Key:  Family Grapsidae (Shore crabs)

Taken primarily from  Kozloff, 1987, 1996 p416 (Copyright 1987, 1996, University of Washington Press.  Used in this web page by permission of University of Washington Press)

An excellent alternate, up to date key can be found on page 387 of Wicksten, 2009.
 

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1a	Merus of legs 2-5 much flattened, used for swimming; male without a tuft of hairs on the propodus of the cheliped; oceanic crabs, only occasionally washed onto shore with floating debris or other animals	2
1b	Merus of legs 2-5 not markedly flattened; male with a tuft of hairs on the propodus of the cheliped; common intertidal crabs	3
2a	Carapace decidedly longer than wide; lateral margins convex	Planes cyaneus
2b	Carapace only slightly longer than wide; lateral margins almost straight, but converging slightly posteriorly	Planes marinus
3a	Dorsal surface of carapace with flat transverse ridges, and with red or purple transverse lines on a blackish green background; carapace with 2 teeth on the anterolateral margin (not likely to be found north of Oregon)	Pachygrapsus crassipes
3b	Dorsal surface of carapace without flat transverse ridges and without transverse lines; carapace with 3 teeth on the anterolateral margin	4
4a	Legs fringed with abundant hairs (photo); prevailing coloration generally grayish green; chelipeds without conspicuous purple spots (photo)	Hemigrapsus oregonensis
4b	Legs not obviously hairy; prevailing coloration generally reddish; chelipeds with conspicuous purple spots	Hemigrapsus nudus