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thus requiring negative buoyancy. Encapsulation of the swimbladder in ageneiosids 

 and Hypophthalmus is difficult to account for, given their semipelagic swimming 

 habits (Howes 1983); possibly, they maintain positive bouyancy by a high hpid 

 content in the body, but the selective factors responsible for reduction of the 

 swimbladder remain unknown. Encapsulation has undoubtedly occurred 

 independently in many lineages, as evidenced by the widespread occurrence among 

 unrelated taxa (Alexander 1964, Howes 1983, Stewart 1986). Nevertheless, among 

 doradoids, I know of no other species that have encapsulated swimbladders; thus, 

 this character is uniquely derived within a restricted clade of the Ageneiosidae. This 

 is supported by morphological differences in the encapsulation process; in other 

 catfishes, the capsules are generally formed from a combination of elements that 

 usually include portions of the posttemporal and/or transverse processes of the 

 complex centra (Alexander 1964; Howes 1983). In ageneiosids, the transverse 

 process of the fourth vertebra is modified for the ESA, and the parapophyses of the 

 fifth and sixth centra are fused to the posterior process of the epioccipital; the 

 capsule of the swimbladder is derived from the ventral surface of the complex centra 

 and, possibly, in situ ossification of the peritoneal tunica {sensu Rosen and 

 Greenwood 1970). Howes (1983) placed ageneiosids on the same branch with 

 loricarioids in a hypothesized cladogram of several families, based on the shared 

 condition of an encapsulated swimbladder, despite his discussion of the differences 

 between these taxa. The situation involving swimbladder encapsulation in 

 ageneiosids is analogous to that described by Stewart (1986), in which an 

 intrafamiUal clade of pimelodids (the so-called Callophysus group) has an unusual, 

 partially encapsulated swimbladder, but modified much differently than in any other 

 catfishes. 



; The development of posterior swimbladder caecae in some species may also 



represent a derived state. Among sister taxa, neither Trachefyopterus or 



