Jellyfish are best known as free-swimming marine animals consisting of a gelatinous umbrella-shaped bell and trailing tentacles. The bell can pulsate for locomotion, while stinging tentacles can be used to capture prey. Continue reading to see more.
5. 95% Water
The manubrium is a stalk-like structure hanging down from the center of the underside, with the mouth at its tip. This opens into the gastrovascular cavity, where digestion takes place and nutrients are absorbed. Jellyfish do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. They have limited control over movement, but can use their hydrostatic skeleton to navigate through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are mostly passive.
The body is composed of over 95% water; 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 is referred to as gastrodermis, which lines the gut.
Some jellyfish have ocelli: light-sensitive organs that do not form images but which can detect light, and are used to determine up from down, responding to sunlight shining on the water's surface. These are generally pigment spot ocelli, which have some cells (not all) pigmented. Certain species of jellyfish, such as the Box jellyfish, have been revealed to be more advanced than their counterparts.
The Box jellyfish has 24 eyes, two of which are capable of seeing color, and four parallel information processing areas or rhopalia that act in competition, supposedly making it one of the few creatures to have a 360 degree view of its environment. It is suggested that the two eyes that contain cornea and retina are attached to a central nervous system which enables the four brains to process images. It is unknown how this works, as the creature has a unique central nervous system. The eyes are suspended on stalks with heavy crystals on one end, acting like a gyroscope to orient the eyes skyward. They look upward to navigate from roots in mangrove swamps to the open lagoon and back, watching for the mangrove canopy, where they feed.
Jellyfish reproduce both sexually and asexually. Upon reaching adult size, jellyfish spawn daily if there is enough food. In most species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn.
Jellyfish are usually either male or female (hermaphroditic specimens are occasionally found). In most cases, adults release sperm and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms. In a few species, the sperm swim into the female's mouth fertilizing the eggs within the female's body where they remain during early development stages. In moon jellies, the eggs lodge in pits on the oral arms, which form a temporary brood chamber for the developing planula larvae.
Jellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months. Life span and maximum size varies by species. Jellyfish held in public aquariums are carefully tended, fed daily even when food might be seasonally rare in the wild, and sometimes treated with antibiotics if they develop infections, so may live several years, though this would be very unusual in the sea.
Most large coastal jellyfish live 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter. One unusual species is reported to live as long as 30 years. Another unusual species, T. nutricula, falsely reported as Turritopsis dohrnii, might be effectively immortal because of its ability under certain circumstances in the laboratory to transform from medusa back to the polyp stage, thereby escaping the death that typically awaits medusae post-reproduction if they have not otherwise been eaten by some other ocean organism .
Jellyfish range from about one millimeter in bell height and diameter to nearly two meters in bell height and diameter; the tentacles and mouth parts usually extend beyond this bell dimension. The lion's mane jellyfish, Cyanea capillata, was long-cited as the largest jellyfish, and arguably the longest animal in the world, with fine, thread-like tentacles that may extend up to 36.5 metres (120 ft) long (though most are nowhere near that large).
They have a moderately painful, but rarely fatal, sting. Claims that this jellyfish may be the longest animal in the world are likely exaggerated; some other planktonic cnidarians called siphonophores may typically be tens of meters long and seem a more legitimate candidate for longest animal.