Swimming crab from Thailand

Shot this on the Boonsung wreck near Khao Lak in the Andaman Sea, Thailand.

The best I can come up with is swimming crab. Anyone any better idea?

For more detail go to the YouTube page and choose 480p or 720p.

I now understand from another forum that this is the "crucifix crab", Charybdis (Charybdis) feriata.

Thanks for posting the id Nick