Jellyfish make up one of the oldest groups of creatures that roam the world’s oceans, fossil records showing certain specimens very similar to modern ones existing 500-700 million years ago. Jellyfish are extremely simple animals, consisting of an umbrella-shaped bell and trailing tentacles that capture prey. They swim through the water by pulsating the gelatinous bell and squeezing the surrounding water behind them. Because Jellyfish are invertebrates and very far removed from true fish, biologically, it is becoming increasingly common to call them sea jellies in order to prevent the use of the misnomer suffix, “-fish”
Many species of jellyfish do not have specialized systems for digestion, respiration or even central nervous functions. On the underside of their bell, they have a manubrium, which is a stalk-like structure that features a mouth at the end. Food is passed by contraction of the tentacles up into the mouth where it is digested. Jellyfish do not have gills and have no need for respiration – their skin is so thin that they can oxygenate their body through diffusion of the surrounding seawater. The jellyfish body actually consists of 95%-98% water, with only a small percentage of biological matter serving to support the life of the creature.
Jellyfish generally do not have a nervous system, although many species support a network of loosely connected nerves that serve to control the jelly’s movements and metabolisms. Even in the most complex of jellyfish species, there is no structure even close to that of a mammalian brain, and the jellyfish is limited to floating along and eating whatever it may happen to come across. Some jellies have light-detecting organs that they can use to orient themselves in the dark waters of the ocean.
The tentacles of many jellyfish are extremely potent and dangerous weapons. Certain species of box jellyfish in the class Cubozoa are among the most venomous creatures in the entire world, and the nematocyst stinging mechanism is the world’s fastest biological reaction, with the discharge of poison taking place within a timeframe of a few thousand nanoseconds at impossibly high speed. Because the nematocysts are triggered automatically by ionic pressure, even beached and dead jellyfish can still sting and represent a clear danger to human beings. Most nematocysts can only discharge once before requiring a longer rest time to “recharge”, although very few jellyfish have any control over when and whether the discharge happens.