Which term describes free-swimming organisms such as fish?

• A. Plankton
• B. Nekton
• C. Epilimnion
• D. Hypolimnion

Answer: (B)

Nekton refers to organisms that swim actively. These animals, like fish, are capable of locomotion that allows them to move through water, swim against currents, and cover large distances. This distinguishes them from plankton, which are mostly drifters carried by water movement rather than actively swimming. Plankton includes phytoplankton and zooplankton that rely on currents for transport rather than strong propulsion.

The terms epilimnion and hypolimnion describe layers in a stratified lake—the upper warm layer and the deeper cooler layer, respectively. They refer to lake structure and habitat zones, not to the swimming ability of organisms. Therefore, the best term for free-swimming organisms such as fish is nekton.