Ecology, the Amazon barrier, and speciation in western Atlantic Halichoeres (Labridae)

Recent estimates indicate that about 12% of the Brazilian reef fish species are endemic. Most ichthyologists agree that this endemism is generated by the barrier formed by the freshwater and sediment discharge of large rivers in northeastern South America (mainly the Amazon, Orinoco and their tributaries). However, little is known about the dynamics of this barrier. Recent studies have shown that the barrier can be crossed through deep sponge bottoms on the outer continental shelf off northeastern South America. Moreover, the recent discovery of species regarded as Brazilian endemics in the extreme southern Caribbean is showing that the Amazon barrier is weaker than previously thought.; Two hypotheses were generated to explain the effects of the Amazon barrier on western Atlantic reef fish: (1) The Amazon is a strong barrier to dispersal; but sea-level fluctuations influence the effectiveness of the barrier; and may greatly affect fish species diversity in the western tropical Atlantic. (2) Larval exchange between Brazil and the Caribbean is small but constant; contrasting selection pressures in divergent environments (continental Brazil versus insular Caribbean) may be the central force driving speciation.; To test these hypotheses, a total of 632 samples of seven species of Halichoeres were obtained from 13 locations across the western Atlantic. Although congruent population structure was expected in these species because of their similar dispersal potential and exposure to the Amazon barrier, varying degrees of genetic separation were observed.; The observed distribution of genetic partitions indicates that gene flow exists between environmentally similar, but geographically distant locations. Deep genetic divergences coincided with environmental differences between locations, regardless of distance or dispersal barriers. The pattern drawn from our study indicates that divergent environmental conditions can be as important as (or sometimes more important than) vast distances of unsuitable habitat (either open ocean waters or the area influenced by the Amazon) in producing genetic partitions; and that benthic-stage habitat preferences may be strongly influencing the biogeography of western Atlantic reef fishes.