Tetralia are the black-masked acropora crabs found throughout the south Pacific. Trapezia, although larger and more colorful than Tetralia associate with Stylo's, Poci's, and birds nest sps corals. They do require a densely branched cluster corals, but I have seen them in fairly small frags. Published studies reveal that they protect their coral host from predators and when removed, coral mortality increases. Red bugs do not do well in Acropora that have resident Tetralia. Well worth purchasing for your SPS tank....Jim Z.