FISHERY BULLETIN: VOL. 79, NO. 4 



have previously used precaudal length (Fitch and 

 Craig 1964), total length (Bigelow and Schroeder 

 1948), and fork length (Stillwell and Casey 1976). 



The prominent horizontal head grooves (Fig- 

 ures 3, 4) that are characteristic of A. super- 

 ciliosus are present in all specimens we examined, 

 but are better developed in the large subadults 

 and adults than in the two fetuses taken from 

 SHG-A2. The grooves were not indicated in a 130 

 cm TL, free-living specimen figured by Bigelow 

 and Schroeder (1948); but we suspect that they 

 were overlooked on this shark although we were 

 not able to examine it. Fitch and Craig (1964) 

 first called attention to these grooves in A . super- 

 ciliosus and noted that similar grooves are also 

 found in teleosts, in the swift, mesopelagic louvars 

 (Louvarus) and escolars (Lepidocybium). They 

 speculated that the grooves might aid in hydro- 

 dynamic flow, thus enabling the bigeye thresher 

 to maneuver more rapidly. Head grooves are 

 absent or poorly developed in other species 

 of threshers. 



Another characteristic of the bigeye thresher, at 

 least at sizes above 130 cm TL, are the huge, 

 vertically elongated, fleshy orbits, which are ex- 

 panded onto the dorsal surface of the head and 

 provide the shark with a dorsal, binocular visual 

 field (Figures 4, 5). The eyes, head grooves, and 

 bulbous, elongated snout of A. superciliosus give 

 its head a unique, upward-looking, crested 

 or helmeted appearance. The eyelids (Figure 5) 

 apparently change shape with growth, as our two 

 fetuses, and a 130 cm specimen in Bigelow 

 and Schroeder (1948) have relatively enormous, 

 circular lids without the anteroposterior shorten- 

 ing seen in larger individuals such as the 161 cm 

 immature female pictured by Bass et al. (1975). 

 This change in lid shape is also seen though to a 

 lesser degree in A. pelagicus, in which fetuses and 

 small, free-living specimens have circular eyelids 

 and adults more vertically oval lids (Compagno 

 unpubl. obs.). 



COLOR 



The bigeye thresher is often described as gray 

 (Cadenat 1956; Garrick and Schultz 1963; Bass et 

 al. 1975). Bigelow and Schroeder ( 1948) stated that 

 the bigeye thresher is "Dark mouse gray above 

 and hardly paler below...," but we suggest that 

 this coloration is true for preserved material and 

 not living or freshly killed specimens. Nakamura 

 ( 1935) noted that a freshly killed bigeye thresher 



Figure 5. — Lateral view of the left eye of Alopias superciliosus 

 (SHG-A7) showing the keyhole shape which may be an adapta- 

 tion for increasing the dorsal binocular fields. The vertical 

 distance between upper and lower eyelid is 101.5 mm. Photo: 

 S. Spielman. 



is purple above, and we observed a violet to 

 purplish cast above fading to creamy white below 

 on the body of the 356 cm TL Miami specimen 

 (SHG-A2). S. Kato and L Nakamura® stated 

 that fresh Central Pacific and eastern Atlantic 

 bigeye threshers are purple-brown or gray-browoi 

 dorsally, white, grayish or whitish brown below. 

 In the Miami and Central Pacific specimens a 

 metallic silver or silver blue-green sheen was 

 present on the sides at the level of the gills and on 

 the flanks, as in A. pelagicus (Bass et al. 1975) 

 and A. vulpinus (Compagno unpubl. data). The 

 ventral surface of the paired fins and the caudal 

 fin is oulined in dark gray. 



VERTEBRAE 



Vertebral counts have been used as an impor- 

 tant character in teleost systematics for many 



''Susumu Kato (see footnote 3) and Izumi Nakamura (see 

 footnote 4), pers. commun. to L. J. V. Compagno, 1978. 



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