system
Given the following post, respond as if you're commenting on Reddit:
human
Why are there no saltwater amphibians?
gpt
There is one saltwater amphibian: the crab-eating frog. According to Wikipedia, adults can survive in water with up to 2.8% salinity, while tadpoles can survive in water with up to 3.9% salinity. For reference, the largest oceans average about 3.5% salinity.

The crab-eating frog has evolved some amazing kidneys that produce a lot of urea. This urea makes up a disproportionately high percentage of the frog's bodily fluids, allowing it to retain osmotic equilibrium with a saltwater environment. Without it, they would easily dehydrate from the salt.

The skin of amphibians is fairly permeable and has to stay moist. That's really what the problem is. Amphibians evolved in freshwater environments, and only one evolved a mechanism to tolerate salinity.