Fish scales are external coverings found on most fish, acting as a protective barrier. They grow from the skin, forming a diverse array of shapes and sizes depending on the fish species. Scales contribute to the survival of fish in their aquatic environments.
The Core Materials of Scales
Fish scales primarily consist of a blend of organic and inorganic components, working together to provide both flexibility and strength. The main organic material is collagen, a protein that forms a fibrous layer, contributing to the scale’s resilience and pliability. Alongside collagen, scales also contain calcium phosphate. This mineral component provides hardness and rigidity to the scales, similar to the composition of bone and teeth. Some scales may also contain minor amounts of keratin.
Distinct Scale Types and Their Makeup
The diverse world of fish includes several distinct scale types, each with a unique structure and composition tailored to the fish’s lifestyle.
Placoid scales
Placoid scales, characteristic of cartilaginous fish like sharks and rays, are small, tooth-like structures often called dermal denticles. They possess a central pulp cavity, analogous to a tooth, surrounded by a layer of dentine and an outermost enamel-like substance called vitrodentine, making them hard and abrasive.
Ganoid scales
Ganoid scales, found on ancient fish such as gars, are hard, rhomboid-shaped plates that interlock like armor. These scales feature a bony basal layer, a layer of dentine, and are covered by ganoine, a unique, glassy, enamel-like inorganic bone salt that provides exceptional toughness.
Cycloid scales
Cycloid scales are common on many ray-finned fish, including salmon. These scales are smooth, thin, and rounded, overlapping like roof tiles.
Ctenoid scales
Ctenoid scales are similar to cycloid scales in their overlapping structure and composition, also found on many ray-finned fish like perch. The key distinction for ctenoid scales is the presence of comb-like teeth, or ctenii, along their posterior edge, which gives them a rough texture.
Scale Formation and Purpose
Fish scales originate from the dermal layer of the skin and continue to grow throughout a fish’s life. As a fish grows, its scales also expand, often forming concentric growth rings, similar to tree rings, which can be used to estimate the fish’s age.
The materials and structures of fish scales enable them to serve several biological functions. Scales provide protection against predators, parasites, and physical abrasion in the aquatic environment. The hardness derived from calcium phosphate contributes significantly to this protective role.
Scales also play a role in hydrodynamics, with their overlapping arrangement reducing drag and aiding efficient movement through water. The flexibility provided by collagen supports this function, allowing for smooth body movements. Additionally, scales can contribute to osmoregulation by helping to prevent excessive water loss or gain, although this is a less prominent role compared to protection and movement.