Ecology and behaviour of holoplanktonic scyphomedusae and their interactions with larval and juvenile fishes in the northern Gulf of Mexico

Pelagia noctiluca is a venomous, globally distributed holoplanktonic scyphomedusa that periodically forms aggregations in coastal environments, yet little is known about its ecology and behaviour in the northern Gulf of Mexico (nGOM). Using a high resolution plankton imaging system, we describe the patch characteristics of Pelagia medusae in relation to fine-scale biological and physical variables during two summers at shallow (∼25 m, 2016) and deeper (∼45 m, 2011) sampling areas on the nGOM shelf. At the deeper site during the day, average Pelagia medusae concentrations just underneath a surface plume of fresher water (10–25 m) ranged from 0.18 to 0.91 ind. m−3, with a Lloyd’s patchiness index of 13.87, indicating strong aggregation tendencies (peak fine-scale concentration reached 27 ind. m−3). These patches were often associated with horizontal gradients in salinity, and concentrations of several zooplankton taxa (e.g. chaetognaths, hydromedusae, siphonophores, and ctenophores) were significantly negatively correlated with Pelagia medusae abundance (p < 0.0001, Spearman correlations). Although larval fish abundance was not correlated with Pelagia medusae on the 1 m3 scale (19.25 m horizontal distance), larval and juvenile fishes between 0.6 and 2.0 cm aggregated underneath the bell of some Pelagia medusae during the daytime only, even within hypoxic waters. Vertical distributions collected on a diel cycle demonstrated that Pelagia medusae perform a reverse diel vertical migration constrained by low salinity near the surface. These data suggest that salinity changes drive the distribution of Pelagia medusae vertically and horizontally, and when sufficient concentrations are present, medusae are capable of exerting a top-down effect on the abundances of their zooplankton prey. For zooplankton with high visual acuity, such as larval and juvenile fishes, the relationship with Pelagia medusae may change on a diel cycle and depend on the sensory ability of potential prey.