What kind of crab is this (ID small blue crab)
Hi- Thanks so much for contacting us about the small blue crab you found on the north Oregon Coast. Your clear photo is helpful. At first we thought this could be a hermit crab that escaped from an aquarium. But after consulting fish and wildlife professionals in Oregon and searching the internet, we believe this is a rather beat-up little NW crab that may be going into molting. Hence the tail-like flap. The closest we can guess is that this is a Flattop crab, petrolisthes eriomerus. Closer inspection could distinguish it from another, similar species, the Flat porcelain crab.
You can read a little bit about the crabs here: http://www.centralcoastbiodiversity.org/flattop-crab-bull-petrolisthes-eriomerus.html
Most of us haven't seen a brilliant blue crab on our beaches before and we enjoyed studying your photo and learning about these little crabs. Our coastal waters are full of interesting life. Keep learning!