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Normally referred to as a shrimp but in fact Gammarus Pulex are freshwater amphipods.
Anatomy of an amphipod - thanks to wapedia
Gammarus are are useful indicator species due to their requirement of oxygen rich water and are a welcome find in our samples but in Northern Ireland , where they were introduced as a supplementary food for trout by well meaning but misguided anglers, gammarus have become an abundant invasive species displacing the native species.
These are complicated creatures, they have gills and fourteen legs on their thorax and twelve appendages on their abdomen which help with the circulation of water over the gills and swimming.
Gammarus hatch fully developed from eggs and are able to reproduce rapidly. Adults can produce two generations in a year. Their fecundity enables them to be harvested for commercial use where they're sold as fish food -
Only two species of gammarus are found in freshwater in Britain, Gammarus Pulex and Gammarus Lacustris which is found in Scottish lochs. The native Irish freshwater species, Gammarus Duebeni is found in brackish water on this side of the Irish sea.
Parasites infect gammarus and induce a colour change and behavioral differences in their hosts. These changes make the hosts more prone to predation demonstrating the principle of oddity selection - which goes some way to explain how anglers flies work. The parasitic infestation manifests itself in the form of a bright orange spot in the centre of gammarus and attracts fish to eat it in preference to its unifected neighbours.
Subsequently the parasite lives on inside the fish that ate the gammarus which in this unlucky situation is described as an intermediate host.
There's an account of an interesting experiment to study the effects parasites have on gammarus here.
The main predators of Gammarus are fish - trout and salmon parr, bullheads and stone loach all eat them as do minnows and sticklebacks, it's a widely available and important food both in stilwaters and rivers.