Their blood flows in the opposite direction (countercurrent exchange) of the water over the gills in able to remove the maximum amount of oxygen from the water. They use a swim bladder to adjust their buoyancy by changing the volume of gas in the bladder to keep an overall constant volume for the bladder at different depths.
Their blood circulates in the opposite direction (countercurrent exchange) of the water over the gills to extract the maximum amount of oxygen from the water. They use a swim bladder to regulate their buoyancy by altering the volume of gas within the bladder to maintain a consistent volume for the bladder at various depths.