“The fact that jellyfish have survived for 650 million years despite not having brains is great news for stupid people.” Unknown
Jellyfish (or jellies) are non-polyp forms of the phylum Cnidaria and are marine animals that have a gelatinous “body” (umbrella) and tentacles (and are not actually fish because they are not vertebrates). They contain between 95%and 98% water and most of the umbrella mass is a gelatinous material (the jelly) called mesoglea which is surrounded by two layers of protective skin. The top layer is called the epidermis, and the inner layer the gastrodermis, which lines the gut. The skin is so thin that the body is oxygenated by diffusion and thus jellyfish don´t need a respiratory system. The body can pulsate for locomotion and the tentacles are used to catch prey.
There are jellyfish that are exclusively marine (scyphozoans) and others that live in freshwater (hydrozoans). Since they have been around for possibly 700 million years, they are the oldest multi-organ animals. A group of jellyfish is called a bloom or a swarm.
Jellyfish are called medusa in Spanish and Italian, a term also used by Linnaeus in 1752. In biology the term refers to the form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. The upper or aboral surface is called the exumbrella and the lower surface is called the subumbrella, where the mouth is located. The mouth may be partially closed by a membrane spreading inward from the side (called the velum). Jellyfish are very energy efficient swimmers, moving through the water by radially expanding and contracting their bell-shaped bodies to push water behind them. They pause between the contraction and expansion to create two vortex rings. The mesoglea is so elastic that the expansion is powered exclusively by relaxing the bell, which releases the energy stored from the contraction. By doing so, the second vortex ring rolls under it and begins to spin faster. This sucks in water which refills the bell and is pushed up against the center of the body, giving it a secondary boost forward (http://www.kurzweilai.net/jellyfish-energy-efficiency-to-improve-bio-inspired-robotic-designs-for-navy).
Most jellyfish don´t have eyes, but they have ocelli, organs that can detect light, which they use to distinguish up from down. Some species of jellyfish have better vision, like the box jellyfish, which has 24 eyes (two of them can see color) and four parallel information processing areas (rhopalia) that act in competition, supposedly making it one of the few creatures to have a 360-degree view of its environment.
The sea wasp box jellyfish, or marine stinger, from Australia is the most poisonous jellyfish in the world. They have fifteen tentacles, extending up to ten feet, and on each of them there are around half a million darts that are full of venom. Each jellyfish has enough venom to kill up to 60 adults. The venom may cause cardiovascular arrest within minutes and also attacks the nervous system and skin cells. It is said that the pain can be so severe that it may result in shock leading to drowning or heart failure on its own. If a sting is survived, the pain can go on for weeks.
1. a free-swimming marine coelenterate that is the sexually reproducing form of a hydrozoan or scyphozoan and has a nearly transparent saucer-shaped body and extensible marginal tentacles studded with stinging cells
2. a person lacking backbone or firmness (source: Merriam Webster).
The English word jellyfish comes from the words jelly and fish, called for its structure.
Dutch kwal comes from Middle High German qualle, which has the same root as kwellen, to swell.
Spanish and Italian medusa comes from Greek mythology. Medusa was one of the Gorgones, terrible and very ugly female monsters. Medusa was the only mortal one and on her head she had many snakes. Anyone who would look her in the eye would immediately turn into stone. The term referred to the animals because of their shape with tentacles that look like Medusas head with the snakes.
Jellyfish in other languages:
Dutch: de kwal
Spanish: la medusa
Italian: la medusa