Gammarus pulex
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| Gammarus pulex (Linnaeus, 1758) |
Gammarus pulex, sometimes called "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.
The head has compound eyes, which give them all-round vision so that they can see predators, and well-developed pairs of fist and second antennae, which are whip-like and 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 dominant organism in chalk streams. They prefer flowing water, such as in streams, but they are also found in lakes and ponds, normally at the water's edge. They are highly intolerant of polluted waters or ones with low oxygen content.
They are most densely found in rivers with pondweed on the rocks on the riverbed, or where there is loose moss or loose stones. They do not survive well 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 do have to swim against the current to reach new food sources. As such, populations decrease with increasing velocity, but when water level is low, they will move into the middle of the river, where the current is higher, partly due to better rock cover and partly because river velocity is still relatively low here so they can stand to swim against the current for the sake of getting more oxygen.[citation needed]
By moving their legs they also circulate fresh, oxygen-rich water over their gills. They transport oxygen in hemocyanin, which is a copper based respiratory pigment in their blood.
They survive through the winter and autumn by feeding on dead leaves that fall into the stream, although it is less clear what they feed on during the spring and summer when these materials are no longer present in abundance. They obtain energy by shredding dead leaves and plant matter to feed on the biofilm (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 biofilm (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 in chalk streams have been shown to feed by coprophagy, which may constitute an important recycling pathway for organic matter there.
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. They have an impervious exoskeleton to reduce loss through diffusion, but this also means that there is less of an area to perform active transport over. To combat this they take in water at the gills and absorb the ions in their digestive system. They have an advantage over other freshwater organisms as they can also reabsorb ions through the excretory tubule.
It is because they take up so many ions that pollution is so detrimental to them, as poisonous ions would quickly be absorbed into the shrimp.[citation needed]
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 velocity temperature, composition of the substratum, liability of flood or draught, competition from other organisms, predation and shade.
