Red Leg Crab, Yellow Watchman Goby

So my neighbor called me and wants me to come over and take a few pictures of his saltwater fish this weekend, I went once all ready, and I have to say...It's way harder then i thought it would be, here is one of the better one's from the last time I went. I shot it with a Nikon D300 with a Sigma 50mm 2.8 set at 2.8