Oxygen sustains virtually all animal life. Yet, many animals live in environments where oxygen is limited. The striped catfish (Pangasionodon hypophthalmus) is native to waterways in Southeast Asia where heavy pollution lowers oxygen levels significantly. Fortunately, the striped catfish is one of a few fish species with a unique superpower – they have the ability to breathe air. When oxygen levels in the water fall too low, these catfish swim to the surface and take big gulps of air. Oxygen from the air then diffuses into the blood, allowing the fish to survive where other species would perish. However, molecules that are carried in solution tend to diffuse from an area of high concentration to low concentration, placing the gulped oxygen – now at high concentrations in the fish's blood – at risk of being lost to the oxygen-depleted waters in their environment through the fish's disproportionately large gills. In a new study led by Magnus Aaskov from Aarhus University, Denmark, a team of scientists from across the globe (including from University of North Texas, USA, Can Tao University, Vietnam, and Nagasaki University, Japan) aimed to understand the fate of the freshly gulped oxygen carried in the striped catfish's blood.
To measure the extent of oxygen loss across the gills, the team used a technique called bimodal respirometry, in which the oxygen consumption of an air-breathing fish can be measured both underwater and above the water's surface. Aaskov and the team discovered that when the water was well oxygenated, only 7% of the oxygen that the catfish consumed came from its above-water gulping routine. But hold your breath for the twist – catfish in poorly oxygenated water obtained 105% of their oxygen from air. The researchers explained that aerial oxygen uptake exceeds 100% because fish take up enough oxygen to meets the body's needs, plus extra to account for oxygen lost across the gills in water. Contrary to what we may have learned in biology class, stiped catfish lose a mere 5% of gulped oxygen across the gills.
The researchers then decided to take their study a step further and determine whether unique anatomical features in the striped catfish were responsible for minimizing oxygen loss across the gills. Based on a hunch that the catfish would shunt blood away from their gills in low-oxygen water to minimize oxygen loss, the team made delicate casts of the bloodstream in the fish's gills and body. Then, they scrutinized the casts with micro-computed tomography (micro-CT) to reveal minute details in the blood flow pathways in the fish's bodies. The researchers also used ultrasound scans of living catfish in well-oxygenated and oxygen-depleted water to better visualize the movement of blood.
Interestingly, the researchers found a ‘shunt’ in the catfish's circulatory system that could theoretically redirect blood away from the gills, potentially reducing oxygen loss in oxygen-deprived waters. However, much to their surprise, the team discovered that catfish were losing significant amounts of carbon dioxide in oxygen-deprived water, indicating that blood was not being shunted away from the gills – which would prevent oxygen from being lost the oxygen-depleted water – because blood needs to flow through the gills to eliminate carbon dioxide from their bodies. This discovery deepens the mystery surrounding how air-breathing fish manage to retain precious oxygen in aquatic environments in which the lifegiving gas is remarkably scarce. Once again, scientists are left puzzled by the wonders of life concealed in the depths – and at the surface – of the underwater world.
