Abstract

Osmoregulation in aquatic species relies on the relationship between solute concentrations within the body and the outside medium. Marine elasmobranchs maintain their plasma hyperosmotic to saltwater, which facilitates passive water influx and largely eliminates the need for drinking.^1–3

Cownose rays (Rhinoptera bonasus) are the most numerous elasmobranch species displayed in aquaria, and management of Benedeniella posterocolpa is a pressing concern in most collections.^1,4,5 Although hyposalinity treatments are stressful for marine elasmobranchs, they can be useful for removal of parasites by creating an osmotic shock.^4–6 Anecdotal information regarding R. bonasus tolerance of freshwater (FW) exposure varies.

Two R. bonasus presented for management of a severe capsalid monogenean infection in a mixed species habitat at an aquarium. Weekly FW dips were elected as treatment given prior ineffectiveness of repeat praziquantel baths. The first treatment was tolerated well, however emergency treatment was required after the second dip. Bloodwork revealed a marked lactic acidosis, hemoconcentration, hyperproteinemia, electrolyte abnormalities, hyperglycemia, and a drop in BUN and plasma osmolality in one ray. Abnormal values normalized following supportive care. Increased packed cell volume, hyperlactatemia, metabolic acidosis, and hyperglycemia were attributed to a severe stress response.⁷ A selective loss of urea resulted in decreased plasma osmolality to minimize concentration gradients and prevent excess influx of water.^2,3,8–10 Given the severity of the stress response and associated complications, FW dips should be utilized with caution in this species. To the author’s knowledge, this is the first report documenting tolerance of and physiologic responses to FW dips in R. bonasus.

Acknowledgements

We are grateful to Adrienne Dandy and Ashley Plunckett for their technical assistance and sample preparation; Drs. Tonya Clauss, Michelle Davis, Sarah Miller, and Jessica Comolli for consulting on this case; and the Georgia Aquarium Fish and Invertebrate husbandry team for their assistance with case and animal management.

Literature Cited

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7.  Skomal G, Bernal D. 2010. Physiological Responses to Stress in Sharks. In: Carrier JC, Musick JA, Heithaus MR, editors. Sharks and Their Relatives II: Biodiversity, Adaptive Physiology, and Conservation. Boca Raton (FL): CRC Press. p 459–484.

8.  Dowd WW, Harris BN, Cech Jr JJ, Kültz D. 2010. Proteomic and physiological responses of leopard sharks (Triakis semifasciata) to salinity change. Journal of Experimental Biology 213: 210–224.

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10.  Morash AJ, Mackellar SRC, Tunnah L, Barnett DA, Stehfest KM, Semmens JM, Currie S. 2016. Pass the salt: physiological consequences of ecologically relevant hyposmotic exposure in juvenile gummy sharks (Mustelus antarcticus) and school sharks (Galeorhinus galeus). Conservation Physiology 4:1–13.