Stable isotopes and metals and metalloids in the smallest Megamouth Shark (Megachasma pelagios) reported to date from the Atlantic Ocean.

The Megamouth Shark is a circumtropically distributed filter-feeding species with only some records from the Atlantic Ocean available to date. In this study, a very young specimen, possibly a neonate, found floating off Pedra do Sal Beach, Piauí, Brazil, in 2018, was analyzed for stable isotopes and metals and metalloids. This research opens a window for investigating aspects of the biology of this scarcely studied shark at this developmental stage in the Western Equatorial Atlantic Ocean. Stable isotope values were δ13C = -18.66 ± 0.15‰ and δ15N = 5.93 ± 0.09‰, likely reflecting maternal isotopic transfer to the embryo. An exploratory Bayesian mixing model (MixSIAR) indicated an isotopic baseline driven by smaller zooplankton, reflecting feeding in oligotrophic tropical waters during gestation. Among the 65 assessed elements, 41 are reported for the first time for the species, with their concentrations aligning with maternal transfer. A relatively low Hg concentration (0.015 mg kg-1) suggests minimal environmental bioaccumulation. The high Al (116 mg kg-1) and Pb (0.11 mg kg-1) levels suggest maternal offloading via yolk or uterine fluids. Total As was relatively high (4.6 mg kg-1), likely reflecting ocean-basin-specific biogeochemical baseline and mainly organic chemical forms typical of deep-water elasmobranchs. Finally, the findings reinforce the importance of continuous monitoring in the Atlantic Ocean to expand and consolidate knowledge on the ecology and ecotoxicology of this species.