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Placoid Scales vs. Cycloid Scales: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on February 19, 2024
Placoid scales are small, tooth-like structures found in sharks, providing hydrodynamic advantages; cycloid scales are smooth, overlapping scales in bony fish, aiding in flexibility.

Key Differences

Placoid scales, characteristic of sharks and rays, have a tooth-like structure with a central pulp cavity, providing durability and reducing drag. Cycloid scales, found in many bony fish, are smooth and circular, offering flexibility and protection without hindering movement.
The composition of placoid scales includes dentin and an enamel-like substance, making them hard and abrasive. Cycloid scales, composed primarily of bone-like material, are less rigid and have a more flexible structure.
Placoid scales grow with the fish, embedding into the skin, which aids in hydrodynamics and predator defense. Cycloid scales, however, overlap like roof tiles, providing a streamlined structure that enhances swimming efficiency.
In placoid scales, the outer layer is replaced as the fish grows, maintaining their sharpness and effectiveness. Cycloid scales grow in size and number as the fish ages, adapting to the changing body size without losing their functional properties.
The presence of placoid scales is a key identifying feature of cartilaginous fish, such as sharks and rays. In contrast, cycloid scales are a distinctive characteristic of many species of bony fish, reflecting their evolutionary adaptations.

Comparison Chart

Found In

Sharks and rays (cartilaginous fish)
Many bony fish


Tooth-like with a central pulp cavity
Smooth, circular, and overlapping


Dentin and enamel-like substance
Bone-like material


Reduce drag, aid in hydrodynamics
Provide flexibility, streamline movement


Embed into skin, grow with the fish
Overlap like tiles, grow in size and number

Placoid Scales and Cycloid Scales Definitions

Placoid Scales

Tooth-like scales in sharks and rays.
The rough texture of a shark's skin is due to placoid scales.

Cycloid Scales

Smooth, circular scales on many bony fish.
Cycloid scales are evident in the flexible movement of salmon.

Placoid Scales

Hard, enamel-covered structures on cartilaginous fish.
Placoid scales give sharks an advantage in speed and agility.

Cycloid Scales

Characteristic scales of evolutionary adaptations in bony fish.
Cycloid scales reflect the evolutionary journey of many fish species.

Placoid Scales

Abrasive scales aiding in predator defense.
The placoid scales make it difficult for predators to grasp sharks.

Cycloid Scales

Overlapping scales providing streamlined structure.
The cycloid scales on trout facilitate efficient swimming.

Placoid Scales

Scales embedded in the skin, growing with the fish.
Placoid scales are integral to a shark's streamlined shape.

Cycloid Scales

Flexible, bone-like scales aiding in fish movement.
Cycloid scales contribute to the agility of bony fish in water.

Placoid Scales

Hydrodynamic scales unique to cartilaginous fish.
Placoid scales are a distinct feature differentiating sharks from bony fish.

Cycloid Scales

Scales growing in size and number with the fish.
As the carp grows, its cycloid scales adapt in size accordingly.


What are cycloid scales?

Cycloid scales are smooth, circular scales found on many bony fish.

Are cycloid scales flexible?

Yes, cycloid scales are flexible and aid in fish movement.

What material are placoid scales made of?

Placoid scales are made of dentin and an enamel-like substance.

Can placoid scales be sharp?

Yes, placoid scales can be abrasive and sharp.

What are placoid scales?

Placoid scales are tooth-like structures found on sharks and rays.

Do placoid scales overlap?

No, placoid scales do not overlap but are embedded in the skin.

How do cycloid scales aid in swimming?

Cycloid scales provide a streamlined structure for efficient swimming.

What fish have cycloid scales?

Many bony fish, like salmon and carp, have cycloid scales.

Do placoid scales grow with the fish?

Yes, placoid scales embed in the skin and grow with the fish.

How do cycloid scales affect fish flexibility?

Cycloid scales overlap, enhancing flexibility and movement in water.

Do placoid scales aid in hydrodynamics?

Yes, placoid scales reduce drag, aiding in hydrodynamic efficiency.

Can cycloid scales be easily damaged?

Cycloid scales are relatively robust but can be damaged by rough handling.

Are cycloid scales found in freshwater or saltwater fish?

Cycloid scales are found in both freshwater and saltwater bony fish.

How do cycloid scales grow?

Cycloid scales increase in size and quantity as the fish grows.

Do placoid scales offer any protection?

Yes, they provide physical protection and aid in predator defense.

Are cycloid scales unique to certain fish species?

Cycloid scales are characteristic of many, but not all, bony fish species.

Are placoid scales unique to sharks?

They are unique to cartilaginous fish, including sharks and rays.

Do cycloid scales grow in size or number?

Cycloid scales grow both in size and number as the fish ages.

What is the purpose of the enamel in placoid scales?

The enamel provides hardness and protection to placoid scales.

Are placoid scales visible to the naked eye?

Placoid scales are small but can be felt as a rough texture.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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