3rd November: Jellyfish Day

It's Jellyfish day! So here are ten things you probably didn't know about jellyfish.

1. They're not fish. Fish are vertebrates, jellyfish are not. Pedantic people and scientists refer to them as "jellies" or "sea jellies".
2. They don't have Brains - they have nerve nets. They don't have Blood, Hearts or respiratory systems. They get their oxygen by diffusion through their thin skin. Most do not have eyes, just light sensitive organs. That said, some species, including the box jellyfish, have 24 eyes, including two capable of distinguishing colours, meaning it has 360 degree vision. A box jellyfish also has 60 anal regions.
3. Approximately 95% of their body is Water.
4. A group of jellyfish is called a bloom, swarm or smack. A bloom of jellyfish can shut down a nuclear reactor by clogging up the intake pipes.
5. Jellyfish can reproduce sexually, or they can clone themselves. Their life cycle typically goes through a number of stages. Eggs hatch into larval planulae, which attach themselves to something and become stationary polyps. They can remain in this state for years, feeding continuously. Some species arrange themselves into colonies in which individuals share a common stomach. The next stage is the ephyra, which swims free and develops into an adult jellyfish. The adult jellyfish is called a medusa after the Greek monster with Snakes for hair.
6. They have lived on earth for millions of years - longer than fish, insects or flowers; they were around before Dinosaurs. They have survived five mass extinction events, so they could be around for much longer than most other things living on the planet. What's more, scientists are currently studying a species of jellyfish which can, in adverse conditions, return to its polyp state, which is like an adult human becoming a small child again, effectively making itself immortal.
7. The smallest jellyfish are in the genera Staurocladia and Eleutheria. They have bell disks from 0.5 mm to a few millimetres in diameter, with short tentacles. Many of these tiny creeping jellyfish cannot be seen in the field without a hand lens or microscope. The largest is either the lion's mane jellyfish, arguably the longest animal in the world. Its fine, thread-like tentacles can be up to 36.5 metres (120 ft) long; or Nomura's jellyfish, which lives in the waters of Japan, Korea and China. They can reach 200 centimetres (79 in) in bell (body) diameter and about 200 kilograms (440 lb) in weight.
8. Jellyfish can be deadly. The uncoiling of the jellyfish’s stingers is one of the fastest actions in nature - they shoot faster than a bullet from a gun. Their stings can cause anything from mild irritation to death within minutes, depending on the species. The most deadly is the box jellyfish. A single box jellyfish has enough venom in it to kill 60 people. More people are killed by box jellyfish per year than by Sharks. A sting from one can stop a person's heart in two minutes. Stings from the less lethal varieties are best treated by getting out of the water and washing the wound with salt water (not fresh water as that can make it worse), and scraping it off with a knife or a credit card. Contrary to popular belief, peeing on a jellyfish sting is not effective.
9. In some parts of the world, jellyfish are considered a delicacy. In Malaysia, people call them “music to the teeth.” The most commonly eaten jellyfish is the Cannonball jellyfish, and they are prepared by drying them out. Jellyfish are also harvested for collagen, which is used to treat rheumatoid arthritis.
10. Jellyfish are the most energy efficient swimmers of all animals. They move by expanding and contracting their bell-shaped bodies to push water behind them.