Ultrasound Comparison of the Liver to Spleen as an Evaluation of Liver and Body Condition in Elasmobranchs
Abstract
Elasmobranchs use hepatic lipids to store the majority of their energy reserves and for buoyancy.2 The mass of the liver relative to total body mass in elasmobranchs is used to assess general body condition;5 the larger the liver the more ‘fit’ the fish6. This evaluation is typically done postmortem5-7,11 but antemortem tools to evaluate body condition include physical measurements3,4 and serum biochemical markers (triglycerides)3. These methods are indirect and argued as imprecise.6 We developed a method to evaluate liver condition for the southern stingray, Hypanus americanus, using ultrasonography. The method is derived from human lipidosis evaluation, which uses liver to kidney echo-density comparisons to diagnose disease.8,12,13 Here, we capture an image of the liver and spleen in the same focal depth plane and measure pixels in a region of interest (ROI),10 which is expressed as a liver to spleen ratio (LSR). Because the spleen does not store lipids, its echogenicity is relatively constant regardless of nutritional state and can therefore be used to normalize liver echogenicity,9 which changes relative to lipid content. The LSR for managed care stingrays was significantly higher than wild stingrays, indicating higher liver echodensity. Preliminary data suggests ultrasound-derived LSR may allow for quantitative antemortem indices of liver and body condition in southern stingrays. Species ecology, living conditions and seasonality impact liver size and lipid content. Extension and validation of the technique to include a variety of sharks and rays is needed to understand if this tool can be applied broadly to all elasmobranch species.
Acknowledgments
The authors thank the animal husbandry and Animal Health teams at The Seas with Nemo and Friends at Epcot and Castaway Cay, Bahamas (Disney’s Animals, Science and Environment); SEA LIFE (Orlando, Charlotte, Michigan and Kansas City); and Ripley’s Aquarium of Canada. North Carolina Aquariums; Florida Aquarium and Georgia Aquarium for all of their hard work in the feeding, care, management, handling, and welfare of the animals; as well as their skilled sample collection. We also thank the staff at the Bimini Biology Field Station Foundation for wild sample collection efforts. Special thanks to David Reese, DVM, Diplomate ACVR MRCVS, VetCT, Telemedicine, Cambridge (UK) who helped form this process. This study was supported in part by the Bahamas Department of Marine Resources, the National Institute of General Medical Sciences (NIGMS) Grant Number P20GM104932, and COBRE, CORE-NPN, Chemistry Research Core.
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