Abstract
We surveyed trematode parasites infecting 227 gulls from southern New Zealand, comprising 113 red-billed gulls (Chroicocephalus scopulinus) and 114 black-backed gulls (Larus dominicanus), using morphological and molecular approaches. Fifteen trematode species were recovered, including six species new to science: Mesorchis carsonae n. sp., Acanthoparyphium argillamartelliarum n. sp., Himasthla davisi n. sp., Gymnophalloides hutsonae n. sp., Levinseniella trisepsilon n. sp., and Microphallus lagruei n. sp. Nine species occurred in both gull hosts; the remaining six were recorded from only black-backed gulls. New genetic data were generated for 12 species, including all new species, enabling confirmation of species identities and links to larval stages in intermediate hosts, thereby resolving or advancing knowledge of several life cycles. Several parasites represent new host or geographic records, including the first named species of Diplostomum, Gigantobilharzia, Levinseniella, Gymnophalloides, Microphallus and Mesorchis reported from New Zealand. Parasite burdens were generally low, but species richness was high, underscoring gulls as important definitive hosts supporting substantial hidden biodiversity. This study provides the first comprehensive baseline survey of trematode communities in New Zealand gulls and highlights that parasite diversity remains underestimated even in comparatively well-studied hosts. These findings reinforce the importance of incorporating parasites into biodiversity inventories and conservation frameworks.