A recent study has challenged a long-held understanding that sharks do not produce cortisol, challenging decades of research on how these animals regulate their metabolism and respond to environmental changes.
Published in the Proceedings of the Royal Society B, the study from the University of Manitoba and Bamfield Marine Sciences Centre in Canada provided the first molecular evidence of cortisol in the blood of North Pacific spiny dogfish (Squalus suckleyi).
According to the researchers, investigations in the 1960s and 1970s led scientists to conclude that sharks and rays cannot produce cortisol and instead rely on a different corticosteroid called 1α-hydroxycorticosterone. Any traces of cortisol were attributed to “immuno-cross-reactivity with this unique steroid,” the study explained.
But researchers involved in the study challenged that assumption. They examined the sharks’ interrenal glands, organs involved in producing steroid hormones. There, they found genetic transcripts of the Cyp17a1 gene which codes for the enzyme involved in producing cortisol precursors. However, the researchers emphasized that more investigation is needed to establish the enzyme’s activity.
Shark experiments reveal previously undetected hormone
For the experiment, researchers collected North Pacific spiny dogfish off the coast of British Columbia and exposed them to diluted seawater for 48 hours as part of a low-salinity stress challenge before returning them to normal seawater. Even before exposure to diluted seawater, cortisol was detectable in only one shark.
Its presence became more widespread during recovery. They conducted extensive blood sampling, using high-performance liquid chromatography to separate compounds before measuring their concentrations, and found cortisol in two blood samples.
After 72 hours of recovery, researchers found that cortisol was undetectable in all but one dogfish. Despite this, researchers did not find significant differences in average cortisol levels across the recovery period. In the end, the experiments showed that cortisol can circulate in these sharks’ blood.
But why was cortisol so difficult to detect?
In the interrenal tissue, researchers found genetic material of Rhodococcus bacteria, according to Phys.org. They proposed that the bacteria might influence cortisol metabolism, which may explain why the hormone has been difficult to detect.
“Thus, it remains uncertain whether detection of F (cortisol) in circulation is a result of an impaired microbial community or a corticosteroid response itself,” the researchers stated in the study. To assess what might have caused this reaction, they considered the anesthesia they used and the sharks’ repeated handling.
Lastly, the researchers examined whether cortisol affected glucose regulation and the balance of water and dissolved substances in shark blood. Six sharks received cortisol implants dissolved in coconut oil, while six others received coconut oil alone. They found a marked reduction in glucose among cortisol-treated animals, but they did not detect differences at every sampling interval.
Cortisol and corticosterone hormones are responsible for the regulation of energy balance, metabolism, and immunoregulation, which may explain why introducing cortisol affected glucose regulation in the sharks.
The researchers suggested several hypotheses for why this happened, but cortisol’s purpose remains unproven. They concluded that the findings may open new avenues for investigating hormone regulation in sharks and rays.