From a distance this may look like a boulder coral but close up you can see the distinctive cone shaped houses of this coral. If you look closer you may see some of the other creatures that live inside it.
Date Mussels (Lithophaga simplex) are completely encased by the skeleton of the coral and you can only see the ends of their siphons sucking in and filtering out food from the water. These mussels act as fertilisers for the coral, they excrete their waste and the corals eat it and then the algae inside the coral use it as fertiliser (like the horse manure I put on my vegie patch).
Even more exciting than this are the hermit crabs. A group of hermit crabs that have given up shells and use holes in coral for houses. These crabs have specialised feather-like antennae that act like nets and scoop up the food that floats by.