The Gulf of Trieste, a hotspot for gelatinous zooplankton and their digenean parasites?

Gelatinous zooplankton is increasingly recognized as a key component of marine ecosystems due to its multiple trophic interactions. Our study investigated the relationship between gelatinous zooplankton and digenean parasites, focusing on the role of jellyfish as second intermediate hosts between benthic gastropods, the first hosts, and pelagic fish, the definitive hosts. These parasitological data could provide important insights on the trophic position of jellyfish and the potential interactions between fish and gelatinous organisms. We examined, for the first time in the Gulf of Trieste, the occurrence of digenean parasites in common gelatinous species, including Aurelia sp., Rhizostoma pulmo, Aequorea forskalea, and the ctenophore Mnemiopsis leidyi. All species were infected by digeneans regardless of sampling location or season. Parasites exhibited noticeable tolerance to seasonal environmental variability, since water temperatures ranged from 9.2°C in winter to approximately 25°C in summer. Two species, belonging to the family Lepocreadiidae, were preliminarily identified through morphological and molecular analyses: one putatively assigned to the genus Clavogalea, recorded for the first time in a jellyfish host and in the Mediterranean Sea, and one belonging to the genus Opechona, previously reported in jellyfish from extra-Mediterranean areas. High parasite prevalence (>80%) and infection intensity (reaching hundreds of individuals per host) were consistently observed across all gelatinous species. From a broader perspective, these results support the hypothesis that gelatinous zooplankton is a component of fish diet in the area, since trophic interactions are necessary for parasites to complete their life cycles.