Ever wondered how fish cope with sudden temperature drops? Scientists took a close look at pacu fish, exploring how they handle intermittent cold exposure. These fish were first acclimated to a cozy 28°C. Then, they were split into two groups – one stayed at 28°C, while the other experienced two quick dips to 16°C, each lasting 24 hours with a 5-day break in between. Researchers checked in at six different times: right after acclimation, 24 hours post-first cold exposure, after the 5-day recovery, again 24 hours post-second cold exposure, and twice more after recovery.

When the temperature dropped, the fish’s stress response, immune system, and antioxidant defenses kicked into gear. Interestingly, they also started burning fat from their belly stores and kept triglyceride levels steady in their bloodstream. After each cold dip, the fish’s cortisol levels surged, their white blood cells (like leukocytes) were more active after the second cold exposure, and their immune system tried to balance out with increased lysozyme activity during the recovery phases. Plus, more monocytes and granulocytes (types of white blood cells) were spotted in their blood circulation.

The fish’s antioxidant enzymes, catalase (CAT) and superoxide dismutase (SOD), also ramped up their activity after each cold exposure. Glutathione peroxidase (GPx) was especially active after the second cold exposure compared to the fish that didn’t experience the chilly temperatures. All in all, these subtropical fish showed they could bounce back from cold snaps pretty well.

Cold Snap Survival: How Pacu Fish Adapt and React

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