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Why can some crocodiles live in saltwater as well as freshwater?

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Illustration of an estuarine or saltwater crocodile (Crocodylus porosus) with its mouth open, showing scaly skin and powerful limbs, alongside a scale bar for size reference. — crocodile Estuarine, or saltwater, crocodile (*Crocodylus porosus*), found in brackish waters from Sri Lanka, India, Bangladesh, and Myanmar east to the Solomon Islands and Vanuatu and south to Australia.

Some crocodiles, such as the saltwater crocodile and the American crocodile, are able to inhabit both saltwater and freshwater environments because they are euryhaline—meaning they can tolerate a wide range of external salt concentrations.

While many aquatic organisms must constantly manage osmotic pressure to prevent the loss or gain of water through their skin and respiratory surfaces, these crocodiles possess the physiological flexibility to thrive in varying salinities. In saline environments, animals face a constant influx of ions and loss of water, whereas in freshwater, they face the opposite challenge of losing ions and gaining excess water. To maintain internal balance, or osmoregulation, these crocodiles have evolved mechanisms to regulate their internal water-mineral content despite external fluctuations.12

It is important to note that not all crocodiles share this ability; for example, the freshwater crocodile is primarily restricted to inland freshwater environments.3