Freshwater Cyclops – Species Profile

Female freshwater Cyclops. Source: wikimedia.

“Cyclops” is a common genera of copepod organisms found in freshwater habitats. The name comes from the mythical Greek creature, known for having only one eye, a characteristic shared by these minute creatures. Under magnification, this eye can be easily seen as a large red or black dot on the head. Cyclops are easy to spot on aquarium walls as tiny, white, tadpole-shaped specks that move in quick, jerky movements.

Care Level: Easy
Preferred Water: pH 6 to 8, soft to hard
Ideal Temperature: 64 to 72 °F (12.8 – 22.2 °C)
Maximum Size: 0.5mm to 5mm, depending on species
Average Lifespan: 3 months

Here you can just barely make out the red eye of this adult cyclops. You can also see the two “saddle bags” carrying eggs. Because of this last feature, we can identify this one as a female.


Cyclops are found around the world in many freshwater environments and some brackish environments. They prefer shallow, slow moving water where they can easily swim in search of food.


Cyclops are mostly scavengers, feeding on plant matter, algae, decaying flesh, and sometimes preying on smaller creatures like paramecium.

A female (left) and male (right) freshwater cyclops. Source: wikimedia


Cyclops are sexually dimorphic, meaning males and females have different physical characteristics. Both sexes have a pair of antennae, but the male’s are curved to grip the female while mating. After mating the female deposits eggs on two sacks on either side of the tail. These resemble saddle bags, and are visible with the naked eye. Each egg develops into a free-swimming nauplius, which appears as a small white speck on the aquarium wall.

A single Cyclops nauplius. Source: wikimedia
An adult cyclops (bottom) with three nauplii.

In the Aquarium

Cyclops are an excellent supplemental food source for fish and fry. The nauplii are particularly good for small egg-layer fry. Cyclops can be cultured in containers of green water and fed to fish as needed.

A large number of Cyclops in the aquarium can be a sign of overfeeding, but in a well-planted aquarium some Cyclops can be expected. They can be removed mechanically through tight mesh filters. Small predatory fish species have been used successfully to control Cyclops populations in drinking water reservoirs, and could also work in the aquarium. In my own experience, dario dario make quick work of an out of control population of Cyclops.

As a Pest Species

Cyclops have the potential to act as hosts for fish tapeworm and other parasitic diseases. It is for this reason that people have endeavored to control their populations in places like India.

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