NOTE Weng and Block: Diel vertical migration in Alopias superaliosus 



225 



10 15 20 25 



Temperature (C) 

 10 15 20 25 



5 10 15 20 25 30 



50 



100-1 



-jT 150 

 n 



Q_ 



q 200 -I 

 250 



300 -I c 



350 



Figure 3 



Temperature-depth profiles characterizing the thermal habitat of two bigeye thresher sharks. (Al Profiles of the 



Gulf of Mexico taken with a bathythermograph ( ) sampling at 1-m intervals deployed from the fishing vessel 



after the tagging event, and by the pop-up satellite archival tag (O) during the first day it was attached to the 

 bigeye thresher shark. The two profiles are similar, indicating that the pop-up satellite archival tag is capable 

 of characterizing thermal habitat. (B) Average temperature-depth profile for the 60-day track of the bigeye 

 thresher shark in the Gulf of Mexico, showing a mixed layer shallower than 50 m and a thermocline extending 

 beyond 400 m where waters were 10°C. The curve was fitted by using a LOWESS function and error bars are 

 1 SD, because 1 SE bars are invisible at this scale. (C) Average temperature-depth profile for the 27-day track of 

 the bigeye thresher shark in the Hawaiian Archipelago, showing a shallow mixed layer a thermocline extending 

 to approximately 600 m where waters were 6°C. Curve was fitted by using a LOWESS function and error bars are 

 1 SD, because 1 SE bars are invisible at this scale. 



the shark spent most of the daytime below the maximum 

 gradient of the thermocline where temperatures were ap- 

 proximately 10°C. On 25 April and 25 May 2000 the shark 

 spent two hours of the day in waters between 4°C and 6°C. 

 The Hawaii shark showed a similar diel vertical migration, 

 with a lesser contrast between day and night (Fig. 2, C and 

 D). The shark's modal nighttime depth was between 10 m 

 and 50 m, whereas its modal daytime depth was between 

 400 m and 500 m (Fig. 2C). The temperature-depth profile 

 for the Hawaii shark ( Fig. 3C ) indicated that it spent night- 

 time above the thermocline and daytime below it. 



The bigeye thresher shark possesses a large arterial 

 plexus between the posterior part of the eye and the wall 

 of the orbital sinus, which appears to be a rete mirabile 

 (Fig. 4). The orbital rete is bathed in venous blood from the 

 orbital sinus and its anterior surface is contoured to the 

 posterior surface of the eye. The sources of venous input 

 to the orbital sinus remain unknown but are most likely 

 within the surrounding extraocular muscles, which are 

 large and comprise numerous aerobic muscle fiber types, 

 and the retina. The rete shown in Figure 4 measures 72 

 mm by 49 mm by 19 mm. A reduced structure of similar 

 form is also found in the pelagic thresher shark, but is not 

 present in the common thresher. The orbital rete of the 

 bigeye and pelagic threshers is larger in absolute size and 



occupies a greater cross sectional proportion of the orbital 

 sinus than the lamnid orbital rete noted by Burne (1924). 

 The arterial vessels form a finer and more orderly mesh- 

 work than those in the lamnid sharks (Block and Carey, 

 1985; Tubbesing and Block, 2000) and appear similar in 

 physical structure to the mammalian carotid rete used for 

 brain cooling (Baker, 1982). 



Discussion 



Observations of the biological features of the bigeye 

 thresher shark are rare and our knowledge of the species 

 is based primarily on incidental catches in fisheries. Using 

 pop-up satellite archival tags we were able to record behav- 

 ior for a total of 87 days, and for individual periods up to 60 

 days without recapturing or following the study animals. 

 We observed a pronounced diel alternation between warm 

 shallow waters and cool deep waters and a rete mirabile 

 that may confer physiological benefits during deep dives by 

 stabilizing brain and eye temperatures. 



The depth data obtained for the bigeye thresher shark 

 shows a striking pattern of diel vertical migration. The big- 

 eye thresher shark's vertical movement pattern is distinct 

 from those of most other sharks for which observations 



