Feeding in Eptatretus cirrhatus: effects on metabolism, gut structure and digestive processes, and the influence of post-prandial dissolved oxygen availability

Hagfishes are characterised by feeding behaviours that may include long intervals between meals, and a hypoxic feeding environment inside decaying carrion. The effects of feeding on metabolism (oxygen consumption rate), gut mass and morphology (gut somatic index, gut epithelium mucosal thickness), and digestive function (maltase and peptidase activity) were examined in the New Zealand hagfish, Eptatretus cirrhatus. The influence of post-prandial hypoxia on oxygen consumption rate was also investigated to replicate the immersive feeding environment. Fed hagfish displayed a 1.9-fold increase in peak oxygen consumption relative to sham controls. This elevation in post-prandial oxygen consumption continued for 72 h, during which the energy cost of digesting the meal (specific dynamic action; SDA) was 2.1 kJ. Oxygen consumption rate increased when the post-prandial environment was hypoxic, a response suggesting a lack of hypoxia tolerance in this species. Feeding did not alter gut somatic index (percentage of digesta-free gut mass to whole body mass), but there was an increase in the mucosal thickness of the gut epithelium. Maltase activity in the gut was unchanged by feeding, but the activity of gut peptidases was increased significantly, consistent with a protein-based diet. These data indicate that some postprandial responses of New Zealand hagfish are similar in nature to those seen in other animals, but this species does not exhibit the extreme post-prandial physiological and biochemical changes that are observed in other intermittently-feeding vertebrates.