Frogs are a good example of a species that thrives in marshes.
They have several adaptations for the aquatic life that enables them to survive in wet environments.
All frogs have long back feet with extensive webbing between their toes. This lets frogs swim very well by increasing their surface area and thus allowing them to push water behind them. This is what propels the frog forward.
Frogs do not swallow water. Instead, they absorb it through their skin. A large network of blood vessels is also found here. This allows frogs to breathe through the skin too. This is very useful and it allows frogs to stay underwater for long periods of time.
Frogs' eyes are also a very useful adaptation. Placed on top of their head, they enable the frog to see any potential predator while having the rest of their body below the water's surface. Furthermore, the eyes are large and rounded allowing them to look in all directions at once as well as seeing very well in dim conditions.
