Waterlouse
Asellus aquaticus
The waterlouse is a widespread freshwater crustacean belonging to the order Isopoda. It has a dorsoventrally flattened body with seven pairs of legs and two long pairs of antennae. It is commonly found in stagnant or slow-moving waters, living on the bottom among leaf litter and detritus. It is highly adaptable to low oxygen levels and organic pollution.
Details
Identification
Flattened body, grey-brown color with pale spots, two pairs of long antennae, seven pairs of walking legs, two forked uropods at the end of the body.
Social behavior
Solitary, but often occurs in very high individual densities; exhibits distinct mating guarding by the male.
Diet
Omnivore and detritivore; feeds mainly on dead plant parts, leaf litter, fungi, and bacterial films.
Hunting strategy
Gathering and grazing; searches the bottom of the water body for organic material.
Spawning substrate
Eggs are carried in a brood pouch (marsupium) on the underside of the female.
Overwintering
Overwinters actively at the bottom of the water body, often buried in mud or under organic material.
Ecology
Ecological role
Important decomposer; accelerates the breakdown of organic matter and serves as an important food source for fish and amphibians.
Natural predators
Fish (e.g., trout, perch), dragonfly larvae, diving beetles, newts, water shrews.
Competitor species
Other detritivores such as Gammarus species (scuds), especially in better water quality conditions.
Ecosystem service
Supporting the self-purification of water bodies by decomposing leaf litter and organic remains.
Threats
Extreme chemical toxicity (e.g., insecticides), loss of riparian vegetation and leaf litter input.
Scientific profile
Profile
Distinguishing features
The pleotelson (fused abdominal segments) is shield-like, enlarged, and wider than it is long. The uropods at the end of the body are distinctly biramous and project backwards. Unlike scuds (Gammarus), water lice do not swim on their sides but move by crawling upright. Males possess modified first and second pleopods for copulation.
Diet
Primarily dead plant material (leaf litter) and the associated fungal hyphae and bacterial biofilms; additionally periphyton (algal growth) and occasionally carrion.
Reproduction
Sexual reproduction with distinct precopula (male carries female for several days). The female deposits eggs into a ventral marsupium (brood pouch), where development occurs directly (without a larval stage). Juveniles leave the marsupium as fully formed small isopods.
Role in food web
Important decomposer; converts coarse particulate organic matter (CPOM) into fine particulate organic matter (FPOM), making energy available for higher trophic levels. Key species in the nutrient cycling of woodland waters.
Protection & threats
Main threats
No immediate threat; the species is highly adaptable and often benefits from anthropogenic eutrophication and moderate warming.
Conservation measures
No specific conservation measures required; the species is widespread and present in high abundances.