Spatiotemporal & cross-tissue brevetoxin analysis in juvenile blacktip sharks post-red tide exposure

Brevetoxins (PbTx), neurotoxins produced by the marine dinoflagellate Karenia brevis during harmful algal blooms (red tides), pose significant threats to marine fauna and coastal communities in the Gulf of Mexico. While acute toxicity is well-documented, the chronic, sub-lethal accumulation of brevetoxins in higher trophic levels, particularly in elasmobranchs, remains poorly understood. Despite monitoring efforts that typically confirm bloom presence or absence, the cumulative dose experienced by resident fauna, a product of bloom intensity (cell density) and duration, is rarely quantified across tissue types. In this study, brevetoxin concentrations are quantified via competitive ELISA and compared between brain, gill, stomach, liver, and muscle tissue from juvenile blacktip sharks (Carcharhinus limbatus) opportunistically collected between 2020 and 2025 in the Greater Tampa Bay Area of Florida. This analysis investigates tissue-specific bioaccumulation, temporal dynamics relative to bloom events, and the utility of muscle biopsies as a non-lethal monitoring tool. Results from aggregate analyses suggest that absolute brevetoxin concentrations (ppb) have no monotonic relationship with time since last bloom exposure. However, within-individual paired analyses reveal significant shifts in toxin distribution over time. Notably, the relative difference in brevetoxin concentration between brain and stomach tissues increases significantly with days post-exposure, suggesting a redistribution of neurotoxins between digestive and neural compartments. Conversely, muscle concentrations correlate poorly with all other tissue types, indicating that muscle biopsies may not be a reliable proxy for evaluating internal brevetoxin burden. Together, these findings indicate that, rather than total toxin load simply decreasing following a bloom, the internal partitioning of these toxins may evolve over time, particularly towards neural and hepatic tissues. This has significant implications for sub-lethal impacts on sharks, highlighting the need for chronic exposure monitoring in conservation efforts.