Stable isotope fractionation between maternal and embryo tissues in the Bonnethead shark (Sphyrna tiburo)

Abstract

Evaluating tissue fractionation between mothers and their offspring is fundamental for informing our interpretation of stable isotope values in young individuals and can provide insight into the dynamics of maternal provisioning. The objectives of this study were to investigate the isotopic relationships between maternal reproductive (i.e., yolk, yolk-sac placenta) and somatic tissues (i.e., muscle and liver) relative to embryos in the Bonnethead Shark Sphyrna tiburo, to evaluate the fractionation of stable carbon (δ13C) and nitrogen (δ15N) isotopes between these tissues. Additionally, we examined intra-uterine variability in the isotopic relationships to ascertain whether this species may exhibit variable nutrient allocation. Embryos showed similar magnitudes of enrichment in 13C (i.e., Δδ13C, difference between adult and embryo) relative to adult tissues (Δδ13C = ~1.0‰). However, embryos were depleted in 15N relative to adult muscle tissues (Δδ15N = −1.0‰), a finding that contrasts Δδ15N values reported for other placentotrophic sharks. Embryo-muscle Δδ15N was correlated with length, supporting the contention that the magnitude of enrichment between embryonic and maternal tissues results from the shift from yolk to placental feeding. Embryo δ15N and Δδ15N values showed significant intra-uterine variability; a result not observed for δ13C and Δδ13C values. The contrasting patterns in fractionation among placentotrophic sharks highlight the importance of evaluating these relationships across elasmobranch taxa with consideration for different tissues, reproductive strategies and stages of gestation. The divergent findings support future evaluation of stable isotope relationships between mothers and offspring for purposes of estimating inherent isotopic variability and how this variability may inform physiological and dietary mechanisms.