Opercular stain

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Macropod , the dark opercular spot on the gill cover is clearly visible.

Two altum-angelfish , one with (left) and one without (right) visible opercular spot.

The opercular spot is a small, round spot on the gill cover of many fish species (e.g. in macropods and many cichlids ).

The opercular spot can serve to delimit a taxon . For example, within the labyrinth fish it occurs only in the genus of macropods and serves as a generic feature in them, even if it is only recognizable under the microscope in one species ( black macropod ).

In other fish, such as angelfish , it is only visible when excited.