Jellyfish bites are a painful experience caused by the STINGS of jellyfish tentacles. The sting is caused by the release of venom from the jellyfish's nematocysts, which are located on the tentacles. The venom causes a burning sensation and can cause swelling, redness, and itching. The best way to treat jellyfish STINGS is to rinse the affected area with seawater and then apply a cold compress or vinegar to the area.