

condition is widespread among many unrelated catfish lineages (Lundberg and 

 Baskin 1969); thus, its occurrence in ageneiosids is convergent. 



Coloration of the caudal fin is of considerable use in identifying species of 

 Ageneiosus, in spite of extensive intraspecific variation in body and fin coloration. 

 Typically, the distal margin of the tail is black in^. brevifilh and the large bodied, 

 fork-tailed species (A.pardalis,A. ucayalensis,A. valenciennesi, andy4. vittatus). 

 That is generally the extent of pigmentation in A. brevifilis, but in the other species 

 there is usually a crescentic spot at the base of the upper lobe, and, occasionally, a 

 similar spot at the base of the lower lobe. In A. vittatus, both spots are always 

 prominent, and serve to identify this species. A separate apomorphic state is found 

 in A. atronasus, in which the tail has a distinctive stripe in each lobe. 



Sexual Dimorphism and Reproductive Biology 



Undoubtedly the most unusual and least understood aspect of ageneiosid 

 morphology and ecology concerns their unique reproduction. Together with the 

 Auchenipteridae, these catfishes exhibit, during their reproductive periods, 

 pronounced sexual dimorphism of the head, barbels, and dorsal and anal fins. The 

 seasonal development of nuptial structures in males is correlated with a unique 

 mechanism of internal fertilization in the various taxa involved, discussed below in 

 greater detail. A few early authors made cursory remarks about sexual dimorphism, 

 but it is clear from the hterature that many mvestigators failed to recognize or 

 understand the significance of these dimorphic characters. Consequently, some 

 descriptions were based on diagnoses involving differences in the characters listed 

 above, and thus resulted in species names that are synonyms of previously described 

 taxa. Such descriptions probably resulted from inadequate series of sexually 

 dimorphic specimens available to the various authors. In the following discussion, a 

 brief review of references to sexual dimorphism is presented first, followed by 



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