O is for opilio

I was recently asked how to distinguish between snow crabs (Chionoecetes opilio) and Tanner crabs (Chionoecetes bairdi). The person knew the difference in eye color thanks to this post: snow crabs have beautiful green eyes while Tanners have blood red eyes. Another way to tell the difference is to read their lips epistome margins. Opies have flat epistomal plates while bairdis have M-shaped plates:

snow crab on top, Tanner crab on bottom

I like to picture the snow crabs making an "O" sound when I look at their faces as if they were saying "OOOOO-pilio"!

Um, so let's say "meatballs" represent the red eyes? Yes.
(Sorry, I couldn't think of a good "M" word.)

There are a couple other differences: generally adult Tanners seem to be larger than adult snow crabs (at least in my experience) and their bodies are shaped a bit differently. Snow crabs have rounder bodies (their carapace length-to-width ratio is about 1) while Tanners are packing wide loads (their length:width is less than 0.945).

carapace shape and epistome margin from left to right:

pure Tanner crabs, three variations on hybrids, and then pure snow crabs

That may seem like a small difference, but if you're on a boat measuring hundreds upon hundreds of crabs, you start to know who is who before you even check their eye color and epistomal plates. You can also get a feel for something awry, like if you have a wide guy but with green eyes. Then you check his epistome margin and it looks like this:

not quite flat, but not quite M-shaped...

That's right, you've got a hybrid! So now you know, and knowing is half the battle.