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How do fishes survive in hot springs or extremely cold polar waters?

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A porcupine fish in the Andaman Sea — fish *Diodon hystrix* is a species of porcupine fish. These fishes are short and broad-bodied, with large eyes and spines covering their skin. Like puffer fishes, they inflate their bodies when threatened.

Fishes are ectotherms, meaning their body temperature is largely determined by their environment. To survive in extreme temperatures, they have evolved specialized biochemical and physiological adaptations.1

Surviving Cold Polar Waters

In freezing polar environments, many fishes prevent internal ice formation by altering their body chemistry. They utilize cryoprotectants—substances that lower the freezing point of body fluids—to inhibit the growth of ice crystals in their cells and tissues. For example, ray-finned fishes in polar regions maintain high internal salt concentrations and produce specialized glycoproteins that act as biological antifreeze.123

Surviving Hot Springs

In high-temperature environments like desert hot springs, which can exceed 38 °C (100 °F), fishes face the risk of protein damage. To combat heat stress, these animals produce heat-shock proteins. These proteins help stabilize other cellular proteins, preventing them from denaturing—a process where a protein loses its functional molecular structure due to excessive heat.14