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Fishery Bulletin 102(1) 



Figure 4 



Orbital rete of a bigeye thresher shark, showing the highly developed arterial network. The 

 rete was injected with latex so that the arterial structure (72 mm by 49 mm by 19 mm) could be 

 photographed. The structure of the rete and its position in the orbital sinus suggest that it may 

 be a heat exchanging vascular plexus. Retention of metabolic heat in the eyes and brain would 

 buffer these sensitive organs from the large ambient temperature swings that occur as a result 

 of the bigeye thresher shark's diel vertical migrations. A smaller but similar structure is found 

 in A. pelagicus but not in A. vulpinus. 



exist. In satellite or acoustic tracks, diel vertical migra- 

 tion was not observed for white sharks (Carcharodon car- 

 charias; Carey et al., 1982; Goldman and Anderson, 1999; 

 Boustany et al., 2002), salmon sharks (Lamna ditropis; 

 Block et al. 3 ), shortfin mako (Isurits oxyrhynchus; Carey, 

 1982b; Holts and Bedford, 1993), blue (Prionace glauca, 

 Carey, 1982b; Carey and Scharold, 1990), sixgill (Hexan- 

 chus griseus; Carey and Clark, 1995), tiger (Galeocerdo 

 cuvier; Tricas et al., 1981; Holland et al., 1999), Pacific 

 angel (Squatina californica; Standora and Nelson, 1977), 

 whale [Rhincodon typus; Gunn et al., 1999), or scalloped 

 hammerhead sharks (Sphyrna lewini; Klimley, 1993). 



Diel vertical migration has been observed in the sword- 

 fish (Xiphias gladius; Carey and Robison, 1981; Carey 4 ), 

 the megamouth shark (Megachasma pelagios; Nelson et 

 al., 1997), and the school shark iGaleorhinus ga/eus; West 



Block, B.A., K.G.Goldman, and J. A. Musick. 1999. Unpubl. 

 data. Hopkins Marine Station of Stanford University. 120 

 Oceanview Boulevard, Pacific Grove, CA 93950. 

 Carey, R G. 1990. Further acoustic telemetry observations of 

 swordfish. In Planning the future of billfishes; proceedings of 

 the second international billfish symposium, 1-5 August 1988, 

 Kailua-Kona, Hawaii (R. H. Stroud, ed.), p. 103-122. National 

 Coalition for Marine Conservation, 3 North King St., Leesburg, 

 VA 20176. 



and Stevens, 2001). Carey and Robison ( 1981) and Carey 4 

 studied swordfish in both the Pacific and Atlantic Oceans, 

 acoustically tracking fish that moved from the surface at 

 night to over 600 m during day. A megamouth shark showed 

 a strong diel vertical migration when tracked acoustically 

 off southern California (Nelson et al., 1997) with shallow 

 nighttime and deep daytime distribution in a vertical range 

 of 20 m to 160 m. West and Stevens (2001) studied school 

 sharks in southern Australia using archival tags and noted 

 that they ascended in the water column at night. 



The ambient temperature at the modal day- and night- 

 time depths of the two bigeye thresher sharks differed by 

 15° to 16°C, requiring them to be eurythermal. The sharks 

 spent most of the nighttime in shallow waters warmer 

 than 20°C and commonly spent 8 or more hours during 

 the daytime in deep waters cooler than 10°C. The coolest 

 waters occupied had temperatures between 4°C and 6°C. 

 The bigeye thresher sharks tracked in our study spent a 

 higher proportion of their time in waters below 10°C than 

 did white sharks (Carey et al., 1982; Boustany et al., 2002) 

 and mako sharks (Carey and Scharold, 1990; Klimley et 

 al.,2002). 



The presence of a rete mirabile in the cranial region 

 may indicate a mechanism for heat conservation. Heat 

 conservation in the brain and eyes would enable the big- 



