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How do body muscles and tail fins propel fishes through water?

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A porcupine fish in the Andaman Sea — fish *Diodon hystrix* is a species of porcupine fish. These fishes are short and broad-bodied, with large eyes and spines covering their skin. Like puffer fishes, they inflate their bodies when threatened.

Fishes propel themselves through water primarily through the rhythmic contraction of their trunk musculature. This body musculature is organized into chevron-shaped segments called myomeres, which are attached to the vertebrae.12

When these muscles contract in alternating waves along the right and left sides of the body, they create undulatory flexures that travel from the head toward the tail. These waves cause the body to bend, producing a powerful side-to-side sweeping motion. This movement culminates in the caudal fin (tail fin), which acts as a primary engine for forward propulsion, often providing up to 85 percent of the total thrust in many species.134

While the caudal fin is the most powerful component for rapid, sustained swimming, the entire body contributes to the propulsive force as each undulating segment oscillates. In contrast, the paired fins (pectoral and pelvic) are generally utilized for slower, precision movements, such as maneuvering or investigating food.132