Comprehensive analysis of the complete mitochondrial genome of Didymozoidae parasites infecting yellowfin tuna ( <i>Thunnus albacares</i> )

In this study, parasites were isolated from yellowfin tuna ( Thunnus albacares ) and subjected to morphological examination, histological sectioning, and molecular identification. Morphological observations highlighted that the parasite exhibits a total length ranging from 1.5 to 4 mm, characterized by a pale-yellow coloration. Its body is distinctly segmented into anterior and posterior sections, and it is found parasitizing the gill arches and intestinal cecum of the yellowfin tuna. The parasite’s reproductive structure features a uterus, testes, vitelline ducts, vas deferens, and a collective genital apparatus, accompanied by a minimized digestive system. Such findings hint at its possible classification within the Didymozoidae family. The complete mitochondrial genome was determined to have a length of 16,468 base pairs (bp), comprising 32 genes—including 12 protein-coding genes, 19 tRNA genes, and one rRNA gene—along with an uncharacterized gene region that spans 4,289 bp. Notably, it lacks the ATP8 gene, three tRNA genes, and the rrnL gene. Additionally, the COX1 gene shared 88.54% sequence similarity with Koellikeria sp. , while the 18S rRNA and 28S rRNA genes exhibited 97.24% and 96.81% similarity with Didymozoid sp . and Didymosulcus sp. , respectively, all within the family Didymozoidae. These findings support the classification of the parasite within Didymozoidae and suggest that it may represent a potentially novel genus closely related to Didymozoid and Didymosulcus . This study reports for the first time the mitochondrial genome of an unknown genus of Didymozoidae parasites, enriching the mitochondrial gene information database for this species and providing foundational data for further identification of Didymozoidae parasites.