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Why do fast-swimming fishes have folded and overlapping muscle segments?

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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.

In jawed fishes, the myomeres—the segments of axial muscle—are folded into a complex, zigzag pattern that resembles a sideways "W". This structural adaptation is directly related to the evolution of more powerful swimming capabilities.1

The folding and overlapping of these segments allow a single myomere to produce curvature across a significant portion of the fish's body when it contracts. Because these segments project deeply and overlap with multiple anterior and posterior neighbors, the force generated is more effectively distributed. Consequently, faster-swimming fishes exhibit a higher degree of this folding and overlapping; for instance, in the tunny, a single myomere may overlap with as many as 20 others. This arrangement enables the fish to produce much greater thrust through body undulations than could be achieved by the movement of fins alone.1