20-25mm long shrimp like invertebrate swimming on its side in fast flowing stream with silty margins .Dislikes light drops into silt on being illuminated.
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I've just realised the site is in Ireland, so I've removed my agreement as it may not be G. pulex. The native riverine Gammarus in Ireland is the freshwater form of G. duebeni, which I think is called G. duebeni celticus. G. pulex has been introduced and is replacing duebeni. So either species is possible. They are easy enough to separate under the microscope, based on leg spines and shape of the plates that cover the legs bases, but I can't do it from the photos.
thanks for your comments,there were hundreds of these in the headwaters of a small fast moving stream that that drains a boggy area near me.I have no knowledge of them other than that I see them . I may be able to get some things examined in more detail by our local biodiversity unit who are interested in some of what I find from a data base perpective ,if so I will update my observations.
Thanks again foxy.
Lat/Lng: 52.6, -7.9
OS grid ref: SF0616