Asellus Aquaticus (Freshwater Isopod) Facts

Asellus Aquaticus Portal

Asellus Aquaticus is the scientific name of a small crustacean also known as freshwater isopod, water louse, aquatic pillbug, or aquatic sowbug. There are different species of freshwater isopods, with the most popular genus being the Asellus spp. and the Caecidotea spp. These creatures are not to be confused with Amphipods (commonly known as “scuds”), which have a much flattered appearance. These fascinating invertebrates are becoming increasingly popular as aquarium pets and they are relatively easy to keep – having requirements not too dissimilar from your average freshwater shrimp, but being able to withstand a greater variety of environmental conditions!

Freshwater Isopod life cycle, size and appearance

Freshwater isopods have four maturing stages: juvenile, maturing adults, young adults, and older adults. Newborns have an initial size of about 0.1 inches (3mm), and develop into maturing adults in about a month from hatching. The maximum body size can then reach 0.5 inches (1.2 cm) in males and 0.4 inches (1 cm) in females, although temperature and quality of water can significantly impact how much an adult will end up growing – with specimens found in clear waters averaging significantly larger sizes than those living in polluted environments. The life span of asellus aquaticus varies from nine months to up to twenty. In this case, lower temperatures will actually increase the life span, while slowing growth rate.

The resemblance with pillbugs and sowbugs is due to their characteristic segmented body complete with seven pairs of legs. They are often brownish or silvery in color, but black varieties also exist. 

Freshwater Isopod reproduction and breeding

Asellus aquaticus reproduce sexually under most water conditions. A male specimen will often place himself on top of a female, guarding her from other males until she is ready to undergo a molting process. While molting, the female water louse will expose part of her exoskeleton, allowing the male to fertilize her eggs. He will then proceed to guard the female until the exoskeleton is replaced, preventing other males from mating with her. The number of eggs produced depends both on the size of the female and the quality of the water. Each female can give birth to a new brood almost on a monthly basis when temperatures are warm enough, at around 70°F (21°C).

Breeding freshwater isopods is a no-brainer task for aquarists. As long as temperatures do not fall below 60°F (15°C), 

Natural Habitat

In nature, asellus aquaticus have both cave-dwelling and surface-dwelling populations that can also interbreed. They are widespread across the globe and typical of tinned or lentic environments of North America, Europe and Russia. They are very easy to find around ponds and other bodies of water that do not have strong currents – unlike other crustaceans like shrimp, freshwater isopods are not skilled swimmers and prefer to stay close to the surface.

What do freshwater isopods eat?

Freshwater isopods feed on decaying vegetation, microscopic algae and microorganisms such as fungi and bacteria that are present on decaying leaves. In an aquarium tank, they are bottom-dwelling and will feed on any leftover fish food as well as decaying plant matter and biofilm. In a planted self-sustaining ecosystem you may not need to feed them at all.

PS. We are maintaining a list of online sellers of asellus aquaticus worldwide. Already keeping some? You may want to have a look at our in-depth freshwater isopod care sheet.

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