Abstract

A cluster of nine cases of thyroid hyperplasia was documented in marine tropical teleosts housed in a public aquarium. Over a 9.5-month period, thyroid hyperplasia was diagnosed in nine out of 32 (28%) fish necropsied. Reef teleosts including a yellow tang, a six-spot goby, an ocellated dragonet, a leafy seadragon, three short-snouted seahorse, and two lined seahorses, housed in two different buildings, were affected by this condition. Subopercular growths were seen in reef fish but no external lesions were detected in the Syngnathidae. Histological lesions were characterized as follicular thyroid hyperplasia in the pharyngeal connective tissue. The death of five out of nine fish was attributed to the hyperplastic thyroidal disease. Iodine concentration was undetectable in non-ozonized water (<0.01 mg/L)^a. This observation suggests a link between these thyroidal hyperplastic lesions and an environmental iodine deficiency. This syndrome was unusual in this institution; thyroidal hyperplastic change was not detected in any of the 317 tropical marine fish necropsied prior to this event. The only significant modification in the husbandry was a change, 18 months prior to the first case, of the commercial salt mix brand used to make artificial seawater. The iodine content in this replacement salt mix^b was five times lower than in the salt mix^c used before. This case series suggests that levels of iodine in the commercial salt mix currently used is insufficient to maintain thyroid homeostasis in tropical marine teleosts, at least under the husbandry provided in this institution.

a. Multitest Iodine & iodide, Seachem Laboratories, Madison, GA, USA.
b. Crystal Sea Marinemix, Marine Enterprises, Baltimore, MD, USA.
c. Instant Ocean Sea Salt, Instant Ocean, Blacksburg, VA, USA.

Acknowledgements

The authors wish to thank the animal health technicians and aquarists at the Aquarium du Québec that have been involved with the care of these fish.

* Presenting author, + Student presenter