Pelagia noctiluca is a widely distributed scyphozoan jellyfish species. Despite extensive studies, detailed information on its distribution and ecology in the deep sea is lacking. Thanks to the analysis of large-scale ROV footage from 660 sites spanning a 1100 km latitudinal gradient and almost 1785 m in depth, a total of 168 P. noctiluca specimens were recorded, mostly as single individuals, although some sites showed high abundances. Jellyfish were observed down to 500 m, with the highest records occurring between 108 and 190 m. All individuals observed below ∼195 m were dead, suggesting a bathymetric threshold for living specimens. About 22% of jellyfish were swimming in the water column above the seafloor, while the majority were close by or lying on it, particularly on rocky and coralligenous substrates. Many interspecific interactions were documented, mainly involving structuring anthozoans (86%), in which jellyfish were frequently trapped. Crustaceans and the echinoid Stylocidaris affinis interacted mostly with dead or moribund jellyfish (almost 86% of the records), likely scavenging. Overall, this large-scale dataset confirms a broad distribution of P. noctiluca across the Mediterranean basin and provides the first in situ evidence of interactions between this species and deep-sea benthic fauna, likely as a food subsidy. The findings emphasise the ecological relevance of jellyfish in pelagic-benthic coupling and highlight the need for further surveys, particularly in winter and on soft sediments, to fully assess their trophic role and seasonal dynamics.
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