Redescription of the Larval Trematode Taxon Monilicaecum ventricosum (Hemiuroidea: Didymozoidae) from Type Host Cololabis saira, with Inference of the Definitive Host Based on DNA Analysis

The larval trematode taxon Monilicaecum ventricosum Yamaguti, 1942 (Didymozoidae) was redescribed from its type host, the Pacific saury Cololabis saira (Brevoort, 1856), collected from the North Pacific off Eastern Hokkaido, Japan. Because didymozoid larvae and adults differ markedly in morphology, linking these two stages using morphological data alone has traditionally been difficult. Consequently, the monotypic genus Monilicaecum Yamaguti, 1942 is currently regarded as a collective group, similar to genera established solely on larval forms of other trematodes. However, recent advances in molecular studies have enabled researchers to link larval and adult stages, clarify life cycles, and undertake more robust taxonomic evaluations. In this study, newly collected larval specimens were examined morphologically, and DNA barcodes (28S, ITS, and cox1) were generated to confirm their identity as M. ventricosum through morphological comparisons with type materials. These sequences were compared with those available in public databases to determine the corresponding adult stages and definitive hosts. Although no identical adult sequences were detected, M. ventricosum showed a close genetic relationship with four didymozoid species parasitizing the Australian yellowfin tuna, Thunnus albacares (Bonnaterre, 1788), suggesting that its definitive host is likely a species of Thunnus South, 1845. These findings provide essential molecular data for M. ventricosum, contribute to the taxonomic stability of Didymozoidae, and offer a foundation for future studies aimed at elucidating species boundaries and life-cycle connections within this group.