NOTE Weng and Block: Diel vertical migration in Alopias superaliosus 



227 



eye thresher shark to prolong its foraging time beneath 

 the thermocline, as we observed for both of the sharks 

 tagged in our study. The retina and brain are extremely 

 temperature sensitive in most vertebrates and the large 

 changes in depth and temperature recorded would impose 

 significant effects on the biochemical processes occurring in 

 these tissues (Block and Carey, 1985; Block, 1994). Delayed 

 responses to retinal stimulation can be caused by cooling, 

 whereas increased noise and random firing of neurons can 

 be caused by warming — both responses having adverse 

 affects on sensory function (Konishi and Hickman, 1964; 

 Friedlander et al., 1976; Prosser and Nelson, 1981). 



Anatomical and physiological adaptations to warm the 

 brain and eyes have evolved independently in divergent 

 pelagic fish lineages, including the lamnid sharks (Block 

 and Carey, 1985), billfishes of the Xiphiidae and Istiophori- 

 dae (Carey, 1982a; Block. 1983) and some scombrid fishes 

 (Linthicum and Carey, 1972). A cranial rete mirabile also 

 has been identified in mobulids (Schweitzer and Notarbar- 

 tolo di Sciara, 1986) and is thought to be a heat exchanger 

 (Alexander, 1995, 1996). Although it is premature to sug- 

 gest that the orbital rete of the bigeye thresher shark is a 

 heat exchanger without direct evidence of elevated tissue 

 temperatures in the brain and eyes, the structure is larger 

 than the rete mirabile of lamnid sharks, for which elevat- 

 ed brain and eye temperatures have been demonstrated 

 (Block and Carey, 1985). The anatomical arrangement of 

 an arterial plexus in an orbital sinus is correlated with 

 heat conservation strategies in other vertebrates (Baker, 

 1982). The phylogenetic relationships of the alopiid and 

 lamnid sharks (Compagno, 1990; Naylor et al., 1997) sug- 

 gest that endothermic traits evolved independently in the 

 two families. 



This note presents new information on the depth and 

 ambient temperature preferences of the bigeye thresher 

 shark based on observations of two individuals, as well as 

 the anatomy of the orbital rete mirabile, which appears to 

 function as a vascular heat exchanger. Behavior of many 

 organisms varies with ontogeny, season and location; 

 therefore the present study should be considered as only 

 the beginning of an understanding of the bigeye thresher 

 shark's physical habitat preferences and adaptations to 

 temperature change. Further studies on individuals of 

 different sizes and in different regions will enhance our 

 understanding of the behavior, and morphological and 

 physiological adaptations, of the bigeye thresher shark to 

 variations in temperature. 



Acknowledgments 



This research was supported by grants from the National 

 Marine Fisheries Service, the National Fish and Wildlife 

 Federation and the Packard Foundation. The authors wish 

 to thank Captain David Price and crew of the FV Allison, 

 and Captain John Bagwell and crew of the FY Silky. Shana 

 Beemer provided scientific assistance on the cruise and 

 Captain McGrew Rice assisted in tagging and releasing the 

 Gulf of Mexico shark. This research was conducted under 

 Scientific Research Permit TUNA-SRP-2000-002, issued 



by the Office of Sustainable Fisheries, National Marine 

 Fisheries Service, Silver Spring, MD 20910. 



Literature cited 



Alexander, R. L. 



1995. Evidence of counter-current heat exchanger in the 

 ray, Mobu la tarapacana (Chondrichthyes: Elasmobranchii: 

 Batoidea: Myliobatiformes). J. Zool. (Lond) 237:377-384. 



1996. Evidence of brain-warming in the mobulid rays, 

 Mobula tarapacana and Manta birostris (Chondrichthyes: 

 Elasmobranchii: Batoidea: Myliobatiformes). Zool. J. Linn. 

 Soc. 118:151-164. 



Baker, M. A. 



1982. Brain cooling in endotherms in heat and exercise. 

 Annu. Rev. Physiol. 44:85-96. 



Bigelow, H. B.. and W. C. Schroeder. 



1948. Sharks. In Fishes of the western North Atlantic, part 



one (A. Parr and Y. Olsen, eds), p. 59-546. Sears Found. 



Mar. Res., Yale Univ., Mem. 1. 

 Block, B. A. 



1983. Brain heaters in the billfish. Am. Zool. 23:936. 

 1991. Endothermy in fish: thermogenesis ecology and 



evolution. In Biochemistry and molecular biology of 

 fishes. Volume 1: Phylogenetic and biochemical perspec- 

 tives (P. Hochachka and T. Mommsen, eds.), p. 269-311. 

 Elsevier, Amsterdam. 

 1994. Thermogenesis in muscle. Annu. Rev. Physiol. 56: 

 535-577 

 Block, B. A., and F. G. Carey. 



1985. Warm brain and eye temperatures in sharks. J. 

 Comp. Physiol. B. Biochem. Syst. Environ. Physiol. 156: 

 229-236. 

 Bone, Q., and A. D. Chubb. 



1983. The retial system of the locomotor muscles in the 

 thresher shark tAlopias uulpinus). J. Mar. Biol. Assoc. 

 U.K. 63:239-242. 

 Boustany, A. M., S. F. Davis, P. Pyle, S. D. Anderson, 

 B. J. Le Boeuf, and B. A. Block. 



2002. Satellite tagging: expanded niche for white sharks. 

 Nature 415:35-36. 

 Buencuerpo, V., S. Rios, and J. Moron. 



1998. Pelagic sharks associated with the swordfish, Xiphias 

 gladius, fishery in the eastern North Atlantic Ocean and 

 the strait of Gibraltar. Fish. Bull. 96:667-685. 

 Burne, R. H. 



1924. Some peculiarities of the blood vascular system of 

 the porbeagle shark (Lamna comubica). Philos. Trans. 

 R. Soc. Lond. B. Biol. Sci. 212B:209-257. 

 Carey, F. G. 



1971. Warm bodied fish. Am. Zool. 11:135-143. 



1982a. A brain heater in the swordfish {Xiphias gladius). 



Science 216:1327-1329. 

 1982b. Warm fish. In A companion to animal physiology; 

 5th international conference on comparative physiology; 

 Sandbjerg, Denmark, July 22-26, 1980 (C. R. Taylor. K. 

 Johansen and L. Bolis, eds.), p. 216-234. Cambridge Univ. 

 Press, Cambridge, England; New York, NY. 

 Carey, F. G, J. G. Casey, H. L. Pratt, D. Urquhart, J. E. McCosker, 

 J. A. Seigel, and C. C. Swift. 



1985. Temperature, heat production and heat exchange in 

 lamnid sharks. In Biology of the white shark (J. A. Seigel 

 and C. C. Swift, eds. ), p. 92-108. Mem. South. Calif. Acad. 

 Sci., vol. 9, Fullerton, CA. 



