The megalops stage crab fly patterns that I have used,(sorry no picture), have been size #10 or #12 or even #14 to #16, (closer to actual size But still huge compared to the naturals), tied on shrimp/scud hooks, very, very sparse. I use mono eyes, either store bought or home burned mono tips, about 1/8" to 1/16" diameter. If anything I like the eyes to be a little "overstated" or abnormally large. Even at 1/16" your mono eyes will be larger than the real ones. And I will wrap a thread base on the hook, from behind the eye to well into the bend, and then tie in one frond of CDC, grey or tan, at the "tail". And then wrap the thread to behind the eye again. Then I will tie in two light tan or grey CDC fronds, in much the same manner we would tie in calf tail hair, as stand up, split wings for a Wulff type dry fly. I set these CDC "wings" very close to the eye of the hook, at 90 degree sideways angles to the hook. A variation in this is to use a very sparse, thin dubbing, such as possum. I like it with dubbing. But it has to be impossibly sparse. This is a matter of creating a ghostly, impressionistic, translucent fly, rather than a full bodied opaque fly.
If you look at the megalops stage crabs in the water, in a natural setting, often they are barely visible as individuals. Once they group up they appear as "clouds" or groups of greyish or beige colored "swimmers", though they are not very fast. You can be looking right at them and miss them. Larger pocket estuaries, tide pools, lagoons, near shore shallows, places with warmer and more plankton rich water etc, can hold huge numbers of these crab juveniles in early to mid summer. And every fish and bird will eat them. Even on a size 16 to 18 hook your imitation will be bigger than the naturals. But it will work, usually on lighter tippet, a 5x or 6x, and dead drifted. I have seen some interesting patterns that were simply a bit of glue gun glue, spread along the hook shank, and a pair of mono eyes, on very small dry fly or shrimp/scud hooks. I do like scud flies, size 12 through 16, in an ash, grey or olive color. And these can double for gamarid amphipods, or "Beach Hoppers", very nicely. We see Cutthroat feeding at the edges for these very commonplace small crustaceans as well.
I have no doubt that crabs in the soft shell state, at any stage of life, would be easy prey for Cutthroat.
my blog:
http://olympicpeninsulaflyfishing.blogspot.com