Aquatic animals respire by

Respiration is the biochemical process in which the cells of an organism obtain energy by combining oxygen and glucose, resulting in the release of carbon dioxide.

Key Points

• A Gill is a respiratory organ found in many aquatic animals, gill extracts dissolved oxygen from water and excretes carbon dioxide. 
• Respiration over the gills of fishes is called Branchial respiration. 
• Branchia is the zoologist's name for gills.
• The Lungs and respiratory system allow us to breathe, they bring oxygen into our bodies and send carbon dioxide out.
•  Air bladder is an internal gas-filled organ that contributes to the ability of many fishes to control their buoyancy, and thus to stay at their current water depth without having to waste energy in swimming.

Hence from the above-mentioned points, it becomes clear that aquatic animals respire by their Gills.