A new genus and species of hagfish from Vietnam: A transitional lineage between Rubicundinae and Myxininae (Craniata: Myxiniformes).

Dolichostomus leucotaenia gen. et sp. nov. is described from two specimens collected from off northern Vietnam. It is characterized by a long, oval mouth opening visible from the ventral view, a feature unique among the order Myxiniformes. It also possesses a single medial nasal-sinus papilla and branchial slime pores positioned external to the gill-aperture series. Moreover, it can be further distinguished by its slender body, a distinct white mid-dorsal stripe, five pairs of gill apertures arranged in a straight line, and ventral aorta not bifurcated. The character states of the relative position of branchial slime pores and gill-aperture series and bifurcation of ventral aorta suggest that D. leucotaenia sp. nov. represents a transitional lineage between the subfamilies Rubicundinae and Myxininae.