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Question:
My 9 year old daughter & I are researching fish scales. We have discovered that there are 4 kinds of scales: 1. Placoid 2. Cosmoid 3. Ganoid 4. Cycloid and Ctenoid We do not understand why Cycloid and Ctenoid scales are considered one type. On further investigation we found that they even look different? Are there really 4 kinds or 5? If so could you explain why?


Replies:
While the classification of scales may seem odd based on appearance, they are grouped into four categories based on their material composition. Placoid scales have a vascualar (blood supply) inner core of pulp, a middle layer of dentine, and a hard enamel-like outer layer made of vitrodentine. Cosmoid scales have2 basal bone layers, a dentine-like cosmine layer, and a layer of vitrodentine. Ganoid scales have one basal bony layer, a dentine layer, and a ganoine layer. The reason why Cycloid and ctenoid scales are in the same group is because they have the same material composition. These scales have a bony surface laer that has an organic framework fused with calcium based salts, and a deep fibrous lyaer made up of mostly collagen. Forgive me if this is extremely indepth! The simple answer is that Cycloid and Ctenoid scales have the same ingredients and that's why they are in one category!

Grace Fields


This answer comes from a fisheries biologist colleague of mine: According to "An Introduction to Fisheies Biology" and "Fishes of Illinois" the principal types of scales of fishes are;

Ganoid - bony plates of sturgeon and rhombic ganoid plates of gars cycloid scales of trout, minnows and most soft-rayed fishes ctenoid scales of perch, bass, sunfish and most spiny-rayed fishes cycloid scales differ from ctenoid scales in that the exposed margin is smooth where as the exposed margin of ctenoid scales have minute spines/teeth called ctenii making them rough to the touch.

placiod scales are found on sharks and rays; and cosmoid scales are found on

lungfish and some fossil fishes. Ganoid plates most likely evolved from cosmoid scales

J Elliott


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