Indeed, the world's 37 species of seahorse are fish, breathing through gills and using a swimbladder to control their buoyancy.
事实上，世界上有37种海马是鱼，它们通过鳃呼吸，用鱼鳔来控制浮力。
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They must also keep swimming to keep from sinking, since most have largely or completely lost the swimbladder, the gas-filled sac that helps most other fish remain buoyant.