Relatively large mysid seen swimming in a rockpool on the lower shore at South Bay, Scarborough
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I agree its a mysid and, as Paula notes, P.flexuosus seems a good bet for an ID as its big and common.
Nevertheless, after seeing the picture I went back to the literature and only succeded in reminding myself why I have always had difficulty with identifying mysids in the lab and under a microscope (and the near impossibility of getting the ID right when looking at underwater video).
They are greatly neglected and,as far as I know the last complete work on them was the 1951 Ray Society monograph by Tattersall & Tattersall.
Lat/Lng: 54.26637, -0.38489
OS grid ref: TA052868