Ontogenetic feeding shifts in two thresher shark species in the Galapagos Marine Reserve.

BACKGROUND

The morphology and hunting behavior of thresher sharks make them easily distinguishable. These species are distributed across the Tropical Pacific Ocean feeding on squid and small fish. However, ontogenetic changes in their feeding strategies and habitat use are still unknown in this region.

METHODS

We examined the δC and δN signatures in vertebral collagen from populations of and inhabiting the Galapagos Marine Reserve, focusing on three maturity stages: neonate, juvenile and adult. The vertebrae samples were taken from the seizure of illegal fishing activities carried out by a foreign fleet within the Galapagos archipelago. A total of thirty-three vertebrae from and twenty-one from were analyzed.

RESULTS

Both species displayed significant differences in their δN values ( < 0.001), but not in δC ( = 0.230), suggesting a similar habitat use, but different prey consumption. Throughout their ontogeny, displayed isotopic differences ( < 0.001), where neonates showed lower δC values and higher δN values compared to juveniles, probably because they still reflect the isotopic signatures of their mothers even after the first year of life. This study highlights trophic differences between both species, accompanied by an ontogenetic variation in , aspects that allow us to understand the role of these species within the dynamics of the Eastern Tropical Pacific ecosystem.