Gammarus pulex, sometimes incorrectly called the "common freshwater shrimp", is a freshwater amphipod. The adult Gammarus pulex is typically around 11 mm long (though males can be up to 20 mm), with a curved, brown-yellow body. Sometimes, the genus Gammarus is split up; in this case, the present species would be named Rivulogammarus pulex.
The head has compound eyes, which give them all-round vision so that they can see predators, and well-developed pairs of first and second whip-like antennae, which contain sensory organs for taste and touch to allow them to find food.
The thorax has seven pairs of jointed legs; two pairs are claw-like gnathopods for grasping, and five pairs are for crawling and swimming. The thorax also contains the gills. The exoskeleton covering the head and thorax is rigid and known as the carapace.
The abdomen has three pairs of appendages known as pleopods for swimming and circulating water whilst in situ and three pairs known as uropods for swimming. The exoskeleton over the abdomen is flexible to allow movement.
Gammarus pulex is a common organism in chalk streams. They prefer lotic (flowing) water, such as in streams, but they are also found in lakes and ponds, normally at the water's edge. They are intolerant of polluted waters or reduced dissolved oxygen concentrations, and can therefore be used as an indicator species for water quality.
They are most frequently found in rivers with pondweed on the rocks or the stream bed, or where there is loose moss or loose stones. They are less common in rivers with embedded stones as they typically reside under stones and between the bases of plant stems and roots, though they tend to feed on the upper reaches of the plants. They swim in bursts when they are caught in the current or disturbed. It has been found that they are pushed further by the current when it is slow-flowing than when it is fast flowing. Though they spend most of their time in the shelter of rocks and plants, they have to swim against the current to reach new food sources.
They are principally detritivors, organisms which feed on dead and decomposing plant and animal material, including the biofilms (microbial growths made up of bacteria, fungi, algae and their secretions) that grow on them. It has been shown that Gammarus prefer leaves that are partially decomposed and covered with biofilms (termed conditioned), and this has been taken as evidence that they actually gain most energy from microbial biomass covering the dead leaf rather than the detrital materials themselves. Despite much attention given to their status as shredders, Gammarus also eat small organisms, each other, and have been shown to feed by coprophagy in chalk streams, which may constitute an important recycling pathway for organic matter in such habitats.
Since they live in fresh water they have to expend large amounts of energy using active transport to replace the mineral ions lost through diffusion into the water around them as is the case with all freshwater invertebrates. They have an impervious exoskeleton to reduce losses through diffusion, but this also means that they have a smaller surface area to perform active transport over. To combat this they take in water at the gills and absorb the ions in their digestive system.
Unlike many freshwater invertebrates they have no larval stage, but hatch from the eggs fully developed. They also mature and reproduce very quickly, producing up to two generations every year.
The factors known to regulate their occurrence and distribution are water velocity, temperature, composition of the substratum, liability of flood or drought, competition from other organisms (interspecific competition), predation and shade.