This is a Crown of Thorns star. These guys are NOT a favorite critter of ours as they eat coral, and the spines are venemnous to boot. You can see this particular star has just finished off eating the cauliflower coral behind it and is moving on to it's next meal. Corals are a colonial critter, growing on top of the skeleton of previous polyps. The COT starfish will eat the live outer layer, and the coral may or may not ever recover.
As far as the poison content of the spines of this star... it is not to be messed with. I've run into a lot of people who have tried touching it "just to see" if it's sharp or hurts.... It is sharp, will poke through gloves and skin easily, and will mess the contact site up for months, I know people who have visible damage years later - it's not a simple bee sting, it's nasty.
We're in the midst of a busy spell (what a difference a year makes, last year we were taking the boat out of service for the rebuild and had 4 or 5 charters in May, this year I already have 21 charters lined up for the month) so I won't be posting much 'til mid-week as I have charters day and night 'til then. Pat joined the charter today, as we only took out 2 passengers, and took lots of photos (they saw 2 frogfish, devil and titan scorpions, male and female whitley's boxfish, flatworms, an eagle ray and lots of other neat stuff) while I stayed on dry land to get fingerprinted by the Coast Guard for my Captain's license renewal. Bob and Cathy are both Captains and dive instructors so they ran the boat today - they were probably thrilled to have "the boss"/me off the boat.