The sui generis gut of the hagfish: a focal point of hagfish physiology

A functioning digestive system requires the integration of many organ systems. In hagfishes, by virtue of their unusual digestive physiology, some of these integrative processes differ from those of other vertebrate groups, while the gut itself can have important roles in the function of other body systems. The digestive physiology of hagfishes is particularly influenced by their ecology and their feeding behaviour as scavengers that routinely feed on dead and decaying matter on the ocean floor. In this Commentary, we investigate the integration between the gut of hagfishes and the nervous, muscular, endocrine, immune, ionoregulatory and excretory, cardiovascular, respiratory and integumentary systems, emphasising the sui generis nature of the hagfishes. We also highlight the important consideration of the hagfish phylogenetic position and whether traits are of a primitive or derived nature, leading to different interpretations of digestive physiology in the context of functional evolution or adaptation to unique feeding lifestyles. We highlight putative areas of future investigation that will enhance not only our understanding of hagfish themselves but also the evolution of the highly integrative process of digestion.