A vernal pool is a temporary body of water (wetland) that provides essential breeding habi- tat for certain amphibians – such as wood frogs, and spotted salamanders – and inverte- brates – such as fairy shrimp.

Vernal pools are important as wildlife habitat because of the wide range of species that use them, including turtles, frogs, salamanders, fairy shrimp, clam shrimp, fingernail (or “pill” or “pea”) clams, caddis flies and other aquatic insects.

Vernal pools provide a safer breeding ground for these species than permanent waters because there are no fish to eat the eggs or larvae.

Check out this great resource, the source of the above information:
Identifying and Documenting Vernal Pools in New Hampshire
https://www.wildlife.state.nh.us/nongame/documents/vernal-pool-manual.pdf