Preoperculum
Flat bone; part of the skull (craniofacial) skeleton.
Also known as: Gill Cover Bone, Cheek Bone (Fish)
Classification
Flat bone; part of the skull (craniofacial) skeleton.
Dimensions
Typically L-shaped or crescent-shaped, this bone is thin and plate-like. Its size varies significantly based on the fish species, ranging from less than an inch in small perch to over six inches in large predatory ocean fish.
Key Features
Look for a flat, thin, slightly curved plate that often has a distinct 'corner' or bend, giving it a shape like a boomerang or a wide 'L'. One side is usually very smooth where it sat against the cheek, while the other side may have faint radiating lines or small ridges. The edge that faces the back of the fish is often very sharp or finely toothed.
Similar Bones
It can be confused with the Operculum (the main gill cover), which is usually larger, flatter, and more rectangular. It might also be mistaken for a fragment of a turtle shell, but fish gill bones are much thinner, more translucent, and lack the cellular, 'honeycomb' internal texture found inside turtle bone.
Across Species
In bony fish, this bone is a critical part of the gill cover system. While it remains a simple flat plate in many species, in catfish it is often reinforced and heavy. In some species of sunfish or grouper, the back edge may have fine serrations or spines. This bone is uniquely aquatic; it does not have a direct equivalent in land mammals, as the gill-covering structures were lost or repurposed during the transition to terrestrial life.
Evolutionary History
The preoperculum is an ancient feature of bony fish (Osteichthyes). It emerged as part of a complex pump system that allows fish to move water across their gills without having to constantly swim forward. Over millions of years, as some fish evolved into land-dwelling tetrapods, these gill bones were lost, though parts of the associated jaw structures were modified into different tools for breathing air and hearing.
Photography Tips
Lay the bone on a dark, matte surface to show its translucent edges. Take one photo directly from above to show the overall silhouette, and a second 'profile' shot from the side to show how thin and curved the plate is. Placing a coin or a ruler next to it is very helpful for determining the size of the fish it came from.