Respiratory physiology of Neomyxine biniplicata, the slender hagfish

Neomyxine biniplicata is one of the 79 currently described species of Myxinidae and one of only six species endemic to the waters about the east coast of New Zealand. Whilst perhaps not everyone’s favourite animal species, hagfish are of great benefit to the ocean ecology and recycle decaying matter and return it to the ocean floor as nutrients. Also, hagfish form part of the diet of fish and ocean dwelling mammals that are not deterred by the slimy emissions hagfish use, to such great effect, to defend themselves. There are two sub-families of Myxinidae; the Eptatretinae (eptatretids) and the Myxininae (myxinids), and each occupy different ecological niches. Species of eptatretids are generally found on the seafloor whereas myxinids are specialist burrowers. Adaptation to burrowing can be seen in the external respiratory anatomy of the myxinids which exhibit two external apertures compared to the condition of the eptatretids where each gill is evacuated by a dedicated efferent duct. Although both eptatretids and myxinids experience conditions of hypoxia and anoxia, myxinids are considered better adapted to these conditions and, of the hagfish species examined to date, exhibit a lower critical pressure of oxygen (Pcrit) or the point at which oxygen consumption ceases to be independent of dissolved environmental oxygen. When subjected to closed-system respirometry, specimens of N. biniplicata were confirmed to have a Pcrit of 27mmHg although this was dependent upon environmental temperature and experimental method. The fish were not distressed by conditions of severe hypoxia, in contrast to Eptatretus cirrhatus which were examined in an earlier set of experiments and found to have a Pcrit of 50mmHg at which point the fish became highly agitated. At an oxygen tension of 20mmHg, N. biniplicata exhibited no discomfort or desire to escape and merely lay quietly. However, when confronted with acute temperature alterations that were beyond the normal thermal range N. biniplicata reacted with agitation and escape behaviour. The ventilatory rate increased exponentially at temperatures above 12 °C and slowed in response to immersion in water cooler than 10 °C although the thermal range differed in two seasonal populations examined as did the physical condition of each population. Observations of N. biniplicata in captivity revealed the species to be social in habit and sensitive to light, sound and vibration. The social habit of the fish influenced the results obtained during experiments and this must be considered when planning further experiments. N. biniplicata is a species adapted to a hypoxic environment but are very sensitive to temperature outside a narrow thermal range. Seasonal migration is a possibility