Fish Muscle Physiology

A cross-section of a fish reveals two distinct muscle types. Red muscle - dark, well-vascularised, oxidative - drives sustained cruising. It sits in a strip along the lateral line, drawing oxygen from a dense blood supply and lipid stores. White muscle - the bulk of a filet - is anaerobic, powering explosive burst movements before quickly fatiguing. The ratio of red to white varies by lifestyle: tuna are up to 30% red muscle, while flounder are almost entirely white.

Red (slow-twitch) muscle: aerobic, mitochondria-rich, lipid-fuelled, used for cruising. White (fast-twitch) muscle: anaerobic, glycolytic, powered by glycogen, used for burst escape.

Myotome (nested muscle blocks), red vs white muscle, aerobic vs anaerobic metabolism, hyperplasia (indeterminate growth via cell addition).

Bluefin tuna: dark red muscle throughout. Trout: red strip along lateral line. Bass: mostly white with red lateral strip. Halibut: nearly all white.

Sushi-grade tuna is red muscle - which is why it can be eaten raw safely (aerobic tissue has fewer bacteria). The white muscle of many fish is often lower-grade for the same reason.