Aquatic microscopic animals are called zooplankton. Zooplankton are tiny animals that float in water and are unable to swim against the current. They are a crucial part of the aquatic food chain as they serve as the primary food source for many larger aquatic animals.

Zooplankton can be divided into two categories: holoplankton and meroplankton. Holoplankton are animals that spend their entire life cycle as plankton, while meroplankton are organisms that spend only a portion of their lives in the planktonic stage.

Examples of zooplankton include:

1. Rotifers: These are tiny aquatic animals that use hair-like structures called cilia to move through the water and feed on algae and other small organisms.

2. Copepods: These are small crustaceans that are an important food source for many larger aquatic animals. They are found in both marine and freshwater environments.

3. Krill: These are small shrimp-like animals that are found in large numbers in the oceans. They are a primary food source for many marine animals, including whales, seals, and penguins.

4. Jellyfish: These are gelatinous animals that float in the water and are often found in large numbers in the oceans. They are predators that feed on other zooplankton and small fish.

In summary, zooplankton are aquatic microscopic animals that are an important part of the aquatic food chain. They serve as a primary food source for many larger aquatic animals and play a critical role in maintaining the balance of aquatic ecosystems.