Aquatic respiration is the process whereby an aquatic organism exchanges respiratory gases with water, obtaining oxygen from oxygen dissolved in water and excreting carbon dioxide and some other metabolic waste products into the water.
Sea slugs respire through a gill (or ctenidium)
Posterior view of the gills of a tuna
In physiology, respiration is the movement of oxygen from the outside environment to the cells within tissues, and the removal of carbon dioxide in the opposite direction to the surrounding environment.
Image: Surface tension and lung volume
Image: Curvon 1