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Reef Building Corals

The corals that form reefs (i.e. hermatypic corals) cannot grow where there is sediment in the water, because silt blocks the sunlight that the corals' symbiotic algae need to photosynthesize energy (Cox and Moore 2000).  Reef-building corals also have a hard time growing on muddy bottoms and in waters of reduced salinity.  For these reasons, coral reefs are almost entirely absent from a 2800 kilometer stretch of the Amazon Barrier, from the Orinoco delta to Fortaleza, Brazil (Briggs 1974; Veron 1995), and the barrier seems prevent many coral species from dispersing through it.

The coast of Brazil east of the Amazon Barrier has many species of corals that are not found in the Caribbean and vice-versa. For example, of the 20 species and 13 genera of reef-building corals occurring between Alcantara (2ºS) and Rio de Janeiro, 8 species (40%) and one genus (8% - Mussismilia) are endemic (Maida and Padovanio Ferreira 1997). 

The following species of corals are endemic to the reefs of Brazil east of the Amazon Barrier:

Scleractinian corals:
Favia gravida
Favia leptophylla
Mussismilia brasiliensis
Mussismilia harti
Mussismilia hispida
Siderastrea stellata

Millepora braziliensis
Millepora nitida

Sources:  Maida and Padovanio Ferreira (1997); Nunes et al. (2011)

Ten other Scleractinian coral species and two other Hydrocoral species are also found on Brazilan reefs with the above endemics, however these latter 12 corals also occur in the Caribbean (Maida and Padovanio Ferreira 1997). 

Only one species of reef dwelling-soft coral (Neopongodes atlantica) is found on the reefs east of the Amazon Sea Barrier, and it also is endemic to the east coast of Brazil (Maida and Padovanio Ferreira 1997).

Along the east coast of Brazil, the distribution of coral species is further divided by another smaller barrier of freshwater and sediment entering the sea.  This freshwater originates from the San Francisco River, which drains the state of Bahía, runs along the borders of the states of Sergipe and Alagoas, and eventually enters the Atlantic Ocean at approximately 11º south latitude.  North and south of this entry point, there are differences in the coral species found, yet the numbers of coral species in both areas is approximately the same (Belém et al. 1986; Veron 1995).

Shallow Water Reef Fishes

Almost all reef fishes have larvae that spend some time drifting in the sea as plankton.  However, the time spent in the planktonic stage varies depending on mode of spawning (Floeter and Gasparini 2000).  Pelagic and balistid-type spawners (mainly balistids, monacanthids and tetradontids) have a longer larval period (20-50 days) compared to that of normal demersal-spawners (i.e. fish that spawn on the sea bottom), which is 15-25 days (Thresher 1991; Floeter and Gasparini 2000).  The reduced larval period of demersal-spawners leads to the prediction that they have a more difficult time dispersing across the Amazon Barrier to colonize reefs in the Caribbean.

This prediction turns out to be correct.  Forty-five (12.7%) of the 353 reef fish species found on the Brazilian shelf east of the Amazon Sea Barrier are endemic to the region (Rocha 2003).  Three-fourths of these endemic reef fishes are benthic demersal-spawners, even though only 30% of the reef fishes found in this area are benthic demersal-spawners (Floeter and Gasparini 2000).

Rocky Shore Gastropods

 (Vermeij 1978) states that many open-surface rocky-shore gastropods are hindered by the barrier, but gives no further details.

Effectiveness of the Amazon Barrier

As we have seen, the Amazon Barrier resembles other biogeographic barriers in that it is imperfect, hindering the dispersal of only some organisms.  In his analysis of reef fishes, Rocha (2003) pointed out that, even for the fishes whose dispersal it hinders, this barrier of freshwater and sediment is not as effective as a land barrier. 

For example, he noted that "reef fish species pairs separated by the Amazon, which started discharging large amounts of sediment and freshwater in the Atlantic about 10 million years ago, look more similar to each other than pairs separated by the Isthmus of Panama, which closed about 3 million years ago."  This fact suggests that over long periods of time, there has been some intermittent genetic contact between the fish pairs on each side of the Amazon Barrier. 

Such contact could occur, for example, when sea levels rise or rainfall decreases, changing the volume and distribution of the Amazon's freshwater plume as it enters the Atlantic, and making it easier for occasional adults or larva of the endemic fish species to cross the Amazon Barrier (Robertson et al. 2006; Floeter et al. 2008).  Thus, a land barrier like the Isthmus of Panama seems to promote speciation faster than a less effective barrier like the Amazon Barrier (Rocha 2003).

In this review, we have focused solely on the outflow of the Amazon and Orinoco Rivers into the Atlantic Ocean, and analyzed the way this massive volume of freshwater and sediment acts as a barrier to the dispersal of marine organisms.  However, long before it reaches the sea, the Amazon River also acts as a barrier to the dispersal of certain terrestrial organisms living in the tropical rainforest (eg. Hayes and Sewlal 2004).  In a future review, we will document this additional aspect of the Amazon Barrier.


Allison MA, Lee MT  (2004)  Sediment exchange between Amazon mudbanks and shore-fringing mangroves in French Guiana.  Marine Geology 208: 169-190

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Information about this Review

This review is also available in the following languages:


The author is:  Dr. Paul D. Haemig (PhD in Animal Ecology)

The photograph at the top of the page was taken by Carin Araujo of Venezuela and shows the Brown Pelican, a nearshore marine bird whose breeding range appears to be limited by the Amazon Barrier (for details see Page 1).

The proper citation is:

Haemig PD  2012   The Amazon Barrier. ECOLOGY.INFO 29

If you are aware of any important scientific publications about the Amazon Barrier that were omitted from this review, or have other suggestions for improving it, please contact the author at the following e-mail address: 

director{at} ecology.info

© Copyright 2005-2012 Ecology Online Sweden.  All rights reserved.


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