From surfacing to stranding: The origins and dispersal dynamics of a neustonic siphonophore

Abstract

The Portuguese man‐o'‐war, Physalia physalis , regularly washes ashore on the US East Coast, but the physical and biological processes that determine when and where these animals strand are not well understood. Here we integrate iNaturalist observational data with a biologically‐informed particle‐tracking model to investigate the origins, dispersal, and stranding dynamics of P. physalis in the northwest Atlantic Ocean, building on recently reported population genomic structure. We find that juvenile P. physalis largely appear in Florida and on the Gulf of Mexico coast in winter months, and we use these findings to initialize particle‐tracking simulations based on observed juvenile distributions. Using models that incorporate Physalia drift hydrodynamics and high‐resolution forcing data, we show that P. physalis undergo rapid Gulf Stream transport along the US East Coast, with seasonal reproduction and onshore wind patterns shaping distinct regional stranding peaks, consistent with observed stranding data from iNaturalist. Our results suggest that seasonal Gulf Stream dynamics, including eddy activity, influence P. physalis transport in offshore regions and modulate strandings north of Cape Hatteras. Our study highlights the Gulf Stream's critical role in structuring planktonic distributions in the northwest Atlantic and reveals the transport dynamics that allow sailing species like P. physalis to disperse across vast regions of the ocean.