Hagfish represent the oldest extant connection to the ancestral vertebrates, but their physiology is not well understood. Using behavioural (video), physiological (respirometry, flow measurements), classical morphological (dissection, silicone injection) and modern imaging approaches (micro-MRI, DICE micro-CT), we examined the interface between feeding and the unique breathing mechanism (nostril opening, high-frequency velum contraction, low-frequency gill pouch contraction and pharyngo-cutaneous duct contraction) in the Pacific hagfish, Eptatretus stoutii. A video tour via micro-MRI is presented through the breathing and feeding passages. We have reconciled an earlier disagreement as to the position of the velum chamber, which powers inhalation through the nostril, placing it downstream of the merging point of the food and water passage, such that the oronasal septum terminates at the anterior end of the velum chamber. When feeding occurs by engulfment of large chunks by the dental plates, food movement through the chamber may transiently interfere with breathing. Swallowing is accelerated by peristaltic body undulation involving the ventral musculature, and is complete within 5 s. After a large meal (anchovy, 20% body mass), hagfish remain motionless, defaecating bones and scales at 1.7 days and an intestinal peritrophic membrane at 5 days. O2 consumption rate approximately doubles within 1 h of feeding, remaining elevated for 12–24 h. This is achieved by combinations of elevated O2 utilization and ventilatory flow, the latter caused by varying increases in velar contraction frequency and stroke volume. Additional imaging casts light on the reasons for the trend for greater O2 utilization by more posterior pouches and the pharyngo-cutaneous duct in fasted hagfish.

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