The onset of goiter in several species of sharks following the addition of ozone to a touch pool.

This report describes the onset of goiter in several species of shark following the addition of ozone to a touch pool. A detailed description of a female brown-banded bamboo shark (Chiloscyllium punctatum) that was presented with multinodular goiter is provided. Four other brown-banded bamboo sharks and 11 white-spotted bamboo sharks (Chiloscyllium plagiosum) housed in the same system developed clinical disease consistent with goiter, but two zebra bullhead sharks (Heterodontus zebra) did not. Plasma thyroxine (T4) concentration was 4.64 ng/ml before euthanasia, consistent with a diagnosis of hypothyroidism. The sharks had been chronically exposed to mean (+/- standard error) NO3-N concentrations of 35 +/- 5.12 mg/L before ozonation of the system. Ozonation of aquarium water causes a reduction in environmental iodide, which is required for thyroid hormone synthesis. Nitrate is goitrogenic and would further decrease I- absorption by competitive inhibition. Multinodular goiter is consistent with goiter caused by chronic iodide deficiency. Understanding the interaction between water chemistry and goiter development is critical to development of elasmobranch health management systems.

Nitrate-induced goiter in captive whitespotted bamboo sharks Chilosryllium plagiosum.

Elasmobranch susceptibility to goiter formation in captive environments has been well documented. Until recently, most public aquariums operated under the belief that the etiology of goiter in elasmobranchs was nutritional and specifically caused by insufficient dietary iodine. Recent studies have demonstrated that high environmental nitrate (NO3-N) inhibits the ability of the thyroid gland to utilize available iodide, resulting in thyroid gland overstimulation by thyroid stimulating hormone and ultimately leading to the development of goiter. The objective of this study was to evaluate the effects of high environmental nitrate concentrations on thyroid function in juvenile whitespotted bamboo sharks Chiloscyllium plagiosum. In July 2008, five juveniles (80-150 g) were exposed to a low-nitrate environment (NO3-N concentration < 1 mg/L of water) and five were exposed to an elevated-nitrate environment (NO3-N = 70 mg/L) for 29 d in a flow-through natural seawater system. Nitrate exposure did not affect growth rates (e.g., weight, length, and condition factor) and did not alter free plasma thyroxine concentrations during the 29-d experimental period. However, histological examination of thyroid glands from sharks exposed to elevated nitrate revealed the development of diffuse hyperplastic goiter. With increasing restrictions on water use, most modern aquaria operate as recirculating systems, which results in higher and more chronic nitrate exposure for captive animals. Goiter is one of the most common health problems in captive elasmobranchs, and this study suggests that nitrate exposure is an important factor in the etiology of this disease.