Summary
The Amazon river system is characterized by its great size (it drains an area of about 6.5 million km2), its great depth (to 90 m or more in some places), the flat topography of its drainage basin, the annual cycles of high and low water periods, and the geological structure of the drainage area. The central Amazonian ecosystem is complex and the types of habitat within it are numerous, with broad overlap and intergradation of categories. An especially important factor in the aquatic habitat is water type ("white", "black" or "clear"), a characteristic which is related to the geological character and the flora of the drainage area. Cyclical changes in water level and associated fluctuations in the availability of oxygen are two factors which exert considerable influence over the biology of central Amazonian fishes. The collection sites of Alpha Helix Phase IV are described briefly. The systematics and evolution of most Amazonian fish groups are poorly known; the inability to identify all fishes examined and lack of well-tested evolutionary hypotheses of relationship on which to base comparisons pose problems for the comparative biologist. A brief survey of general aspects of phylogeny and of the natural history of fishes examined by Phase IV is presented, including, when possible, information about habitat preferences, food and other aspects of the biology of the fishes.