Fresh water fish constantly battle to get enough oxygen. But as oxygen levels fall and fish make their gills more permeable to take up the gas, they are at increasing risk of losing sodium to their dilute environment. Retaining salts while satisfying their oxygen demands is particularly challenging for Amazonian oscars, which frequently encounter dangerously low oxygen levels. Having previously found that the fish breathe much harder as oxygen levels fall, but they somehow protect their sodium levels by reducing the flow from their bodies, Chris Wood and an international team of collaborators from Canada, the USA, Belgium, the UK and Brazil decided to take a closer look at salt flows across oscar gills(p. 1949).
Reducing the oxygen levels in the oscars' water and comparing the hypoxic fish with fish that had received a normal oxygen supply, the team measured sodium and water flow in and out of the fish's gills, checked their breathing rates and gill permeability and looked at the fish's gill structure by scanning electron microscopy. Calculating the oscars' sodium flows, they found that the flows in and out of the fish's gills reduced enormously when the oxygen levels fell, protecting the fish's internal sodium levels. Wood and his colleagues explain that shutting down active ion uptake and passive ion leakage to an equal extent would result in significant energy savings for the fish, as ion transport is metabolically costly.
But how do the fish change their gills' sodium permeability while continuing to transport sufficient oxygen? Having looked at the structure of the gills over several hours of exposure to hypoxia, the team suspect that the fish close ion transport channels in the membranes of gill surface cells in order to reduce ion transport and conserve energy. The team saw that pavement cells move over the sodium pumping cells (mitochondria-rich cells), to reduce the mitochondria-rich cells' exposure to water and reduce sodium flows into and out of the gill.