Colony of the symmetrical brain coral, Pseudodiploria strigosa, is a common reef–building coral in the Caribbean. When under stress, these animals may harbor a species of mutualistic microbe, which was recently discovered to be introduced from the Indo-Pacific. This microscopic symbiont, a dinoflagellate, increases the stress tolerance of native corals, but may significantly reduce the animal’s rate of calcification, thus diminishing their role as reef builders.