What is the Difference Between Placoid and Cycloid Scales

The key difference between placoid and cycloid scales is that placoid scales are triangular, rough structures present in cartilaginous fish, while cycloid scales are round, flexible structures present in bony fish.

The exoskeletal covering of vertebrates is made of two types of scales. They are epidermal and dermal. Epidermal scales are well developed in vertebrates such as reptiles, birds, and mammals, while dermal scales are well developed in fish. Placoid scales and cycloid scales are two types of dermal scales. They act as protective structures against predators.

CONTENTS

1. Overview and Key Difference
2. What are Placoid Scales
3. What are Cycloid Scales
4. Similarities – Placoid and Cycloid Scales
5. Placoid vs Cycloid Scales in Tabular Form
6. Summary – Placoid vs Cycloid Scales

What are Placoid Scales?

Placoid scales are tiny, triangular, tooth-like structures that cover the skin of cartilaginous fish. Placoid scales do not grow after an organism reaches full maturity. Placoid scales contain rectangular base plates that are embedded in fish skin. They are also known as dermal denticles because they grow out of the dermis layer of the organism. Just like teeth, they have an inner core, which consists of connective tissues, blood vessels, and nerves. The pulp cavity is nourished by a layer of odontoblasts, which secrete dentine. Dentine is a calcified material, and it forms another layer of scales. These fit between the old scales formed before. The dentine is coated with an enamel-like substance called vitrodentine. Vitrodentine is harder than dentine and is produced in the ectoderm.

Placoid scales are usually packed tightly together. They grow facing backwards and lay flat on the skin. Placoid scales are rough, and the structure is impossible to penetrate. These scales protect fish from predators. The triangular shape reduces drag and increases turbulence when swimming.

What are Cycloid Scales?

Cycloid scales are smooth-edged, uniform structures that cover the skin of bony fish. These scales contain two regions. The inner fibrous layer is composed of collagen, while the outer bony layer is a calcium-based frame. At a cooler temperature, cycloid scales tend to grow closely and slowly, leaving a dark band called the annulus.

Cycloid scales are mostly found in advanced fish, and they give external protection to the organism. They are flexible and have a round outline. They are thicker in the center since collagen is present. The part of the scale that is exposed to the posterior area shows fewer ridges, and the anterior area is embedded in the skin. Cycloid scales are overlapping scales and contain growth rings. These scales continue to grow as the fish grows. This results in a pattern of concentric growth rings on the scale.

What are the Similarities Between Placoid and Cycloid Scales?

• Placoid and cycloid scales are seen in fish.
• They provide external protection.
• Both are dermal scales.

What is the Difference Between Placoid and Cycloid Scales?

Placoid scales are triangular, rough structures present in cartilaginous fish, while cycloid scales are round, flexible structures present in bony fish. Thus, this is the key difference between placoid and cycloid scales. Moreover, placoid scales stop growing once the fish matures with time, but cycloid scales grow with the growth of fish. Furthermore, placoid scales are tightly packed scales, unlike cycloid scales.

The below infographic presents the differences between placoid and cycloid scales in tabular form for side by side comparison.

Summary – Placoid vs Cycloid Scales

Placoid and cycloid scales are present in fish, and they provide external protection against predators. Placoid scales have the unique feature of not growing after an organism is fully matured, while cycloid scales grow along with the growth of the organism, resulting in growth rings. The tough structure of placoid scales is a result of the presence of a calcified substance called dentine. Placoid scales are mainly present in cartilaginous fish, while cycloid scales are present in bony fish. So, this summarizes the difference between placoid and cycloid scales.

Reference:

1. “Galleries: Cycloid Fish Scale.” Nikon’s MicroscopyU.
2. Kennedy, Jennifer. “Learn Why Shark Skin Is so Rough.” ThoughtCo.

Image Courtesy:

1. “Osteichthyes – Carassius auratus (cycloid scales detail)” By MAKY.OREL – Own work (CC0) via Commons Wikimedia
2. “Fossilized scales 2” By Paleopermru – Own work (CC BY 4.0) via Commons Wikimedia