Jellyfish

Jellyfish are not adapted to closed spaces. They depend on currents to transport them from place to place. Professional exhibits feature precise water flows, typically in circular tanks to prevent specimens from becoming trapped in corners. The Monterey Bay Aquarium uses a modified version of the kreisel (German for "spinning top") for this purpose. Jellyfish are becoming a popular trend in home aquariums. It is now possible to buy jellyfish aquariums and live jellyfish online. It is also possible to assemble a jellyfish aquarium for personal use.
• Toxicity
All jellyfish sting their prey using nematocysts, also called cnidocysts, stinging structures located in specialized cells called cnidocytes, which are characteristic of all Cnidaria. Contact with a jellyfish tentacle can trigger millions of nematocysts to pierce the skin and inject venom, yet the sting of only some jellyfish species causes an adverse reaction in humans. When a nematocyst is triggered by contact by predator or prey, pressure builds up rapidly inside it up to 2,000 lbs/sq. inch until it bursts open. A lance inside the nematocyst pierces the victim's skin, and poison flows through into the victim. Touching or being touched by a jellyfish can be very uncomfortable, sometimes requiring medical assistance; sting effects range from no effect to extreme pain to death. Because of the wide variation in response to jellyfish stings, it is wisest not to contact any jellyfish with bare skin. Even beached and dying jellyfish can still sting when touched.
Scyphozoan jellyfish stings range from a twinge to tingling to savage agony. Most jellyfish stings are not deadly, but stings of some species of the class Cubozoa and the Box jellyfish, such as the famous and especially toxic Irukandji jellyfish, can be deadly. Stings may cause anaphylaxis, which can be fatal. Medical care may include administration of an antivenom.