221 



Diel vertical migration of the 



bigeye thresher shark (Alopias superci/iosus), 



a species possessing orbital retia mirabilia 



Kevin C. Weng 



Barbara A. Block 



Tuna Research and Conservation Center 

 Hopkins Marine Station of Stanford University 

 120 Oceanview Boulevard 

 Pacific Grove, California 93950 



E-mail address (for K. C. Weng): kevin cm wengia'stanford edu 



The bigeye thresher shark {Alopias 

 superciliosus, Lowe 1841) is one of 

 three sharks in the family Alopiidae, 

 which occupy pelagic, neritic, and 

 shallow coastal waters throughout the 

 tropics and subtropics (Gruber and 

 Compagno, 1981; Castro, 1983). All 

 thresher sharks possess an elongated 

 upper caudal lobe, and the bigeye 

 thresher shark is distinguished from 

 the other alopiid sharks by its large 

 upward-looking eyes and grooves 

 on the top of the head (Bigelow and 

 Schroeder, 1948). Our present under- 

 standing of the bigeye thresher shark 

 is primarily based upon data derived 

 from specimens captured in fisheries, 

 including knowledge of its morpho- 

 logical features (Fitch and Craig, 1964; 

 Stillwell and Casey, 1976; Thorpe, 

 1997), geographic range as far as it 

 overlaps with fisheries (Springer, 1943; 

 Fitch and Craig, 1964; Stillwell and 

 Casey, 1976; Gruber and Compagno, 

 1981; Thorpe, 1997), age, growth and 

 maturity (Chen et al., 1997; Liu et al., 

 1998), and aspects of its reproductive 

 biology (Gilmore, 1983; Moreno and 

 Moron, 1992; Chen et al.. 1997). 



Limited information on the move- 

 ment patterns of bigeye thresher 

 sharks has been obtained from mark- 

 recapture studies by using conven- 

 tional tags. The longest straight-line 

 movement of a conventionally tagged 

 bigeye thresher shark to date is 2767 

 km from waters off New York to the 

 eastern Gulf of Mexico (Kohler and 

 Turner, 2001). The bigeye thresher 

 shark has been captured on longlines 

 set near the surface at night (0 m to 65 

 m, Fitch and Craig, 1964; Stillwell and 



Casey, 1976; Thorpe, 1997; Buencuerpo 

 et al., 1998) and at 400 m to 600 m 

 during the day (Nakamura 1 ). There 

 is no published information available 

 regarding its habitat and behavior, al- 

 though Francis Carey tracked a bigeye 

 thresher with an acoustic tag for six 

 hours (Carey 2 ). 



Endothermy is a rare trait in fishes 

 and has been demonstrated only in 

 tunas (Thunnini), billfishes (Xiphiidae, 

 Istiophoridae), and lamnid sharks 

 (Lamnidael (Carey and Teal, 1969; 

 Carey, 1971, 1982a; Block, 1991). In all 

 endothermic fishes, the blood supply 

 to aerobic tissues such as slow-twitch 

 swimming muscle, visceral organs, 

 extraocular muscles, retina, and 

 brain occurs by counter-current heat 

 exchangers known as retia mirabilia. 

 The vascular supply reduces heat loss 

 to the environment and enables heat 

 conservation in metabolically active 

 tissues (Carey, 1971). Lamnid sharks 

 have retia mirabilia in the circulatory 

 anatomy supplying the slow-oxidative 

 swimming muscles, viscera, brain, and 

 eyes (Burne, 1924; Block and Carey, 

 1985; Tubbesing and Block, 2000). In 

 many lamnid species, tissue tempera- 

 tures significantly above ambient have 

 been recorded from freshly captured 

 specimens and through telemetry stud- 

 ies of swimming animals (Carey, 1971; 

 Carey et al., 1981, 1982, 1985; McCos- 

 ker, 1987; Goldman, 1997; Tubbesing 

 and Block, 2000). 



The anatomy of alopiid sharks sug- 

 gests that endothermy may occur in 

 this family. The bigeye thresher and the 

 common thresher (Alopias vulpinus) 

 have centrally located slow-oxidative 



muscle and primitive retia mirabilia 

 supplying blood to them (Carey, 1982b: 

 Bone and Chubb, 1983). Burne (1924) 

 noted a coiling of the pseudobranchial 

 artery supplying the orbit and cranial 

 regions in the common thresher. No 

 internal tissue temperature measure- 

 ments have been taken for free-swim- 

 ming thresher sharks to ascertain 

 whether heat is conserved in oxidative 

 tissues. A freshly caught bigeye thresh- 

 er shark was found to have a body-core 

 thermal excess of 4°C (Carey, 1971); 

 thus the species may have the ability 

 to conserve metabolic heat. 



In this study we present electronic 

 tagging data on the movements, div- 

 ing behavior, and habitat preferences 

 of the bigeye thresher shark based on 

 two individuals studied with pop-up 

 satellite archival tags. In addition, 

 we provide a brief description of the 

 orbital rete mirabile of the species. 

 The presence of this highly developed 

 rete mirabile within the orbital sinus 

 suggests a physiological mechanism 

 to buffer the eyes and brain from the 

 large temperature changes associated 

 with diel vertical migration, potentially 

 conferring enhanced physiological per- 

 formance. 



Materials and methods 



The movements of two bigeye thresher 

 sharks were monitored with pop-up 

 satellite archival tags (PAT tag version 

 2.00, Wildlife Computers, Redmond, 

 WA; Gunn and Block, 2001; Marcinek 

 et al., 2001). The first shark was cap- 

 tured on a longline set in the Gulf of 

 Mexico at 26.5°N, 91.3°W on 12 April 



1 Nakamura. I. 2002. Personal commun. 

 Institut National des Sciences et Technolo- 

 gies de la Mer. 28 rue 2 Mars 1934, 2025 

 Salammbo. Tunisia. 



2 Carey. F. G. (deceased). 1990. Personal 

 commun. Woods Hole Oceanographic 

 Institution, Woods Hole, MA 02543. 



Manuscript approved for publication 

 15 August 2003 by Scientific Editor. 



Manuscript received 20 October 2003 

 at NMFS Scientific Publications Office. 



Fish. Bull. 102:221-229 (2004). 



