Endoliths are a prominent bioerosive component of intertidal marine habitats, traditionally thought to be formed by a few cyanobacteria, algae and fungi. Using molecular techniques, however, we found that endoliths from Mona Island, Puerto Rico, were of high diversity, well beyond that reported in traditional studies. We also found evidence for substrate specialization, in that closely related cyanobacteria seem to have diversified to specialize recurrently to excavate various mineral substrates