The curious case of the chemical composition of hagfish tissues--50 years on

Modern hagfishes are considered to be the most primitive of the living craniates and along with their close jawless agnathan relative, the lamprey, take us back an astonishing 500 million years to the base of the vertebrate evolutionary tree. The unique osmoconforming strategy of the hagfish, whereby the osmotic constituents of the blood plasma bear more of a resemblance to marine invertebrates than vertebrates, has been classically depicted in comparative physiology textbooks for many years. Fifty years ago in this journal, Bellamy and Chester Jones [Bellamy and Chester Jones, 1961. CBP 3, 173-183] published a paper on the chemical composition of the tissues of the Atlantic hagfish, Myxine glutinosa. This publication was one of a flurry of papers published in the 50s, 60s and early 70s focused on describing the ionic and osmotic components of this bizarre fish. Here we take a retrospective look at the research that has taken place on these intriguing animals prior to and following the Bellamy and Chester Jones manuscript, focusing on tissue chemical compositions, the possible role of amino acids, and our current view on ion regulation, metabolism and hypoxia tolerance.