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named after its discoverer, Johannes Miiller (the history of studies of the ESA were 

 summarized by Tavolga 1962). There are ligaments extending between the anterior 

 face of the expanded plate and the posterior occipital region, which, when 

 contracted, cause oscillations in the volume of the swimbladder. The ESA is 

 involved in sound production and has been well studied in ariids (Tavolga 1962), but 

 relatively unstudied in the remaining taxa. The presence of an ESA has been used 

 widely as a defining character for the neotropical doradoids, and some authors have 

 used it at a wider level of universality to suggest interfamilial relationships (Chardon 

 1968, Curran 1989). , . ;. 



Taxa with an ESA usually have large swimbladders. Because several 

 ageneiosids have encapsulated swimbladders, some investigators have erroneously 

 assumed that an ESA was absent in ageneiosids (Tavolga 1962, Howes 1983). 

 However, all ageneiosids do have an ESA although its structure varies between 

 those species with large swimbladders and those with encapsulated swimbladders. 

 In all species with large, unencapsulated swimbladders, the Mullerian ramus is a 

 relatively large, discoidal plate closely apposed anterodorsally to the swimbladder 

 peritoneal tunica (Figs. 17, 18a-c). In species with an encapsulated swimbladder, 

 however, the Mullerian ramus is relatively reduced in size (Fig. 18d), although it 

 maintains a ligamentous connection with the posteroventral margin of the nuchal 

 shield (Royero 1987). In those species with enclosed swimbladders, there is a large 

 foramen in the anterodorsal wall of the capsule where the Mullerian ramus contacts 

 the soft tissue of the swimbladder. 



Including six for the anterior elements that are fused to form the Weberian 

 complex, and one for the fused preural centra, counts of total vertebrae in 

 ageneiosids range from 39 to 58, with most species tending toward the upper end of 

 this range. Howes (1983:36) noted that few comparative data exist for vertebral 

 counts in siluroids, although he indicated a "mean siluroid count" of about 40-45; 



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