In this post, we will answer the question “Do all fish have scales?” and discuss what are the fish that have scales and the ones that do not have scales.
Do all fish have scales?
No, not all fish have scales. Scales are flattened plates that protect the animals. They can be present in fish, reptiles, and birds. However, there are many fish species without scales or covered in different structures.
What are scales?
In fish, the scales are formed by keratin, the same protein that forms nails, hairs, and skin. The scales function is to protect the fish externally. The scales also assist in the fish locomotion in the water.
Scales functions include mainly protection. The hard scales can avoid parasites infestations and bruises. Additionally, they become slick, which can help the fish to avoid predator attacks.
Types of scales
Fish with different habitats and lifestyles normally present different scale types. Scales can vary in type, sometimes in size, also according to the fish sex.
Sharks and rays have placoid scales. These scales do not vary in size as the fish grows. In this case, new placoid scales are added in between the older scales to cover the grown skin. They can be also called denticles. Similar to teeth, the placoid scales have a vascular inner area, called pulp; a middle layer composed of dentine, and the external layer is formed by vitrodentine, as a hard enamel-like layer.
Cosmoid scales are the ones found in Lungfish and some fossil fish. They are very similar to placoid scales and consist of 2 basal bony layers, one middle layer of cosmine, and the external layer of vitrodentine. These scales are believed to have evolved from fused placoid scales. The Cosmoid scales grow as the fish gets bigger, basal layers obtain new bone.
The Ganoid scales are considered Cosmoid modified scales. They are present in Bowfin, Gars, and Sturgeons. They have a basal layer of bone, a middle layer of dentine, and ganoine on the outside layer. They appear to be joint and articulated to allow the fish to move.
Cycloid and Ctenoid Scales
The majority of bony fish have Cycloid and Ctenoid scales. They are disposed of in a manner in which they become imbricate, with overlapping areas. This arrangement allows fish to move without limitations due to the scales. Ganoid and Cosmoid scales can provoke some movement limitations.
The difference between the Cycloid and the Ctenoid scales is that the first ones have smooth margins, while the other have spiny margins. Both of these scales are composed of an outer layer rich in calcium salts, and an inner layer composed of collagen.
These scales have evolved from ganoid scales. They have lost the ganoine and made the bony layer thinner. These scales also grow according to the fish growth. They form concentric growth rings similar to trees. Thus, in some species, it is possible to determine the age of the fish by its scales.
Fish with scales
Most of the fish species have scales. It is possible to highlight some of them:
Fish without scales
Many fish are scaleless. These fish are a member of 5 different groups, they are:
The sea lamprey is a scaleless fish. They are members of the Petromyzontiformes group.
This animal is known for being agnate, fish that lack jaws. They have concentric rows of teeth to attach to other animals skin and feed on other fish blood. They can grow up to 1 metre long and inhabit water in the Atlantic and the Pacific.
Similarly to the sea lamprey, these fish are also agnate. Hagfish are also scaleless fish. They have muscles fibbers embedded within. Differently from the lamprey, they do not have a suction type mouth. They have awesome sensory organs. These detect smell and touch.
The Ratfish is one of the Chimaeriformes group. They are also called rabbitfish. They have a peculiar looking due to their large head and small body.
They live in the Atlantic Ocean and the Mediterranean Sea. Their diet is based on algae, small fish, and crustaceans.
The conger eel is a member of the Anguilliformes group. This species can reach up to 2 metres in length. They have huge mouths and eyes. They have an incredible healing ability.
Black bullhead catfish
The Black Bullhead catfish is also a scaleless fish. They are a member of the Siluriformes group. They have a thick layer of dark mucus. Similarly to the other catfish species, the Black bullhead catfish also have barbels. They inhabit rivers and their diet is based on other fish.
The Freshwater blenny is a member of the Perciformes group. They are tiny scaleless fish. They inhabit the river and their diet is based on crustaceans and smaller fish.
Other fish without scales
– Zebra catfish;
– Tiger sorubim;
– Common Sturgeon;
– Mola-mola – Sunfish.
Why did these fish lose their scales?
Certainly, the lack of scale in some species is related to the evolutionary process.
It is possible to relate the lack of scales to hydrodynamics, to the shape of some fish bodies and the need for mobility freedom. Among them, we can mention the eels.
Do sharks have scales?
Yes, sharks do have scales. Sometimes people even forget that sharks are also fish. The shark scales are also called denticles and their distribution follows a diamond pattern. These scales are very small and quite different from other fish scales. In addition to the scales, sharks also have a thick fibrous dermis.
In this post, we answered the question “Do all fish have scales?” and also pointed what are the fish do not have scales.
If you have any thoughts or doubts, feel free to drop us in a comment below!
Frequently Asked Questions (FAQs): Do all fish have scales?
What fish do not have scales?
Some fish species do not have scales, they are the clingfish (Gobiesocidae family). They have other types of skin cover, a thick mucus layer.
Do Tuna have scales?
Yes, Tuna have scales. However, they are reduced in number.
Do any catfish have scales?
No, catfish do not have scales. Their skin is often covered in mucous or bony plates, similar to body armour.
Do Salmon have scales?
Similar to most fish, Salmon have scales covering their skin. The scales avoid bruises, predators, and parasites from attacking the fish skin.
Do tilapia have scales?
Yes, similarly to most of the fish species, Tilapia have scales. The Tilapia scales are covered by layers of collagen forming the scale bulk.
Do scales grow back?
In general, fish scales grow back. However, the unprotected area can become the entrance to parasites and also make the fish vulnerable to predators.
McGrouther, M. 2019. Fish scales. https://australian.museum/learn/animals/fishes/fish-scales/
Nesbitt, M. 2020. Do All Fish Have Scales? – Types of Scaleless Fish. https://www.animalwised.com/do-all-fish-have-scales-types-of-scaleless-fish-3526.html
Ramel, G. Fish Scales 101: Placoid, Ganoid & Other Types Of Scale Explained. https://www.earthlife.net/fish/scales.html