Digenean parasites as novel tracers of predation on jellyfish

Cnidarian medusae and ctenophores were long considered trophic dead ends due to their low nutritional value and they are often overlooked in the digestive tracts of predators. Traditional and emerging methods of analysing diets, including gut content analysis, stable isotopes, and metabarcoding, have limitations in identifying jellyfish consumption. This review explores the potential of using endoparasitic trematodes as novel trophic tracers of jellyfish predation. Trematodes, which require multiple hosts to complete their life cycles, can serve as natural dietary tags, with their presence in definitive hosts reflecting consumption of infected intermediate hosts, including jellyfish. First, we systematically compiled published records of trematode metacercariae in jellyfish and supplemented these with new, unpublished records from off eastern Australia. To identify potential jellyfish predators, we then examined the published literature for reports of definitive hosts of trematode species known to infect jellyfish as metacercariae. We identified 230 definitive host-parasite associations involving 64 putative trematode species. Molecular and morphological analyses linked these parasites to 140 fish species across 48 families, confirming some known predators of jellyfish (e.g. some carangids) and identifying some previously unrecognised potential jellyfish predators (e.g. some scombrids). While host specificity of digeneans in jellyfish and fish hosts varies, some trematodes show strong potential as indicators of jellyfish predation. However, the accurate identification of metacercariae (to the level of individual species) remains a key limitation, underscoring the need for integrative taxonomic approaches that incorporate molecular data. This review highlights the promise of trematodes as powerful tools for elucidating jellyfish predation and improving our understanding of marine food webs.