Tuna swim constantly and never fully stop, since they need water flowing over their gills.

The longer version

Constant swimming drives water into the open mouth and across the gills, a method known as ram ventilation. Stop moving and the supply falters, so there is no true rest. The body is built for endurance: a crescent tail, retractable fins and a streamlined shape that trims drag.

Red muscle packed along the flanks burns slowly for cruising, and counter-current heat exchange in the blood vessels keeps swimming muscle and eyes warmer than surrounding water, letting the animal hunt deep and cold. Lift from stiff pectoral fins supplies buoyancy, replacing the swim bladder most bony fish rely on.

Where this entry sits

Oceans and Sea Life looks at the salt water that covers most of the planet, from crashing waves and tides to the darkest deep-sea trenches. Expect notes on coral, fish, whales, sharks, jellyfish, shells, and the strange creatures that live far below the surface. The full list sits on the Oceans and Sea Life page, where every entry is listed in order.