Scales
Schematic cross-section: scales are embedded in the dermis and covered by the epidermis and its protective mucus. © dive.explo360.fr — All rights reserved. Reproduction prohibited without permission.
The skin has two main layers:

The epidermis, the living outer layer in contact with water, which contains mucus-producing cells, among others.

The dermis, beneath the epidermis, which contains blood vessels, nerve endings and, in many bony fish, scales embedded in the dermis.

Scales can overlap like roof tiles while retaining a degree of flexibility.

INVISIBLE ARMOUR

The surface of many fish is covered by a thin layer of mucus produced by epidermal cells.

This mucus forms a first interface between the fish and its environment. It contributes notably to defence against certain pathogens and parasites and protects the body surface against mechanical damage.

Its properties and composition can vary between species.

1

Protection against infection

Mucus contains various molecules contributing to the organism’s defences. It also forms a physical barrier limiting direct contact between tissues and microorganisms in the water.
2

Protection of the body surface

The mucus layer forms a moist, slippery surface. It helps limit mechanical damage and abrasion when the fish comes into contact with sand, rocks or other surfaces.

It can also influence friction between the body and water.

3

Defence against predators

In some species, skin secretions may contain unpleasant or toxic substances.

A very slippery surface can also make a fish harder to grasp.

4

Protection of wounds

When part of the skin is damaged, mucus continues to form a barrier between the injured area and the outside environment. It thus contributes to the conditions needed for healing.
5

Chemical information

Secretions produced by fish can also play a part in chemical interactions between individuals, notably in certain situations involving reproduction or danger.

EXCEPTIONAL SKINS

Exceptional skin
Moray eel skin
Conger and moray eels have skin without the visible scales characteristic of many other fish. Their surface is covered with abundant mucus.

This very flexible, slippery skin is particularly suited to their way of life. These fish can move through rocky cavities and crevices while limiting direct friction against the walls.

Mucus also helps protect their epidermis.

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