Abstract

The spotted ratfish (Hydrolagus colliei) is a chimaera (Holocephali) which naturally occurs in the north-eastern Pacific Ocean; it is the most common species among chimaera kept in aquariums worldwide, and is classified as “least concern” by the IUCN.¹ Although information regarding capture, transportation, and husbandry of holocephalans is growing, there is still a knowledge gap concerning their medical management and clinical pathology.² Currently, there are still few published hematology studies in chondrichthyans (mainly in sharks and rays), which have shown large interspecific differences.^3,4 In this study, three adult spotted ratfish (one female, two male) which have been maintained for over twenty years at Oceanário de Lisboa were captured and anesthetized individually for veterinary checkups using MS-222 at 50 ppm dilution, which provided light anesthesia and allowed for the safe handling and blood collection of all animals, with mean induction and recovery times of 5 and 20 min. Blood samples of 1 ml were collected from the caudal vasculature of each animal via lateral approach, then they were directly introduced into a 1-ml lithium heparin tube and were processed for hematology analysis within 30 minutes from collection. The following parameters (mean; methodology) were determined: packed cell volume (15.55%; microhematocrit tube centrifugation), plasma total solids (6.4 g/dl; manual refractometry), total erythrocyte count (0.67 10¹² cells/l; manual count in Neubauer chamber), total leukocyte count (21.17; manual count in Neubauer chamber using), and leukocyte differential count (lymphocytes 74.4%, fine eosinophilic granulocytes 10.7%, coarse eosinophilic granulocytes 9.3%, monocytes 5.8%; manual count of blood smears stained using Diff-Quick).

Acknowledgments

The authors would like to thank David Ruiz García for providing us with images from the chimaeran dissections which helped in the choice of the venipuncture site and to Kadie Anderson for her advice with the anesthetic protocol.

*Presenting author

Literature Cited

1.  Barnett L, Ebert D, Dagit D. Hydrolagus colliei (spotted ratfish). The IUCN Red List of Threatened Species. 2015. https://Dx.Doi.Org/10.2305/IUCN.UK.2015-4.RLTS.T60191A80678052.En.

2.  Tozer H. Husbandry of spotted ratfish, Hydrolagus colliei. In: Smith M, Warmolts D, Thoney D, Hueter R, editors. The Elasmobranch Husbandry Manual: Captive Care of Sharks, Rays and Their Relatives. Columbus, OH: Ohio Biological Survey; 2004:487–491.

3.  Arnold J E, Delaune A. Elasmobranchs. In: Brooks MB, Harr KE, Seelig DM, Wardrop KJ, Weiss DJ, editors. Schalm’s Veterinary Hematology. Seventh edition. Wiley; 2022:1166–1175. https://doi.org/10.1002/9781119500537.

4.  Walsh CJ, Luer CA. Elasmobranch hematology: identification of cell types and practical applications. In: Smith M, Warmolts D, Thoney D, Hueter R, editors. Elasmobranch Husbandry Manual: Captive Care of Sharks, Rays, and Their Relatives. Columbus, OH: Ohio Biological Survey; 2004:307–323.