They are quite invisible against the background of the bottom, but after you find the first crayfish, it is easier to detect the following. Still, they do not always hide in holes, periodically quietly crawl along the bottom, walk about their crayfish affairs, apparently. So if you want to catch fresh crayfish, welcome) Dolomite quarry near the city of Shchelkovo, Moscow region.
It's better to watch the photo in high resolution.
OLYMPUS E-M5 MarkII
Exposure time: 1/320 sec
Aperture: F 2
Sensitivity: ISO 1000
Focal length: 8 mm
35 mm equivalent: 16 mm
You can also see my photos in my blog LJ and in my profile on NatGeo