Loricariids and some other bottom dwelling fish have pupils that appear to be round in shape.
Loricariids seem to be able to control the shape of their pupil with a flap-like extension to the iris called the dorsal iris operculum. When dilated the pupil appears round, while when constricted it appears curved like a crescent.
This action seems confined to bottom-dwelling fishes such as plecs, flounders, skates and rays. The exact function is not fully known, but the flap might help break up the distinctive shape of the eye and aid camouflage.
Alternatively, the flap may protect the retina from strong light.
Observations of visual ganglion cell patterns in the loricariid retina indicate plecs see best directly behind and in front of them. When moving plecs between aquariums, it is perhaps best to approach them with a hand or net from above.
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