Gross anatomy of the Pacific hagfish,Eptatretus burgeri, with special reference to the coelomic viscera

Abstract

Hagfish (Myxinoidea) are a deep-sea taxon of cyclostomes, the extant jawless vertebrates. Many researchers have examined the anatomy and embryology of hagfish to shed light on the early evolution of vertebrates; however, the diversity within hagfish is often overlooked. Hagfish have two lineages, Myxininae and Eptatretinae. Usually, textbook illustrations of hagfish anatomy reflect the morphology of the former lineage, especially Myxine glutinosa, with its single pair of external branchial pores. Here, we instead report the gross anatomy of an Eptatretinae, Eptatretus burgeri, which has six pairs of branchial pores, especially focusing on the coelomic organs. Dissections were performed on fixed and unfixed specimens to provide a guide for those doing organ- or tissue-specific molecular experiments. Our dissections revealed that the ventral aorta is Y-branched in E. burgeri, which differs from the unbranched morphology of Myxine. Otherwise, there were no differences in the morphology of the lingual apparatus or heart in the pharyngeal domain. The thyroid follicles were scattered around the ventral aorta, as has been reported for adult lampreys. The hepatobiliary system more closely resembled those of jawed vertebrates than those of adult lampreys, with the liver having two lobes and a bile duct connecting the gallbladder to each lobe. Overall, the visceral morphology of E. burgeri does not differ significantly from that of the known Myxine at the level of gross anatomy, except for the number of branchial pores.

Gene expression in the liver of the hagfish (Eptatretus burgeri) belonging to the Cyclostomata is ancestral to that of mammals

“…The liver is an important metabolic organ common to vertebrates, and may have been acquired by vertebrate ancestors during evolution. The hagfish, which is an earliest diverged lineage among vertebrates (the Cyclostomata) and lacks many characteristics of jawed vertebrates such as jaws and vertebral centra (Higuchi et al, 2019; Kuraku & Kuratani, 2006; Muramatsu et al, 2023; Ota et al, 2011), has an ancestral liver structure histologically resembling that of the tetrapod but characterized by its tubular structure (two‐cell cords) surrounded by hepatic sinusoids, and the ancestral condition of the portal triad with periportal biliary ducts or abundant ductules (Umezu et al, 2012). The morphology of the hagfish is regarded to be highly derived, including the degeneration of eyes due to adaptation to the deep sea (Dong & Allison, 2021; Gabbott et al, 2016).…”