Stable-isotope comparisons between embryos and mothers of a placentatrophic shark species
Stable nitrogen (15N) and carbon (13C) isotopes of Atlantic sharpnose shark Rhizoprionodon terraenovae embryos and mothers were analysed. Embryos were generally enriched in 15N in all studied tissue relative to their mothers' tissue, with mean differences between mother and embryo 15N (i.e. Δ15N) being 1·4‰ for muscle, 1·7‰ for liver and 1·1‰ for cartilage. Embryo muscle and liver were enriched in 13C (both Δ13C means = 1·5‰) and embryo cartilage was depleted (Δ13C mean = −1·01‰) relative to corresponding maternal tissues. While differences in 15N and 13C between mothers and their embryos were significant, muscle 15N values indicated embryos to be within the range of values expected if they occupied a similar trophic position as their respective mothers. Positive linear relationships existed between embryo total length (LT) and Δ15N for muscle and liver and embryo LT and Δ13C for muscle, with those associations possibly resulting from physiological differences between smaller and larger embryos or differences associated with the known embryonic nutrition shift (yolk feeding to placental feeding) that occurs during the gestation of this placentatrophic species. Together these results suggest that at birth, the 15N and 13C values of R. terraenovae are likely higher than somewhat older neonates whose postpartum feeding habits have restructured their isotope profiles to reflect their postembryonic diet.
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