Metabolic rate of Western Atlantic hagfish (Myxine limosa): Effects of temperature, time of day and conspecific slime.

We investigated the metabolic rate of Western Atlantic hagfish (Myxine limosa Girard, 1859) in response to cold temperatures, time of day and conspecific defensive slime exposure. Closed-chamber respirometry was performed during acute stepwise cooling from 11 to 1°C (11, 9, 7, 5, 3 and 1°C), over a 24-h period, and before and after exposure to conspecific slime. Overall, the metabolic rate decreased with temperature. However, there was a significant breakpoint at 8.4°C (95% CI: 7.2-9.6°C), with low temperature sensitivity (Q10 = 1.3) below and high sensitivity (Q10 = 59.0) above this temperature. Time of day had no significant impact on M. limosa metabolic rate, possibly due to a lack of light-dark cycles in their deep-sea habitats. Exposure to conspecific slime was associated with a roughly 20% decrease in metabolic rate, potentially indicating reduced activity when a predator is present or metabolic suppression under hypoxic conditions from slime entrapment. Absolute metabolic rate of M. limosa scaled with body mass to the power of 0.814 (95% CI: 0.579-1.049), confirming our measurements align with typical animal metabolic rate scaling (exponent of 0.75). Our data, compared with published Eptatretus genus values, contradict the assertion that Eptatretus has lower metabolic rates than Myxine. Our study confirms that, like other hagfishes, M. limosa has a remarkably low metabolic rate (0.43 μmol O2· g-1· h-1 at 10°C) compared to other vertebrates, corroborated by a phylogenetic meta-analysis that revealed significant phylogenetic signal at the node shared by hagfishes and ray-finned fishes. Finally, we found that hagfish metabolism shows previously undescribed non-linear thermal sensitivity and energetic responses under a range of environmental conditions.