When you walk past the tanks in an aquarium store, you see both physostomous and physoclistous fishes. Goldfish, for example, are physostomous, and cichlids (such as the popular convict cichlid) are physoclistous

Question

Conduct a thought experiment comparing the responses of the two kinds of fishes to changes in air pressure. (Don't actually do this because it would be painful for the fishes.) a. Imagine that you put a goldfish and a cichlid into a jar half filled with water and then screwed on a lid to make an airtight seal. Protruding from the lid is a tube to which you can attach a vacuum pump and a valve that allows you to close off the tube. b. Pump some air out of the jar. What effect will doing that have on the air pressure inside the jar? How will each of the fishes respond to that change in pressure? How are those responses related to being physostomous or physoclistous? c. Now open the valve and allow air to enter the jar, returning the pressure quickly to its starting level. How will each of the fishes respond to this change in pressure? How are these responses related to being physostomous or physoclistous?

Answer 1

Pumping air out of the jar reduces the pressure, and that causes the fishes' swim bladders to expand, making them positively buoyant so they float to the surface of the water. The goldfish can burp air out of its swim bladder because it is physostomous, and it quickly restores neutral buoyancy. The cichlid is physoclistous so it must use its gas gland to move oxygen from the swim bladder to the blood. This is a much slower process than burping out the extra volume, so the cichlid floats on the surface for several minutes after the goldfish has resumed its normal behavior.
Opening the valve increases the pressure in the jar, and the fishes' swim bladders get smaller, so they are now negatively buoyant and sink to the bottom of the jar. The goldfish swims to the surface and gulps air into its swim bladder, quickly returning to neutral buoyancy. The cichlid must use its gas gland to release oxygen from its blood into the swim bladder. This is a slow process compared to gulping air, and once again the cichlid remains helpless for several minutes after the goldfish has resumed normal behavior, but this time the cichlid is resting on the bottom of the jar.