New Contracaecum species records in fish and the description of C. sistanense sp. nov. in birds from Iran

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Understanding the richness and distribution of Contracaecum spp. in aquatic ecosystems is important because these zoonotic nematodes circulate through complex food webs involving invertebrates, fish, birds, and occasionally humans. This study investigated the species richness and distribution of Contracaecum spp. in Iranian waters through a literature review combined with morphological and molecular characterisation of adult specimens collected from piscivorous birds. The literature review demonstrated that most previous reports involved larval stages recovered from 39 fish species across 16 families inhabiting freshwater, marine, and brackish environments in 9 Iranian provinces. However, the identity of most larval forms remains unresolved due to the absence of molecular data. Molecular and morphological analyses of adult nematodes collected from the great white pelican, Pelecanus onocrotalus , confirmed the presence of Contracaecum micropapillatum and C. gibsoni in Iran for the first time and led to the description of a new species, Contracaecum sistanense sp. nov. Together with the previously molecularly confirmed record of C. multipapillatum , these findings increase the number of confirmed Contracaecum species recorded in Iran to four. The present study improves current knowledge of Contracaecum spp. in Iran and highlights the need for broader surveys across diverse hosts and geographical regions to better understand the distribution, taxonomy, and zoonotic potential of these parasites.