tissues that are inorganic in chemical form (e.g., otoliths), which are not present in all fishes and less than ideal for answering diet related questions. Eye lens layers (laminae) have recently been validated as a chronological tissue for SIA sampling in fishes. Laminar tissue is primarily composed of a crystallin protein, with isotopic composition (δ13C and δ15N) of laminae reflecting diet and movement across the entire lifetime of an individual. While eye lens studies have primarily focused on teleosts (i.e., ray finned fishes), chondrichthyan (i.e., cartilaginous fishes) physiologies differ regarding concentrations of lipid and urea within their tissues, which can affect SIA. To assess the isotopic differences in eye lens SIA methodology in fishes (chondrichthyan vs teleost), eyes were extracted from 10 adult Leopard Sharks (Triakis semifasciata) and 10 adult Chinook Salmon (Oncorhynchus tshawytscha) from Northern California. Eye lenses were measured during serial sampling from the outer lamina (death) to laminar core (birth), prior to being analyzed via continuous-flow Isotope Ratio Mass Spectrometer coupled to an Elemental Analyzer.

Results/Conclusions

: During laminar sampling, a distinct difference in composition was observed between the outer unformed laminae (cortex) and the inner formed laminae (nucleus). A Mann-Whitney U Test was used to determine that the means of C/N ratios between these different compositions of eye lens laminae (i.e., cortex vs nucleus) were significantly different in Leopard Sharks (p < 2.2e-16), but not in Chinook Salmon (p = 0.1609). With C/N ratios significantly higher in the Leopard Shark eye lens nucleus and lower in the cortex, this may indicate a lipid (carbon rich) or urea (nitrogen rich) effect. A linear regression model found a significant relationship between both C/N and δ13C values (p < 2.2e-16; r2 = 0.46) as well as C/N and δ15N values (p = 1.283e-10; r2 = 0.28) in Leopard Shark eye lens laminae. Although previous findings have found significantly higher lipid concentrations in the nucleus compared to cortex of teleost fish eye lenses, and lipids are known to influence δ13C values, there was no significant relationship found between C/N and δ13C or δ15N values for Chinook Salmon. This may indicate a contrasting lipid vs urea effect that needs to be considered in future chondrichthyan studies utilizing eye lens SIA methodologies.