The effects of variable water salinity and ionic composition on the plasma status of the Pacific Hagfish (Eptatretus...

Hagfish are the most pleisiomorphic extant craniates, and based on the similarity of ionic concentrations between their internal milieu and seawater (SW), they have long been touted as a model for osmo- and ionoconformation. As a result, the lack of direct symmetry between hagfish plasma and the environment with respect to [Na(+)], [Cl(-)], [Mg(2+)], and [Ca(2+)] have been left largely unexplored. In order to determine the capacity of hagfish to regulate their blood compartment, we exposed Pacific hagfish (Eptatretus stoutii) to 24, 32, 40, and 48 g/l salinity for 48 h, as well as to two treatments where a portion of the water [Na(+)] was replaced with either Mg(2+) or Ca(2+) at constant salinity for up to 6 days. Following exposure, we measured plasma ion status, pH, and total carbon dioxide (TCO(2)). As expected, our results indicated that hagfish had no capacity to regulate plasma osmolality, [Na(+)], or [Cl(-)], but they did maintain plasma [Mg(2+)] and [Ca(2+)] nearly constant despite fluctuation of environmental salinity or elevated water [Mg(2+)] and [Ca(2+)] (two- and sevenfold, respectively). Furthermore, exposure to elevated water [Mg(2+)] and [Ca(2+)] resulted in a large increase of plasma TCO(2) with little to no increase of plasma pH. We concluded that hagfish may control plasma [Mg(2+)] and [Ca(2+)] at levels below that of their environment via secretion of HCO(3) (-), similar to the mechanisms described in the intestine of teleosts. We speculate that secretion of HCO(3) (-) likely evolved to maintain plasma [Mg(2+)] and [Ca(2+)] below environmental levels (both of which negatively affect nervous function and muscle contraction if elevated), and was an exaptation for the development of water-absorption mechanisms in the intestine of marine osmoregulators. The ancestors of modern hagfish are thought to have never entered freshwater, thus investigations into their ionoregulatory ability potentially have profound implications regarding the evolution of fishes.