Reconstructing age and growth history of wild green turtles using eye lens radiocarbon

Abstract

Accurate estimation of age and lifetime growth in sea turtles has long been constrained by their exceptional longevity and extensive migration. Bomb 14C dating has recently attracted attention as a powerful tool for age estimation in long-lived organisms. However, its application requires tissues that retain a lifelong isotopic record. We focused on the eye lens—an inert, non-metabolic tissue that persists throughout the life of an organism—and applied bomb 14C dating to estimate age and reconstruct growth histories of wild green turtles (Chelonia mydas). Eye lenses were collected from 48 individuals obtained through legal turtle fisheries, strandings, and mortality in captivity. Age estimation was conducted for eight females and four males using bomb 14C dating. Each lens was dried and scraped layer-by-layer from the exterior toward the core to obtain F¹⁴C measurements representing different body sizes. Using coral-derived F¹⁴C reference records encompassing the turtles’ foraging range, we inferred birth years from lens core F¹⁴C values and determined chronological ages. Inner lens sections consistently exhibited higher F¹⁴C values than outer sections, reflecting the expected temporal decline following the bomb peak. The estimated ages ranged from 34.9 to 45.6 years for females and from 29.5 to 48.1 years for males. These results indicate that females and males reach sexual maturity at approximately 31 and 28 years of age, respectively, which is consistent with the results of previous studies. The oldest individuals suggested a minimum lifespan of approximately 48 years. Radiocarbon measurements were obtained from multiple lens layers in three individuals. Using the allometric relationship between lens size and straight carapace length derived from all samples, we estimated the body size at the time each layer was formed and reconstructed individual growth trajectories. The resulting growth curves, including growth rates and asymptotic body sizes, were consistent with estimates from mark–recapture and skeletochronological studies on C. mydas. This study suggests that the reptilian eye lens provides a chronologically layered structure that remains stable through time, offering a promising framework for retrospective ecological and life-history reconstruction.