

of Fig. 2. Another fish, collected with this individual and exhibiting virtually the 

 same pigmentation, was kept alive in an aquarium for several days, during which 

 time it became darkly mottled over the entire head and body (upper left of Fig. 2). 

 Additional specimens of this species that were taken from relatively clear natural 

 waters, or maintained in aquaria, had a similar mottled pattern (cf. photographs in 

 Kopke 1986 and Burgess 1989). , > •. - 



I believe that the tremendous variation in coloration exhibited by v4. viVtoftts 

 and several other species is primarily attributable to variation in the hydrological 

 composition of the water from which specimens are collected. Those taken from 

 relatively turbid waters, corresponding to the Whitewater classification of various 

 authors, are much lighter in coloration than specimens from clearer rivers. Some 

 species that are found in different water types exhibit color variation between 

 populations; for example, specimens of ^. ucayalemis in the Rio Negro, Rfo 

 Essequibo, and other nutrient-poor, clearwater rivers draining the Guiana shield are 

 overall much darker than conspecifics from Whitewater rivers in other parts of the 

 species' range (in {siCt,A.guianensis was separated from the nominate form almost 

 solely on the difference in color). It is worth noting thsit A. pofystictus, a species 

 confined to the Rio Negro, is one of the darkest and most distinctively colored. In 

 addition to environmentally induced color changes, it may be possible that 

 individuals undergo behaviorally mediated changes. Some auchenipterids are also 

 capable of similar dramatic color changes (D. C. Taphorn, personal 

 communication), but, to the best of my knowledge, nothing has previously been 

 published documenting this phenomenon. . , 



In spite of the extensive intra- and interspecific variation in coloration, 

 certain pigmentation patterns are characteristic of individual species and serve as 

 important identifying features. In the species accounts, general descriptions of 

 coloration are based on the typical condition found in preserved material. While 



