What are zooplankton?

Zooplankton are small, primitive, single-celled or colonial animals that float in the water column, forming a critical part of aquatic ecosystems. They exist as a diverse group of organisms that can range from tiny protozoa to small crustaceans. Zooplankton serve as a vital food source for larger aquatic animals, such as fish and whale species, contributing to the energy transfer within the aquatic food web.

In contrast to other answer choices, small, single-celled organisms in water refer to phytoplankton or certain bacterium, which primarily perform photosynthesis rather than being classified as animals. Large aquatic plants do not fit the definition of zooplankton, as they are not animal organisms and belong to different biological groups, such as macrophytes. Finally, predatory fish, while crucial components of aquatic ecosystems, are not included in the zooplankton category since they are significantly larger and not suspended microscopic life forms. Therefore, the defining characteristic of zooplankton as small, primarily animal-life forms makes the chosen answer accurate.