Final answer:
The answer to the question is 'scales'. These are the thin, hard plates that cover the bodies of bony fishes and serve multiple protective and hydrodynamic functions.
Step-by-step explanation:
The thin, hard plates that cover a fish’s body are scales. Bony fishes, such as tuna and salmon, are covered in overlapping scales that serve multiple functions, including reducing drag during swimming and aiding in osmoregulation. The scales are part of the fish's integumentary system, which serves as an outer protective layer. Unlike bony fishes, members of the Chondrichthyes group, such as sharks, have skin covered with placoid scales made from modified teeth, giving their skin a rough texture used for protection. Contrastingly, tunicates, though classified as chordates, do not have scales but are covered by a cellulose-like material called a tunic.