Houtman Abrolhos
The Houtman Abrolhos is unusual in having a luxuriant and diverse living coral reef at such a high latitude. 194 species in 50 genera have been recorded there, all but two of which are tropical. This is a surprisingly high coral diversity, considering the high latitude of the reef, and the relatively low diversity of other biota. For a full list, see list of corals of the Houtman Abrolhos.
Amazon Reef
The majority of the reef is made up of beds of rhodolith, various species of red algae, which superficially resemble coral. While it has been described as "impoverished", and "not having the biodiversity of some of the more prominent coral reefs of the world", 61 species of sponge and 73 species of fish, in addition to various coral and starfish species, have so far been identified as inhabiting the reef, including staghorn corals, and spiny lobsters. Pockets of coral species, discovered as early as 1999, were found to be similar to those found in the Caribbean Sea, hinting at the possibility that Caribbean corals dispersed to the Amazon Reef. Marine biologists have also entertained the idea that the reef serves as a stepping stone to facilitate dispersal of species from reefs in southern Brazil northwards to the Caribbean. The biology of the reef is mostly dictated by the discharge of the Amazon into the Atlantic. Northern sections of the reef are often shrouded in the shadow of the sediment layer above for half a year on average, producing an environment similar to a "shadow zone". These northern areas are populated with sponges and carnivorous species such as hydroids. Southern sections of the reef, which are covered by the Amazon's plume only three months a year on average, are more populated with diverse coral-centric life, where photosynthesis can occur. It is believed that single-celled organisms are central to the reef's ecosystem, providing the main source of nutrients to sponges, corals and other inhabitants. Fabiano Thompson, along with other researchers on the reef, describe the system as a new class of biome.