Scales Protect Skin

close up underwater photograph of a shark

Biological Strategy

Cartilaginous fishes

AskNature Team

Image: David Clode / Unsplash / Free non-commercial use

Protect From Animals

Animals–organisms that range from microscopic to larger than a bus–embody a wide variety of harms to living systems, including other animals. They threaten through predation, herbivory, defense, and parasitism, and they compete for resources such as water, nutrients, and space. Any given living organism commonly faces threats from a variety of animals, requiring strategies that effectively defend from each. Trout and other bony fish, for example, escape predators by having scales made of very thin, flake-like pieces of bone covered with slippery mucus. They also have behavioral strategies such as camouflage, fast swimming, and twisting and turning to achieve release from a predator’s grip.

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Manage Mechanical Wear

A living system is subject to mechanical wear when two parts rub against each other or when the living system comes in contact with abrasive components in its environment, such as sand or coral. Some abrasive components are a constant force, such as finger joints moving, while others occur infrequently, such as a sand storm moving across a desert. Living systems protect from mechanical wear using strategies appropriate to the level and frequency of the source, such as having abrasion-resistant surfaces, replaceable parts, or lubricants. For example, human joints like shoulders and knees move against each other all day, every day. To protect from mechanical wear, a lubricant reduces friction between the cartilage and the joint.

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Fish

Class Agnatha (“without jaws”), Class Chondrichthyes (“cartilage fish”), Superclass Osteichthyes (“bone fish”): Sharks, eels, snapper, hagfish

The fish are a diverse group, comprising multiple classes within Phylum Animalia. The most well-known classes are Chondrichthyes, which has sharks and rays, and superclass Osteichthyes, which has all bony fish like cod and tuna. Unlike other vertebrates, fish only live in water. They use special adaptations like fins, gills, and swim bladders to survive. Most are ectothermic, meaning their body temperature depends on the water temperature around them. Over half of all vertebrates are fish. They’re found from the bottom of the sea to high mountain lakes.

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The skin of cartilaginous fish is protected by a covering of abrasive placoid scales, called denticles.

“The other main type of fish scales are those known as placoid scales or, more commonly and appropriately, denticles: ‘little teeth’ (diagram d). They are found on the primitive cartilaginous fishes, sharks, skates, and rays (whose skeletons are made of cartilage, not bone). Each denticle grows up from the dermis until its curved tip breaks the skin surface — denticles are not covered with skin as bony scales are. Each denticle, like a human tooth, is made of dentine (tooth ivory) capped with enamel; each has a pulp cavity containing nerves and blood vessels. Denticles are usually small, but may be sharp. Brushing against the skin of a shark, can flay the skin of a swimming man like a particularly vicious sandpaper.” (Foy and Oxford Scientific Films 1982:91)