The interior of these reefs contain many cavities inside the coral from the boring by organisms and from the irregular overgrowths of encrusting organisms. The larger cavities (growth cavities) are due to biological roofing over chiefly by coral. The cavities inside the reef contain a distinctive community of organisms. Near the outside, cavity walls are encrusted with the right valve of the bivalveSpondylus ictericus, coralline algae, ectroprocts, serpulid tubes and the red foraminifer Homotrema rubrum. Deep within the cavities the proportion of wall covered by living organisms decreases to zero.