Impact of long-term captivity on interrenal physiology in New Zealand shortfin eel, Anguilla australis
2022
Milsom, Mia J.
The steroid hormone cortisol is released as the final product of the stress response in teleost fish and is essential in aiding survival. When an animal perceives a stressor, large amounts of cortisol are produced and released into the blood to reallocate resources to various tissues to improve the response to the stressor. While high plasma cortisol levels can be beneficial for short durations, prolonged high levels of cortisol, produced in response to a chronic stressor such as captivity, are known to negatively impact various functions including growth and reproduction.
When exposed to a stressor, silver-stage (migratory) New Zealand shortfin eels (Anguilla australis) have significantly lower plasma cortisol levels than yellow-stage (feeding) eels do, and it was hypothesised that this difference in plasma cortisol levels would be reflected in observable differences in immunoreactivity of the corticosteroidogenic interrenal cells and their nuclear diameters. This thesis first conducted a field study to gain insight into the interrenal physiology of wild eels through measuring the nuclear diameters of the interrenal cells. Immunoreactivity of the interrenal cells to key enzymes involved in cortisol biosynthesis, cytochrome P450 side-chain cleavage (Cyp11a1) and 11β-hydroxylase (Cyp11b), were also measured. The previously reported difference in plasma cortisol level in response to a stressor between silver and yellow stages of A. australis was confirmed, however this difference was not universally reflected in the interrenal physiology. Interrenal tissues from silver-stage eels displayed higher immunoreactivity than in yellow-stage eels. No significant difference was found between the life history stages for either interrenal nuclear diameters or for Cyp11b immunoreactivity, suggesting that there is little difference in levels of interrenal cell activity between yellow and silver eels.
Subsequently, the effect of long-term captivity on interrenal physiology was investigated in silver-stage A. australis. Wild-caught fish were held in tanks for up to eight months, with periodic terminal sampling of individuals at different timepoints. As captivity length progressed, a significant reduction in immunoreactivity for interrenal tissues stained for either Cyp11a1 or Cyp11b were observed, alongside a concurrent decrease in interrenal nuclear diameters. Unusually, plasma cortisol levels did not differ significantly between the different groups, suggesting a potential reduction in cortisol metabolism or increase of cortisol-binding globulin in the plasma to compensate for the apparent reduction in interrenal activity.
This study provides valuable preliminary information into the impacts of chronic stress on interrenal physiology and localise cortisol production in A. australis. As chronic stress is known to negatively impact various tissues and functions of an animal, such as reproduction, understanding the impacts of long-term captivity as a chronic stressor is vital to further aquacultural development of fish such as the shortfin eel.