Stable‐isotope comparisons between embryos and mothers of a placentatrophic shark species

Stable nitrogen (δ¹⁵N) and carbon (δ¹³C) 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 δ¹⁵N (i.e. Δδ¹⁵N) being 1·4‰ for muscle, 1·7‰ for liver and 1·1‰ for cartilage. Embryo muscle and liver were enriched in ¹³C (both Δδ¹³C means = 1·5‰) and embryo cartilage was depleted (Δδ¹³C mean = ?1·01‰) relative to corresponding maternal tissues. While differences in δ¹⁵N and δ¹³C between mothers and their embryos were significant, muscle δ¹⁵N 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 (L_T) and Δδ¹⁵N for muscle and liver and embryo L_T and Δδ¹³C 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 δ¹⁵N and δ¹³C 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.