Disentangling shared and divergent life histories with age-based eye lens isotope records in a coastal shark.

Abstract

Documenting lifetime diet preference and habitat use is critical for understanding how long-lived, mobile species respond to ecological change, yet such data remain sparse - particularly for comparisons within and among individuals, species, and populations. Eye lens layers (laminae) provide a biochemical archive that records these ecological signals sequentially through time, creating a unique opportunity to reconstruct entire life histories. This approach is especially valuable for elusive chondrichthyans (sharks, skates, rays, and chimeras), whose ecological importance as meso- to apex predators contrasts with the logistical challenges of tracking their movements and resource use. Stable isotope analysis (SIA) of vertebral growth bands is the established method for aging and reconstructing isotopic life histories in chondrichthyans, but eye lens laminae represent a promising yet under-verified alternative. We compared δ¹³C and δ¹⁵N values derived from eye lens laminae and vertebral bands of wild leopard sharks (Triakis semifasciata) to evaluate the ecological utility of eye lenses. Eye lens diameter increased linearly with total length (R²  =  0.95); however, because older laminar cells become progressively compacted, we incorporated a compaction index derived from attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy into aging models. This integration yielded age-specific δ¹³C and δ¹⁵N patterns that closely mirrored those observed in vertebral centra. Across ages, δ¹³C offsets between eye lens laminae and vertebrae were inconsistent, whereas δ¹⁵N values were largely concordant, with only modest increases in the outer laminae. To further verify eye lens isotopic records, we compared δ¹³C and δ¹⁵N values from the terminal eye lens lamina and muscle tissue, enabling estimation of age-specific tissue discrimination factors for both isotopes. Finally, eye lens isotope profiles revealed shared and divergent life history patterns across four Central California estuaries. Collectively, our results highlight eye lens laminae as a robust isotopic substrate for reconstructing lifetime ecological context in chondrichthyans.