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 flows against the current of the water over the gills (countercurrent exchange) to maximize oxygen absorption. They use a swim bladder to adjust their buoyancy by altering the gas volume within it, ensuring that the overall bladder volume remains constant at different depths.