Jellyfish are free-swimming, non-aggressive, gelatinous marine animals surrounded by tentacles. These tentacles are covered with sacs (nematocysts) that are filled with poison (venom) that can cause a painful to sometimes life-threatening sting. The marine animals included in the “family” are jellyfish, box jellyfish (sea wasps), Portuguese man-of-war, hydroids, anemones, and fire coral. Jellyfish are found throughout the world. But, the most deadly are found in the Indo-Pacific and Australian waters.