Abstract

Wild caught captive sandbar sharks, Carcharhinus plumbeus, developed ocular lesions consistent with corneal abrasion and scarring and erosive lesions on the rostrum and dorsal fins. Lesions were present both in sharks housed in a circular, smooth quarantine tank and in sharks housed in a mixed species exhibit pool with variable surfaces and had been present for multiple years. Assessment of exhibit enclosure design indicated several design deficits leading to traumatic injury in the habitant sharks. The sharks were observed hitting both the rostrum and dorsal fins on sections of the enclosure but were not observed hitting the eyes on any substrate. Research into the visual capabilities of the sandbar shark, known to inhabit a wide range of habitats, has shown that the species can respond to a broad range of light levels, allowing it to fully function in a range of environments.¹ It has been demonstrated that the eyes of C. plumbeus have a retinal specialization that enhances vision, allowing for improved tracking of predators and prey.¹ Due to their preferred habitat, C. plumbeus place increased importance on sampling the horizontal visual field and have more UV-transparentocular media than other similarly sized sharks.² This adaptation, while enhancing the visual sensitivityof the eye, increases susceptibility to UV damage of ocular tissues.² We hypothesized that ocular lesions were a result of a combination of lack of UV protection in an unnatural environment and that rostrum and dorsal fin lesions were secondary to visual impairment which, combined with enclosure design deficits, lead to self-trauma. Continued self-trauma, secondary to initial UV visual injury, was likely to include ocular trauma. We suggested that the addition of night lighting would allow the sharks better navigation and subsequent improvement of rostrum and dorsal fin lesions and that addition of shade in the outdoor exhibit would prevent and possibly improve UV induced ocular lesions. Existing underwater lighting was turned on in the exhibit enclosure; room lights were left on in the quarantine tank. Ocular and skin lesions were subjectively re-assessed- ocular lesions improved in one of two sharks housed in the quarantine tank and skin lesions healed in both animals. Ocular lesions in the exhibit sharks remained unchanged; skin lesions showed mild to moderate improvement. Our findings suggest that captive sandbar sharks housed require UV protection and that the relationship between UV protection, habitat, and environmental lighting plays an important role in the physiological welfare of captive sandbar sharks. Further study into the ocular disease in this group of captive sharks will include the placement of shade for UV protection in the outdoor exhibit.

Acknowledgements

The author wishes to thank Drs. Heather Wilson Barron of St. Matthew's University School of Veterinary Medicine and Mr. Brian Dann and Mr. Geddes Hislop of the Cayman Island Turtle Farm in Grand Cayman.

References

1.  Litherland L, Collin SP, Fritsches KA. Eye growth in sharks: Ecological implications for changes in retinal topography and visual resolution. Vis Neurosci 2009; 26(4): 397–409.

2.  Litherland L, Collin SP, Fritsches KA. Visual optics and ecomorphology of the growing shark eye: a comparison between deep and shallow water species. J Exp Biol 2009; 212: 3583–3594.