If you want to meet a truly tough fish, buy a ticket to Lake Magadi, Kenya. Here, the Magadi tilapia swim happily in the hot, alkaline waters of the lake, where oxygen levels soar and plummet daily and water chemistry pushes the boundaries of acid–base regulation. Few fish can survive such an extreme aquatic environment and this has attracted comparative physiologists for decades to the town of Magadi, where tilapia are easily studied in the nearby man-made lagoon. But not all Magadi tilapia are created equal, as discovered by Canadian scientist Chris Wood and a team of international colleagues on a recent expedition to Kenya. In fact, life is even more challenging in the fast-flowing hot springs that feed into Lake Magadi's southwestern shores. Remarkably, tilapia live here too.
Wood and crew trekked out to study this isolated population of Magadi tilapia, looking for the toughest tilapia yet. They noted some striking differences between the deep, static waters of the lagoon and the shallow, steamy waters of the hot springs. For example, in a given 24 h period, the lagoon tilapia endure an impressive swing in dissolved oxygen levels from ∼15% to 80% saturation, but their hot spring relatives eat breakfast in anoxia and lunch in super-saturated water! Also, during that same 24 h period, while the lagoon fish sweat it out in 33–36oC water, over in the hot springs, the tilapia's mercury peaks at a whopping 43oC and fluctuates by 11oC throughout the day. So a day in the life of a hot springs tilapia is no small feat, and Wood bet that these fish must have outstanding thermal tolerance and metabolic capacity.
Wood's study started heating up when he measured the upper critical temperatures of the lagoon and hot springs tilapia. When the researchers cranked the thermostat on the lagoon fish, they held out to a maximum temperature of 44.5oC – that's pretty toasty for a poikilotherm. But the hot springs fish toughed out temperatures to 45.6oC, the highest temperature ever recorded for any fish! Scarily, the hot springs fish died at a temperature that was only 2.6oC warmer than that in which they swim daily and that gap will quickly narrow if current climate change trends continue.
Next, Wood and his team began measuring the routine and maximum metabolic rates of fish from the two Lake Magadi habitats and again the hot springs tilapia set records. While routine metabolic rate in all Magadi tilapia is high, the fish Wood’s team caught from the hot springs had metabolic rates that were quadruple the rate of the lagoon fish, putting them on a par with mammals of a comparable size. And, when they made the fish swim to exhaustion to measure their maximum metabolic rates, not only did the hot springs tilapia outperform the lagoon fish in the swim test, their maximum metabolic rate surpassed any previously recorded in a size-matched fish.
The Magadi tilapia remains a perfect fish for studying physiological adaptations to extreme environments and the hot springs population teeters on the edge of that extreme, making them true Olympians among aquatic athletes!