Scales

Scales are the outermost armor of most fish. Four major types have evolved - placoid (sharks), ganoid (gars, sturgeon), cycloid (salmon, carp), and ctenoid (perch, bass). Each type carries an evolutionary and functional signature: sharks feel like sandpaper because their scales are literally tiny teeth; a perch scale has fine spines along the trailing edge; a salmon scale has smooth concentric rings you can count to determine age.

Cycloid and ctenoid scales are thin, flexible bone plates embedded in the dermis, overlapping like roof tiles. Ganoid scales are diamond-shaped, thick, and enamel-covered. Placoid scales (dermal denticles) share tooth-like development including a pulp cavity, dentin, and enamel-like enameloid.

Sharks have placoid denticles that reduce drag - the basis for Speedo Fastskin swimsuit design. Eels and catfish are scaleless. Seahorses have bony plates. Some deep-sea fish have transparent or bioluminescent scales.

A single salmon scale can reveal every migration between fresh and saltwater the fish made - each transition shows as a distinctive ring pattern.