The trait that increases the gas exchange efficiency of fish gills is the
a. Unusually high velocity of blood flowing through the gills due to the pressure required by having only one ventricle
b. Back-and-forth movement of water that maximizes oxygen uptake
c. Largest respiratory surface of all vertebrates
d. High degree of oxygen saturation in water
e. Blood flowing through the gills in the opposite direction of water flowing over the gills
The correct answer is e. Blood flowing through the gills in the opposite direction of water flowing over the gills.
In fishes, counter-current mechanism is used to increase the efficiency of gas exchange.
Other answers are not correct, as fishes are among the most primitive groups of vertebrates, while water cannot dissolve much oxygen.