Infusoria

Infusoria is a word used to describe various freshwater microorganisms, including ciliates, copepods, euglenoids, planktonic crustaceans, protozoa, unicellular algae and small invertebrates. Some authors (e.g., Bütschli) have used the term as a synonym for Ciliophora. In modern, formal classifications, the term is considered obsolete; the microorganisms previously and colloquially referred to as Infusoria are mostly assigned to the clades Diaphoretickes (e.g. ciliates, most algae), Amorphea (e.g. crustaceans and other small animals) and Discoba (euglenids).

In other contexts, the term is used to define various aquatic microorganisms found in decomposing matter.

Aquarium use

Certain microorganisms, including cyclops and daphnia (among others), are sold as a supplemental fish food. Some fish stores or pet shops may have these infusoria available for live purchase, but typically they are sold in frozen cubes—for example, by the Japan-based fish food brand Hikari. Still, some advanced aquarists, with especially large collections of fish, will breed and cultivate their own supplies of the microorganisms.

Infusoria are especially used by aquarists and fish breeders to feed fish fry; because of their small sizes, infusoria can be used to rear newly-hatched offspring of many common (and also less common) aquarium species.