A moon jellyfish (Aurelia aurita) swimming. Moon jellyfish are one of the most common types of jellyfish, and they can be found in cold and temperate waters all over the world. They are typically transparent, with four horseshoe-shaped gonads visible through their bell. Their bell can range in diameter from 2 to 40 cm, and they have tentacles that can trail up to 6 m (20 ft) long.