Fish possess diverse external coverings for survival in aquatic environments. While many species have distinct scales, these rigid plates are not universal. Their presence or absence reflects evolutionary adaptations, offering insight into the remarkable diversity of fish.
The Role and Structure of Fish Scales
Fish scales are small, rigid plates that grow from the fish’s skin. These protective coverings primarily serve as armor, shielding the fish from predators, parasites, and injuries. Scales also contribute to camouflage, helping fish blend into their surroundings. Beyond protection and concealment, scales offer hydrodynamic advantages, allowing fish to move efficiently through water by reducing drag.
Scales are produced from the dermis and composed of collagen and calcium salts, such as hydroxyapatite. There are several categories. Cycloid scales, found on fish with soft fin rays, have a smooth texture and circular edges. Ctenoid scales, common on fish with spiny fin rays, are similar but feature comb-like teeth. Ganoid scales, found on ancient fish like gars, are hard, enamel-like plates, while placoid scales, characteristic of cartilaginous fish, resemble tiny teeth.
Fish Species Known for Their Scales
The majority of bony fish species possess scales. Many familiar fish exhibit cycloid scales, which are smooth and overlap like roof tiles, providing flexibility. Examples include carp, goldfish, salmon, and trout. These scales grow throughout the fish’s life, with growth rings that can be used to estimate their age.
Ctenoid scales, which have a rougher, toothed edge, are prevalent in species like perch and bass. Tuna, for example, have scales so small they might appear scaleless, but possess prominent, thickened scales in specific areas like the corselet, a protective band around the shoulder region. Other scaled marine species include herring, mackerel, snapper, flounder, and tarpon.
Fish Species Without Scales
Many fish species have evolved without traditional scales, developing alternative skin coverings. Catfish typically lack scales, possessing tough, leathery skin or bony plates called scutes. Eels, such as the conger eel and moray eel, are largely devoid of scales, which aids their movement through narrow spaces and burrows. Their skin is often covered with a thick layer of mucus, providing a protective barrier.
Sharks and rays, which are cartilaginous fish, do not have true scales like bony fish. Instead, their skin is covered by tiny, tooth-like structures called dermal denticles. These denticles are structurally similar to teeth, with a central pulp cavity and a layer of dentine. They give shark skin a rough, sandpaper-like texture and help reduce drag, allowing for efficient swimming. Other scaleless or nearly scaleless fish include hagfish and lampreys, ancient jawless fish with smooth, slimy skin, and sturgeons and paddlefish, characterized by rows of bony plates.