Unexpected richness of reef corals near the southern Amazon River mouth

Abstract

Manoel Luis Reefs (00° 52’ S, 44° 15’W),located 180 km north of Sao Luis, offMaranhao State, represent the northernmostoutpost of the Brazilian reef coral fauna, andthe closest well-developed reefs to theAmazon River mouth (Figure 1). In July1998, during a reef fish survey along theBrazilian coast, we found a surprising richness of corals and other reef associatedorganisms in the area, including nine speciesof reef corals and two milleporans, six ofwhich were not previously reported for theNorthern coast of Brazil (Belem et al. 1986,Veron 1996). Despite its proximity to the Amazon Rivermouth, the West flowing Equatorial Currentprovides the region with clear and salinewater. The tidal range is about 6.5 m andvisibility ranged between 20 and 30 m duringour 12 day visit. The reef is approximately y-shaped, with about 10 km in its SE-NEaxis (Figure 1). It is constituted by approximately circular pinnacles, ranging from 50 to 300 m in diameter.The pinnacles rise from 25–30 m depth to near the surface, the tops of some being exposed on the lowesttides. Superimposed coralline algal crusts constitute the framework of the reef, which is sparsely covered withcolonies of Agaricia fragilis(Dana), Favia gravidaVerrill, Madracis decactis(Lyman), Meandrina brazilien-sisMilne Edwards & Haime, Montastraea cavernosa(Linne), Mussismilia hispida(Verrill), Poritescf. ast-reoidesLamarck, Scolymia wellsiLaborel, Siderastrea stellataVerrill, Millepora alcicornis(Linne) and abizarre sub-massive form of Millepora,possibly a new species. Fifty-five percent of the reef coral fauna surveyed iscomposed of species widely distributed in the Western Atlantic and 45 % of Brazilian endemics (Belem et al.1986, Hetzel and Castro 1994). At the time of our visit all coral species were bleached. The Manoel Luis Reefs region, due to its proximity to the Amazon River, may play a major role in geneflow between northern and southern Western Atlantic reef fauna. The presence and great abundance ofCaribbean reef organisms which do not occur along the eastern coast of South America, such as the purple reeffish Chromis scotti,provide additional evidence that the Manoel Luis Reefs may be one of the main faunalstepping stones between the Caribbean and the Brazilian coast (Collette & Rutzler 1977).