How do tuna breathe?

Tuna are obligate ram ventilators, which means that they have to keep water flowing over their gills in order to breathe. They do this by constantly swimming, typically at speeds of 2-3 knots, although they can swim much faster if necessary.

A tuna's mouth is located on the bottom of its head and is shaped like a half-moon. When the tuna opens its mouth, water rushes in and fills the buccal cavity. This cavity is connected to the gills, which are located on either side of the head. The gills are made up of a series of filaments that are covered in tiny blood vessels. As water passes over the gills, oxygen from the water diffuses into the blood vessels and carbon dioxide diffuses out.

Once the water has passed through the gills, it exits through a series of vents located on the top of the head. These vents are also called spiracles.