Quantifying ontogenetic stable isotope variation between dermis and muscle tissue of two pelagic sharks

Abstract

Comparative analysis of isotope values from different tissues can capture temporal variation in the trophic and foraging behavior of difficult to study large marine predators, revealing either uniform or variable ecological roles over time. The isotopic values (δ13C and δ15N) of dermis, and muscle tissue of silky Carcharhinus falciformis and blue sharks Prionace glauca sampled in the northeast central Pacific were analyzed to quantify ontogenetic inter- and intra-tissue isotopic variation. Consistent differences in δ15N values occurred between dermis and muscle tissue for both species (2.5 ± 0.4‰ and 2.1 ± 0.3‰, respectively), while tissue differences in δ13C values were more variable between species (2.3 ± 0.6‰ and 0.7 ± 0.6‰, respectively), likely a result of tissue composition. The overall δ15N and δ13C values of dermis and muscle were highly correlated for blue sharks and for silky sharks with the exception of silky shark δ13C values. This pattern indicates that dermis isotope values are able to provide a proxy for muscle tissue, similar to that previously reported for fin, accepting dermis-specific diet-tissue discrimination factors. Tissue-specific ontogenetic isotopic variation for the silky shark, and the low regression slope value between dermis and muscle δ13C values, however, may suggest that dermis and muscle tissue have different isotopic turnover rates. These data demonstrate that dermis yields valuable isotope data to examine the trophic ecology and feeding/movement behavior of sharks, but further work is required to address dermis-specific turnover rates and diet-tissue discrimination factors.