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Prawns are difficult to photograph in such a way as to demonstrate all the characters needed for a confident identification.
Palemonidae separate from other prawns on the basis of features of joint of the second leg.
Also, within the Palemonidae there are genera other than Palemon which separate on other features too small to be seen in a low magnification picture.
Nevertheless, the most likely prawns to be seen in a UK rockpool are indeed of the genus Palaemon; its just difficult to be certain of the ID from an image.
Lat/Lng: 55.958, -4.799
OS grid ref: NS253774
A prawn I saw in a rock pool.