Jellyfish are not fish at all. They are plankton, and often have little or no swimming skills, mostly just floating along in the direction of the ocean current. They're mainly found in the sea, but a few do thrive in freshwater lakes and rivers as well. There are over ten thousand species of jellyfish, and they've been around for over 650 million years. Some of the well-known ones are 'Lion's Mane', a jellyfish that actually featured in a Sherlock Holmes tale, and Box jellyfish and Irukandji jellyfish. The last two are the deadliest and their stings can be fatal. Many people die every year from getting stung by jellyfish.
Some jellyfish are tiny, some can grow to be seven feet long or more, and some even actually have tentacles that stretch up to 100 - 120 feet and more. Often enough, these miniatures and some of these monsters get washed ashore and die almost immediately. They cannot survive out of water, as nearly 98 % of their body is water content.
The body is usually transparent, gelatinous, shaped like a cup or a bell, and nearly completely filled with water. It can be quite heavy, over 450 pounds or more. It adheres to a radial symmetry, which means that if you halve a jellyfish from any side, the two halves will always be equal.
They come in various colors of various brightness―light blue, deep blue, yellow, purple, lilac, orange-red, etc.―and look rather beautiful when seen from afar. They glow in dark waters. Their very simple nervous system reacts to light and gravity.
A jellyfish has an outer epidermis covering, an inner gastrodermis lining, and, between these two, there's a jelly filling called mesoglea. Then they have a multipurpose gastrovascular cavity and orifice. The former combines the functions of gullet and stomach and intestine, and the latter doubles as a mouth and anus. The orifice is surrounded by tentacles with sensory nerves, but otherwise, it is quite unhampered by the presence of bones or brains.
Heartless too, as you will realize if you get a tad too close. Getting up-close and personal with them is really not a good idea. They tend to sting, and if the deadly venom doesn't kill you in three minutes, it'll make for several days or even weeks of severe pain.
The sting is usually to stun prey or attackers. It comes from the stinging threads in the nematocysts in the cnidoblasts. The cnidoblasts are the cells found on the tentacles. When the tentacles last out, the venom is injected into the prey or the attacker. Their venom is being studied and used in medical research. It may help produce an antidote against other venom, and may also help in treating cancer and various other diseases.
Medusae―adult jellyfish―are either male or female, and have an interesting way to reproduce. After the gonads either produce eggs and sperm, the male Medusa releases the sperm from its orifice, and these swim into the female's orifice and fertilizes the eggs there. On hatching, the resulting Planula attach themselves to rocks or any other solid surface on the sea floor and turn into flower-like polyps. Later the polyps develop into jellyfish. It is related to corals, sea anemones, and the Portuguese Man-o-War.
Jellyfish are eaten by predators like banner fish, sun fish, spade fish, loggerhead turtles, arrow crabs, and other sea creatures. They are also eaten by human beings. Jellyfish salad is considered a delicacy in Japan and China.
Due to certain climatic changes, not to mention the fast disappearing of the animals that preyed on them, there has been an explosion of the jellyfish population. They have been sighted is extraordinarily large numbers around Japan and Australia, and also in certain parts of Europe.