Final Answer:
In a teleost fish, water enters the mouth when the fish opens its mouth, creating a negative pressure that draws water in. This process is facilitated by the expansion of the fish's buccal cavity.
Step-by-step explanation:
Teleost fish, a diverse group that includes most modern fish species, have a unique mechanism for ventilating their gills. Unlike some other fish species that rely on a continuous flow of water over their gills, teleost fish use a method called buccal pumping. When a teleost fish opens its mouth, the buccal cavity expands, creating a decrease in pressure within the mouth.
This negative pressure causes water to rush into the mouth, carrying oxygen along with it. As the fish closes its mouth and the buccal cavity contracts, the pressure increases, forcing the water over the gills and allowing gas exchange to occur.
This buccal pumping mechanism is highly efficient for many teleosts, enabling them to extract oxygen from water even in environments with low oxygen concentrations.
The ability to actively control the flow of water over the gills is advantageous for teleost fish, allowing them to regulate their respiratory processes based on environmental conditions. Overall, the process of water entering the mouth during buccal pumping is a key adaptation that contributes to the success and diversity of teleost fish in various aquatic habitats.