Abstract

Spinal column abnormalities continue to plague captive sandtiger sharks, Carcharias taurus. Initial surveys^1,2,3 and studies⁵ have suggested multiple etiologies, but detailed, systematic approaches to evaluating contributing factors have been limited. In this study, husbandry practices, animal behavior, nutritional physiology, and spinal biomechanics were explored to identify the proximate causes of spinal deformities with the intention of developing better husbandry guidelines that would improve captive sand tiger health and reduce dependence on wild stocks for exhibit specimens.

Data and tissue samples were requested at three stages along the clinical timeline of resident sharks from public aquaria nationwide. Stage I was a history and husbandry survey, coupled with videography of shark swimming behavior. Stage II requested hematological samples and radiography to be collected during health exams. Stage III requested data and tissue samples upon necropsy of an expired or euthanized animal.

Seventeen institutions responded. Preliminary analyses suggest that the occurrence of spinal deformity is not associated with sex, nor with other connective tissue abnormalities commonly seen in this species. However, it is associated with collection locale and method; animals collected off the Rhode Island coast and with pound nets have a disproportionately higher prevalence of disease than animals caught in other areas or with hook-and-line. These trends should be further evaluated by examining the methods of collectors operating in different locales. Once in captivity, spinal deformity usually manifests by a median of 4 years, whereas healthy sharks persist for a median of 10 years in captivity. With regard to captive environments, aquarium length (or diameter) is negatively associated with disease prevalence; i.e., aquaria with smaller lengths (or diameters) have populations with higher disease prevalence. This is informative in light of swimming behavior. In general, captive sharks (regardless of condition) spend a median of 95.5 % of their time actively swimming and only 0.5 % gliding, suggesting abnormal locomotion lacking parity between swimming and gliding that other species naturally demonstrate.⁴ Furthermore, sandtiger sharks with spinal deformity spend significantly less time gliding than healthy sharks. This is coupled with constant lateral stress on the vertebral column due to clockwise or counter-clockwise swimming that accounts for a median of 99.7 % of locomotion of sharks (regardless of condition), generally with a low occurrence of change in direction. The median proportion of time spent swimming in a linear direction is negligible. Affected sharks are more lethargic, characterized by a slower tail-beat frequency. Finally, average Fulton condition factor averaged over time of resident sharks is significantly larger for animals presenting with deformity; i.e., affected sharks have more body mass per unit length.

Biomechanical analyses of sections of vertebral columns and individual vertebrae from healthy and deformed sandtiger sharks were conducted to characterize the mechanical basis of skeletal deformities. Vertebral sections were subjected to bending tests, while individual vertebrae were subjected to compression to failure tests, both using an MTS Mini-Bionix 858 material testing system. Mineral content of individual vertebrae was determined as well. The flexural stiffness (resistance to bending) of vertebral columns from healthy animals was greater than that of deformed animals due to greater second moment of area. Second moment of area is a structural property that measures the distribution of skeletal material away from the central axis of the vertebral column. From these data it was also determined that the force required to buckle the vertebral column was greater in the healthy specimens. The compressive stiffness, yield strength, yield strain, and ultimate strength of vertebrae from healthy animals were greater than those from deformed animals. However, the compressive stiffness and ultimate strength of vertebrae from healthy specimens were still lower than those of most species for which data are available in the literature.⁶

Acknowledgments

We owe a debt of gratitude to the participants of the Shark Spine Study, who graciously provided valuable biological data from their exhibit populations: Adventure Aquarium, Aquarium of the Pacific, Downtown Aquarium in Houston and Denver, Jenkinson's Aquarium, Kattegat Centre, Moody Gardens, Mystic Aquarium and Institute for Exploration, New England Aquarium, North Carolina Aquarium on Roanoke Island, Omaha's Henry Doorly Zoo, Sea World of Orlando and San Antonio, The Seas Aquarium at Epcot (Disney), South Carolina Aquarium, Tennessee Aquarium, and Underwater Adventures at Mall of America.

We'd also like to thank S. Bhansali of the University of South Florida College of Engineering, G. Erickson and P. Gignac of the Florida State University Department of Biological Sciences, and B. Walsh of the National High Magnetic Field Laboratory for their contribution to biomechanical analyses. B. Maciol of the J. G. Shedd Aquarium read leukocyte differentials, C. Miranda of the University of South Florida Clinical Histology Laboratory processed histological samples, D. Murie of the University of Florida IFAS Program in Fisheries and Aquatic Sciences conducted liver lipid analysis, and E. Norkus of the Montefiore Medical Center conducted serum vitamin analyses.

This study was funded by the Association of Zoos and Aquariums Conservation Endowment Fund, the Disney Worldwide Conservation Fund, and the Bernice Barbour Foundation.

References

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