Fish are incredibly diverse, and while most species are covered in scales, a significant minority have evolved different skin coverings or none at all. These alternatives represent varied solutions to the challenges of aquatic life, such as defense, movement, and disease resistance. The outer covering of fish is highly specialized, reflecting the unique environments they inhabit and their evolutionary history. Losing ancestral scales is a consequence of natural selection favoring other protective or hydrodynamic features.
Alternative Skin Coverings
Fish lacking traditional cycloid or ctenoid scales are protected by various biological structures. A common alternative is a thick, tough skin layer often coupled with copious amounts of mucus or slime. This mucus acts as a barrier against external threats, including parasites, bacteria, and abrasive surfaces. The slippery coating also helps reduce surface resistance, allowing for efficient movement through the water.
Some scaleless fish have evolved external dermal bone plates, referred to as scutes or armor, which provide robust protection. These bony plates are not modified scales but represent ossified skin tissue. Certain species develop a rigid outer shell from fused plates that fully enclose their bodies. This heavy, leathery covering serves as a strong defense against predators.
Examples of Truly Scaleless Fish
The order Siluriformes, commonly known as catfish, represents a large group of fish almost entirely devoid of scales. Catfish bodies are often described as “naked,” covered instead by a thick, resilient, skin-like epithelium. In some families, the skin is fortified with bony plates called scutes, creating natural armor.
Eels, such as moray eels, are often considered scaleless, though some species possess tiny, smooth scales deeply embedded in their skin. Their primary defense is thick, durable skin covered by a dense layer of slime-producing cells. Primitive jawless fish, like hagfish and lampreys, also have smooth skin entirely lacking in scales or dermal bone. Hagfish are known for secreting large amounts of protective slime when disturbed, helping them escape predators.
Fish With Specialized Scales
Some fish appear scaleless but possess a unique integument structurally distinct from bony fish scales. Cartilaginous fish, including sharks, rays, and skates, are covered in placoid scales, also called dermal denticles. These structures are not flat, overlapping plates but resemble tiny teeth, each having a central pulp cavity, dentine, and a hard, enamel-like outer layer.
The placoid scales are supported by spines that project posteriorly, giving the skin a rough texture similar to sandpaper. Unlike bony fish scales that grow larger as the fish matures, new dermal denticles are added to cover the growing surface area. This specialized covering contributes to the sharkâs hydrodynamics by creating small vortices that reduce drag and turbulence, facilitating efficient swimming.