Abstract
Clinostomum species are typical trematodes (or flatworms) and zoonotic parasites of humans, fish, and birds. These parasites require at least two definitive hosts, fish and birds, to complete their life cycle. Previous studies indicated that the yellow grub, identified as C. complanatum, first appeared in northern Taiwan around the 1990s, with uncertain origins. This study identified 65 of 2,181 museum fish specimens with leech-like metacercariae across four main river systems (Tamshui, Houlong, Tzengwen, and Xiuguluan Rivers) and documented new infection records in fishes from Beigang, Puzih, Kaoping, and Bie Rivers during subsequent field work. The parasite appears to have established in the Houlong and Tamshui Rivers before dispersing to southern and eastern waterways. COI barcode analysis revealed that most metacercariae belong to C. sinensis with low nucleotide diversity (π = 0.00314353). The closely related haplotypes with insignificant Tajima's D (-1.89473 with p-value = 0.981839) suggest a gentle population expansion after their colonization to Taiwan. Additionally, yellow grub infections were more prevalent in carnivorous fishes (> 60%) compared to omnivorous and algal-feeding fishes. The high infection rates documented in literature and museum specimens suggest that Jhonggang and Houlong rivers represent the primary (or earlier) infection areas from which the parasite subsequently spread throughout Taiwan, highlighting the need for enhanced regulations to protect endangered or cultivated species.