Like skin, scales help protect animals from their environments. But not all scales are created the same. Let’s look at some of the different types of scales in the animal kingdom.
Reptiles are covered in scales. Their scales are made from keratin. Keratin is the same substance that makes our hair. These scales are a continuous shield, like our skin. They are not individual pieces. The tough, dry scales provide protection from predators and help reduce water loss for the many reptiles living in very dry climates.
Scales can vary in size and shape and can be very specialized. For example, the long, horizontal scales on a snake’s belly are designed to help with locomotion, the snake’s ability to move. And the spiky scales on the bodies of crocodiles and alligators give them extra protection from their predators that have very large and sharp teeth.
Some birds’ feet and sometimes their legs are covered in scales. Like reptiles, bird scales are also made from keratin and are a continuous scaly surface. They are often larger in size on the tops of the feet and smaller on the sides and between the toes.
All fish scales are made from the same ingredients as our teeth, dentin, and enamel. They are made up of individual scales, not a continuous covering like found on reptiles and birds.
Fish scales fall into four main categories:
Placoid scales: Placoid scales only grow on cartilaginous fish like sharks, rays and sawfish. (Cartilaginous means made up of cartilage, which is a boneless tissue, like our outer ears.) Their scales are called denticles, and they are layered like our teeth, with hard enamel on the outside, a middle layer of dentin and a center layer of pulp. This type of scale does not grow larger as the fish grows larger. As the fish grows, spaces form between old scales. New scales form to fill in those spaces.
Cosmoid scales: The cosmoid scale is almost entirely found in fossils. You would have to travel to the West Indian Ocean to find the only living fish with these scales. The Latimeria is called a living fossil because it is more closely related to fossils than any other of type of fish living today.
Ganoid scales: Ganoid scales seem to have developed as a cross between placoid scales and cosmoid scales. They have only two layers and are found on species like bowfins, sturgeons, bichirs and gars. These fish are also often considered living fossils or primitive fish. They are related to fossil ancestors and have remained unchanged for a long, long time.
Elasmoid scales: This is the most common type of scale found on fish. It is on most of the bony fish we find in our local ponds, lakes and rivers. There are two main forms within this type: ctenoid and cycloid. If you look closely at each scale, ctenoid scales have a serrated, or toothed, edge on the back edge each scale. The cycloid scales have rounded edges. These types of scales grow as the fish grows. They overlap each other but only have one point of attachment. This helps improve the flexibility of the fish. A single scale can break off without harming the rest of the scales.
Each of these scales is also like a tree’s rings or a mussel’s shell. Scientists can determine the age of the fish, if it has ever been sick and other valuable information from examining just one scale.
Words to know
Cartilage: Firm connective tissue.
Dentin: Hard, dense tissue forming the bulk of a tooth.
Enamel: The hard, white covering on teeth.
Keratin: A fibrous protein in hair, feathers, hooves, claws and horns.
Locomotion: Movement or the ability to move from one place to another.
Scales have many variations. A male and female of the same species might even have scales that look different. Scales can also be different sizes on a fish. Larger scales may increase protection but limit locomotion, so a fish may have smaller scales near the back fin to help them move faster while larger scales may be found on the rest of their bodies.
Do all fish have scales?
Not all fish appear to have scales, but is that actually the case? Most eels are lacking scales except for common eels. They have scales, but you can’t see them. They are so tiny they can only be seen with a microscopic.
Catfish do not have scales at all. Because of this, catfish are sometimes called naked fish. To make up for this, catfish often have tough, leathery skin. Some catfish have developed bony plates on their bodies, but they are formed much differently than true fish scales.
View scales for yourself at Four Rivers
Four Rivers Environmental Education Center is home to a 2,000-gallon fish tank that is home to six species of fish. This allows for a unique opportunity to take a closer look at the elasmoid scales of bony fish living there. Come and check them out when you can!
Follow Willy's Wilderness on Facebook for more kid-friendly nature stories and activities.