Final answer:
Fish post special adaptations to avoid sinking, the most notable is the presence of swim bladder which controls their buoyancy.
Step-by-step explanation:
The adaptations possessed by fish to help avoid sinking primarily involve the use of the swim bladder for buoyancy control. As a gas-filled organ derived as a pouch from the gut, the swim bladder helps control the buoyancy of the fish. Gases of the swim bladder are typically exchanged directly with the blood, providing the required control over buoyancy.
Bony fishes have a unique adaptation which is having gills for respiration which, while don't directly affect buoyancy, provide the oxygen necessary for survival underwater. Gills are thin tissue filaments that are highly branched and folded. When water passes over the gills, the dissolved oxygen in water rapidly diffuses across the gills into the bloodstream.
Additionally, these fish also have a lateral line system that detects vibrations in the water and overlapping scales which not only provides protection but significantly reduces drag when swimming.
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