Global patterns of helminths associated with gelatinous zooplankton: a missing link in marine parasite transmission

Gelatinous zooplankton are abundant predators and prey in marine food webs, yet their role in helminth (parasitic worms) transmission remains poorly resolved. Here we combine a global synthesis of published records with new morphological and molecular observations from the Mediterranean, Red, Celtic, Baltic and North Seas to assess the ecological role of helminths associated with gelatinous zooplankton. We compiled 431 host-parasite association records from 89 sources, including 23 new records, and show that helminth occurrence and richness in gelatinous hosts are concentrated at temperate latitudes, contrasting with the classical latitudinal diversity gradient. Our sampling revealed markedly higher parasite prevalence, abundance and diversity in the Red Sea than in the Mediterranean, while no helminths were detected in gelatinous zooplankton sampled from the Baltic and North Seas. All helminths recorded were larval stages, indicating that gelatinous zooplankton function as key intermediate hosts in marine helminth life cycles. Molecular analyses identified cestodes, nematodes and digenean trematodes associated with cnidarians, ctenophores, and chaetognaths, including the first record of trematode larvae in a pelagic tunicate. Our findings challenge the view of gelatinous zooplankton as dead-end hosts and identify them as overlooked vectors that may shape marine parasite biogeography, food web connectivity and invasion dynamics.