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The shared trait of breathing via gills in bony fish and cartilaginous fish is a famous example of symplesiomorphy. Bony fish are more closely related to terrestrial vertebrates, which evolved out of a clade of bony fishes that breathe through their skin or lungs, than they are to the sharks, rays, and the other cartilaginous fish. Their kind ...
Many aquatic animals have developed gills for respiration which are specifically adapted to their function. In fish, for example, they have: A large surface area to allow as much oxygen to enter the gills as possible because more of the gas comes into contact with the membrane; Good blood supply to maintain the concentration gradient needed
Secondary gills are also present in the unrelated genus Patella, in which they are found as folds within the mantle cavity. Some smaller gastropods have neither true gills nor cerata. The genus Lepeta uses the whole of the mantle cavity as a respiratory surface, while many sea butterflies respire through their general body surface.
Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing countercurrent exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx.
In fish and some molluscs, the efficiency of the gills is greatly enhanced by a countercurrent exchange mechanism in which the water passes over the gills in the opposite direction to the flow of blood through them. This mechanism is very efficient and as much as 90% of the dissolved oxygen in the water may be recovered.
Aquatic animals generally conduct gas exchange in water by extracting dissolved oxygen via specialised respiratory organs called gills, through the skin or across enteral mucosae, although some are evolved from terrestrial ancestors that re-adapted to aquatic environments (e.g. marine reptiles and marine mammals), in which case they actually ...
The ability to breathe through their skin is associated with increased capillary density in their skin. [12] This mode of breathing, similar to that employed by amphibians, is known as cutaneous respiration. [6] Another important adaptation that aids breathing while out of water is their enlarged gill chambers, where they retain a bubble of air.
This allows them, like lungfish, to "breath" in two ways: they can extract oxygen from the water when breathing through their gills, but can also break the water's surface to breathe or gulp air through the pneumatic duct. [35] [36] When performing low-level physical activity, bowfin obtain more than half of their oxygen from breathing air. [37]