What is the system of scales on an osteichthyes?

Cycloid scales: These scales are thin, flexible, and rounded, with smooth edges. Typically, they are found in soft-rayed bony fish, like salmon, trout, and many freshwater species.

Ctenoid scales: These scales are also thin and flexible, but unlike Cycloid scales, they have small spiny projections or cteni along their posterior edges. Ctenoid scales are found in many marine bony fish, including sea bass, flounder, and tuna.

Ganoid scales: Ganoid scales are more primitive and are commonly found in ancient fish species, such as gars, bowfins, and some sturgeons. They are thick and rigid, and they are composed of a layer of bone covered by a layer of enamel-like material called ganoine.

Placoid scales: These scales are unique to cartilaginous fish, including sharks, rays, and skates. Placoid scales are small and tooth-like, with a central pulp cavity. They are derived from the teeth and can be modified to form defensive spines, like in stingrays.

The scales on an osteichthyes play several essential functions:

Protection: Scales provide the fish with physical protection against predators and external injuries.

Body shape and streamlining: Scales help to maintain the fish's body shape, which affects their swimming efficiency and hydrodynamic properties.

Sensation: Scales contain nerve endings that allow fish to sense changes in their surroundings, such as water currents and pressure.

Coloration: Scales contribute to the fish's coloration by reflecting and refracting light, aiding in camouflage, courtship, and communication.

Some fish, like scaleless eels and hagfish, lack scales altogether. In these cases, they have developed alternative protective mechanisms, such as thick mucus layers, tough skin, or specialized dermal structures.