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Why do cephalochordates sink when they stop swimming?
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Cephalochordates, such as the amphioxus, sink quickly when they stop swimming because they lack buoyancy organs. Unlike many bony fish, which possess a swim bladder to maintain their depth, cephalochordates do not have specialized hydrostatic structures to counteract gravity. Consequently, they rely on the active contraction of their myotomes to swim and remain mobile; once this muscular effort ceases, the animal is not buoyant and sinks.123