'■ •r>?rF'— 't--^' vfjjr.y IK- -;vr^.^'W?7Xf'^ 



is also known to harbor a large number of endemic fishes. The relative 

 plesiomorphy of A. piperatus suggests that it may have speciated in the area of the 

 Guiana Shield as a result of vicariance associated with formation of the shield. 



Ageneiosus pofystictus is confined to the Rio Negro. This huge drainage is 

 ecologically and ichthyofaunally distinct, and has a number of other endemic fishes 

 (Goulding et al. 1988). Because the systematics of its fauna has not been vigorously 

 studied, there are currently no zoogeographical hypotheses concerning observed 

 distribution patterns of fishes in the Rio Negro. Presumably, some of the endemism 

 is related to the formation of the Guiana Shield, but there are probably many 

 ecological factors involved in some of the diversification of fishes in this region. 



The remaining species of ageneiosids have relatively widespread 

 distributions. There are no species in the Orinoco basin that are not also found in 

 the Amazon basin. On the other hand, three species are found only in the Amazon 

 basin; v4. brevis,A. atronasus, and A. n. sp. Ageneiosus atronasus andyl. brevis seem 

 to be limited primarily to the upper half of the Amazon basin (as is >1. vittatus, 

 although it also occurs in the Orinoco), whereas ^4. n. sp. appears to be limited to 

 the Rio Negro, lower Amazon, and Rio Amapd. Movement of some species (A. 

 ucqyalensis,A. brevifilis, possibly yl. vittatus) between the Orinoco and Amazon 

 basins almost certainly occurs via the Rio Casiquiare connection. 



Vari (1989a) found that only two species of Curimata do not have 

 distributions overlapping to some degree those of other species of the genus; many 

 are largely sympatric. The situation involving the distributions of most ageneiosid 

 species is very similar to that in Curimata, and, since both groups are principally 

 large river fishes, some parallels are probably the result of similar historical events 

 that led to speciation. Vari (1989a) suggested that the pattern he found in Curimata 

 was best explained by allopatric speciation followed by dispersal and secondary 

 sympatry. Presumably, isolation of clades within hydrographic systems occurred 



