Refer to the paragraph on crucian carp. Gills serve multiple functions in fish in addition to gas exchange. Given the large surface area of gills with lamellae, what is the most likely explanation for why crucian carp cover protruding lamellae in their gills when levels of oxygen are normal?

To avoid Hyper-oxygenation

Explanation:

Crucian carp have the ability to increase gill surface to maximize oxygen intake, or reduce overall area when under specif circumstances. If the fish does not need so much oxygen it will cover the protruding lamellae to reduce oxygen intake and avoid Hyper-oxygenation.