Welcome to TONMO, the premier cephalopod interest community. Founded in 2000, we have built a large community of experts, hobbyists and enthusiasts, some of whom come together when we host our biennial conference. To join in on the fun, sign up - it's free! You can also become a Supporter for just $50/year to remove all ads and gain access to our Supporters forum. Follow us on Twitter and Facebook for more cephy goodness.
It looks like the common box crab, Calappa calappa. The color pattern in variable, but this spotted form is fairly common. It is found throughout the Indo-Pacific and feeds on mollusks by crushing and peeling them.
I saw one of those freediving one night in the Cook Islands. Pretty shy.
I also saw the only slipper lobster I've ever seen on that same night. And three blacktip reef sharks that wanted the fish I had speared. The first two I didn't worry about so much, but I figured that three was pushing my luck a little.
Cool addition, hope it doesn't eat anything any your tank that you really like.