Post-prandial physiology and intestinal morphology of the Pacific hagfish (Eptatretus stoutii)
Hagfishes are unique to the vertebrate lineage in that they acquire dissolved nutrients across multiple epithelia including the intestine, gill, and skin. This feat has been attributed to their immersive feeding behavior that likely simultaneously provides benefits (nutrient rich) and potentially adverse (hypercapnia, hypoxia, high environmental ammonia) physiological effects. Examinations have been conducted of the ex vivo transport capabilities of specific nutrients as well as in vivo effects of the hypothesized feeding environments, yet the physiological effects of feeding itself have never been elucidated. We examined the post-prandial physiology of Pacific hagfish (Eptatretus stoutii), identifying changes in oxygen consumption, acid–base balance, ammonia waste excretion, and intestinal morphology following feeding in captivity. Following voluntary feeding, post-prandial oxygen consumption was significantly elevated (1868 ± 272 µmol kg−1 h−1) 8 h following feeding when compared to control resting metabolic oxygen consumption (642 ± 51 µmol kg−1 h−1) and resulted in a factorial metabolic scope of 2.92. Changes in acid–base status were not observed following feeding in either the excreted components or the caudal blood samples; however, a significant alkalosis was observed 8 h post-feeding in the major intestinal blood vein. Significant increases (16-fold) in ammonia excretion were recorded in 36 h post-fed hagfish. Finally, significant post-prandial increases in intestinal mucosal thickness and microvilli length were observed, with mucosal thickness remaining significantly increased throughout 36 h and the microvilli length returning to fasted lengths by 36 h. These results demonstrate the post-feeding physiology of the earliest diverging extant craniate and identify correlations between physiology and hindgut morphology 8 h following feeding.