FISHERY BULLETIN VOL. 79. NO. 4 



Figure 3. — Three-quarter lateral view of the head of a 159 cm TL immature male Alopias superciliosus (SHG-A4) showing the head 

 grooves and massive "crest" composed of the epaxial musculature. The characteristic large eyes, bulbous snout and flattened 

 interorbital space can also be seen. The crest and grooves are even more pronounced in mature bigeye threshers. Photo: S. Gruber 



above the branchial region, bulbous snout (more 

 tapering in other threshers), nearly flat inter- 

 orbital space (highly arched in other species), 

 huge eyes with lids shaped like an inverted pear or 

 keyhole (in individuals >1,300 mm TL) that 

 extend onto the dorsal surface of the head (Figure 

 4), and a distinct indentation or step in the profile 

 of the forehead at the origin of the head grooves 

 that gives the head a helmeted or crested appear- 

 ance (other thresher species have the forehead 

 convex or flat but not indented; the indentation is 

 less marked in fetal bigeye threshers). In addition, 

 the bigeye thresher has much larger and less 

 numerous teeth than other threshers, e.g., 24/24 

 rows or less (32/29 or more in other species). 

 Tooth row groups represented in the adult denti- 

 tion of the bigeye thresher include anterior 

 and lateroposterior teeth only, without the sym- 

 physial or intermediate teeth found in other 

 species. The bigeye thresher has fewer vertebrae, 



278-308, than other threshers, which have 

 339-472 (Springer and Garrick 1964; Bass et al., 

 1975; unpublished data on all three species). In the 

 monospondylous precaudal region of the vertebral 

 column, the vertebral calcification patterns of the 

 bigeye thresher are simpler than in other species, 

 with fewer radii in the intermedialia and no fusion 

 of their bases (extensively fused in A. pelagicus). 

 The first dorsal fin of the bigeye thresher is 

 positioned more posteriorly on the back than in 

 other species of threshers, with the midpoint of its 

 base much closer to the pelvic fin bases than to the 

 pectoral bases, and with its free rear tip over 

 or slightly anterior to the pelvic origins. In 

 A. pelagicus and A. uulpinus the midpoint of the 

 first dorsal base is usually closer to the pectoral fin 

 bases than to the pelvic bases (occasionally equi- 

 distant between pectoral and pelvic bases), and 

 the free rear tip of the first dorsal is far anterior to 

 the pelvic origins. 



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