The diversity of aquatic life is immense, and while the image of a fish often includes shimmering, overlapping scales, the reality is far more complex. Fish are aquatic vertebrates whose outer coverings are highly specialized adaptations for survival. For many species, scales provide a durable, flexible suit of armor. The integument offers protection, streamlining, and sensory functions.
What Are Fish Scales and How Do They Function?
Not all fish have scales, which highlights the tremendous evolutionary variation within the group. A typical fish scale is a small, hard plate that grows out of the dermis, the inner layer of the skin. These structures are distinct from the epidermal scales of reptiles and are structurally similar to teeth in their composition.
Scales primarily function as a physical barrier, offering protection against abrasions, predators, and external parasites. They overlap like shingles on a roof, allowing for flexibility while maintaining a continuous protective layer. This overlapping pattern also reduces hydrodynamic drag, enabling the fish to move efficiently through the water.
Many fish are also covered in a layer of mucus or slime that coats the scales and skin. This slime layer works with the scales to provide a defense against pathogens like bacteria and fungi. The slime coat also aids in osmoregulation, helping the fish maintain a proper salt-water balance.
Different Types of Scale Structures
Fish scales are categorized into four main types corresponding to different evolutionary lineages.
Placoid Scales
Placoid scales, often called dermal denticles, are found on cartilaginous fish such as sharks and rays. They are structurally homologous to vertebrate teeth, featuring a pulp cavity, dentine, and an enamel-like layer. These scales do not grow in size; instead, new ones develop to fill the space, creating a rough, sandpaper-like texture.
Ganoid Scales
Ganoid scales are characteristic of ancient bony fish like gars and sturgeon. They are thick, diamond-shaped plates that interlock like a jigsaw puzzle, creating an almost impenetrable layer of armor. This armor is composed of bone and covered with ganoine, a hard, inorganic bone salt. This rigid scale type offers superior protection but limits body flexibility.
Cycloid and Ctenoid Scales
The most common scales found on modern bony fish (teleosts) are the flexible cycloid and ctenoid scales. Cycloid scales, seen on fish like salmon and carp, have a smooth outer edge. Ctenoid scales, found on species such as perch and bass, possess small, comb-like teeth or spines along their exposed edge, giving the fish a rougher texture. Both types grow in concentric rings throughout the fish’s life, which scientists use to estimate the fish’s age.
Fish That Are Covered By Skin or Plates
Many fish species have evolved away from traditional scales, instead developing specialized skin or bony plates for protection. Jawless fish, including lampreys and hagfish, possess smooth skin entirely devoid of scales or dermal bone. Hagfish are known for their ability to secrete copious amounts of protective, thick slime when threatened, allowing them to slip away from predators.
Many groups of eels, such as moray eels, are also scaleless, featuring tough, thick skin heavily coated in mucus. This mucus layer protects their skin from physical damage in rocky habitats and provides a defense against disease. The thick, leathery skin often contains a high amount of collagen, providing a durable and flexible covering.
Catfish are a large group defined by the lack of scales, having smooth, naked skin often covered in a thick layer of mucus. This smooth skin is highly adapted to their bottom-dwelling lifestyle. In some species, it facilitates cutaneous respiration, allowing the fish to breathe through its skin. However, some catfish, particularly those in the armored Loricariidae family, possess rows of bony plates or scutes embedded in their skin, offering a different form of body armor.
Another example of alternative integuments is the sturgeon, a primitive bony fish that is largely scaleless. It is armored with five longitudinal rows of large, bony plates called scutes. These scutes are modified ganoid scales and act as a heavy-duty form of protection. Similarly, boxfish and pufferfish possess bony plates or spines that fuse together, forming a rigid, shell-like exoskeleton.