The effects of salinity and hypoxia exposure on oxygen consumption, ventilation, diffusive water exchange and ionoregulation in the Pacific hagfish (Eptatretus stoutii).

Hagfishes (Class: Myxini) are marine jawless craniate fishes that are widely considered to be osmoconformers whose plasma [Na], [Cl] and osmolality closely resemble that of sea water, although they have the ability to regulate plasma [Ca] and [Mg] below seawater levels. We investigated the responses of Pacific hagfish to changes in respiratory and ionoregulatory demands imposed by a 48-h exposure to altered salinity (25 ppt, 30 ppt (control) and 35 ppt) and by an acute hypoxia exposure (30 Torr; 4 kPa). When hagfish were exposed to 25 ppt, oxygen consumption rate (MO), ammonia excretion rate (J) and unidirectional diffusive water flux rate (J, measured with HO) were all reduced, pointing to an interaction between ionoregulation and gas exchange. At 35 ppt, J was reduced, though MO and J did not change. As salinity increased, so did the difference between plasma and external water [Ca] and [Mg]. Notably, the same pattern was seen for plasma Cl, which was kept below seawater [Cl] at all salinities, while plasma [Na] was regulated well above seawater [Na], but plasma osmolality matched seawater values. MO was reduced by 49% and J by 36% during hypoxia, despite a small elevation in overall ventilation. Our results depart from the "classical" osmorespiratory compromise but are in accord with responses in other hypoxia-tolerant fish; instead of an exacerbation of gill fluxes when gas transfer is upregulated, the opposite happens.