Cyclostomes (hagfishes and lampreys) is the only extant clade of jawless vertebrates so-called agnathans. The cyclostome lineage emerged approximately 500 million years ago, and it is a key group in discussing the origin and evolution of vertebrates. Although lampreys and gnathostomes found their way out of the ocean, hagfishes have stayed exclusively marine.

In January 2024, the genome assemblies of the inshore hagfish (Eptatretus burgeri) and the brown hagfish (Eptatretus atami) was published (※1, ※2). These achievements will promote future studies on hagfish biology.

※1  Marlétaz, F., Timoshevskaya, N., Timoshevskiy, V.A. et al. The hagfish genome and the evolution of vertebrates. Nature 627, 811–820 (2024). https://doi.org/10.1038/s41586-024-07070-3.

※2  Yu, D., Ren, Y., Uesaka, M. et al. Hagfish genome elucidates vertebrate whole-genome duplication events and their evolutionary consequences. Nat Ecol Evol 8, 519–535 (2024). https://doi.org/10.1038/s41559-023-02299-z.

To date over 80 hagfish species have been reported from the ocean all over the world. Most of them live in the deep sea and make essential contributions to the oceanic biochemical cycles by eating animal remains on the sea floor.

Hagfish eggs are of oval-shape and reaches 1.4-2.5 cm in longitudinal length. The large egg sizes in turn put limitation on the number of eggs per female (~70 per fish). Moreover, it takes more than a year for embryonic development (※3).  These observations indicate that the fecundity of hagfishes is very low. 

※3  Ota KG, Kuraku S, Kuratani S. Hagfish embryology with reference to the evolution of the neural crest. Nature 446, 672–675 (2007). doi: 10.1038/nature05633.