Freshwater shrimp
Gammarus pulex
The freshwater shrimp is a widespread amphipod in European running waters, characterized by its strongly laterally compressed body and typically curved posture. It primarily inhabits cool, oxygen-rich streams and rivers with gravelly or stony substrates. As an important decomposer (shredder), it plays a key role in the nutrient cycle by breaking down fallen leaves. Its coloration ranges from grayish-brown to olive-green, providing excellent camouflage on the riverbed.
Details
Identification
Laterally compressed body, kidney-shaped eyes, two pairs of antennae, seven pairs of legs (including two pairs of gnathopods), segmented exoskeleton.
Social behavior
Often occurs in very high population densities; during the reproductive phase, they form pairs in so-called amplexus.
Diet
Omnivore, feeds primarily on fallen leaves and detritus, but also consumes algae, biofilms, and occasionally smaller invertebrates.
Hunting strategy
Shredder of organic material and collector; occasionally an opportunistic predator.
Spawning substrate
No external substrate; eggs are carried in the brood pouch (marsupium) on the underside of the female.
Overwintering
Remains active in the water, often buried deep within the interstitial spaces of the sediment; activity is reduced depending on temperature.
Ecology
Ecological role
Central primary decomposer in running waters; converts coarse particulate organic matter (CPOM) into finer material (FPOM) for other organisms.
Natural predators
Brown trout, bullheads, grayling, dragonfly larvae, dippers, and various aquatic beetles.
Competitor species
Other Gammarus species such as Gammarus roeseli as well as invasive species like the killer shrimp (Dikerogammarus villosus).
Ecosystem service
Decomposition of dead organic matter (leaves), self-purification of water bodies, important base of the food chain for edible fish.
Threats
Water pollution (pesticides, fertilizers), river engineering, temperature increase due to climate change, and invasive alien species.
Scientific profile
Profile
Distinguishing features
The primary distinguishing feature from other gammarids is the third uropod: the inner ramus is very short, reaching less than half the length of the outer ramus. Unlike Gammarus roeselii, it lacks dorsal spines or ridges on the segments. The antennae bear dense bundles of setae.
Diet
Decaying leaves (especially with fungal and bacterial biofilm), detritus, algae, and occasionally small insect larvae or conspecifics.
Reproduction
Dioecious. Before mating, the male carries the female for several days in a 'precopula'. Eggs are incubated in a brood pouch (marsupium) under the female's thorax.
Role in food web
Key species in lotic ecosystems; converts coarse organic matter (leaves) into biomass, making it available for higher trophic levels.
Protection & threats
Main threats
Input of insecticides (esp. neonicotinoids), water acidification, structural degradation of water bodies, and displacement by invasive species such as Dikerogammarus villosus.
Conservation measures
Preservation and restoration of near-natural riparian vegetation (leaf input), reduction of diffuse nutrient and pesticide inputs from agriculture, and restoration of longitudinal connectivity.