Jellyfish are a non-polyp form of individuals, of the phylum Cnidaria. Within the phylum, Scyphozoans are considered true specimens of jellyfish, but some hydrozoans that live in freshwater are also included. Their class name comes from the Greek name 'skyphos', which means a drinking cup.
Jellyfish can be found in every ocean in the world. Their lifespan is between three and six months. Jellyfish have two body forms through their life cycle―the polyp stage and the medusa stage. In the polyp stage, they are in the form of a sessile stalk with their mouth and tentacle facing upwards. The second stage of the jellyfish body structure is more recognizable. During this stage, they have an umbrella-shaped body called the bell. This body structure is known as the medusa and tentacles of the jellyfish hang from the border of the bell. The body of these fish comprises 90% to 94% water.
- Jellyfish are made up of an epidermis, gastrodermis, and mesoglea. They do not have a central nervous system,a circulatory system, respiratory system, or a osmoregulatory system. They have an incomplete digestive system and therefore use the same orifice for intake of food and expulsion of waste materials.
- Jellyfish don't have a brain, but they do have a different sensory organ called rhopalia around their bell. The rhopalia and nervous system help these fish to identify light and odor. The jellyfish use their 'nerve net' to detect the touch of another organism.
- A group of jellyfish is called a 'smack'. They feed on small protozoa, large metazoa, and other small fish in the sea. They generally trap these in their tentacles. Thought note that some species of jellyfish do not have tentacles at all.
- The male jellyfish releases its sperm into the water, which then travels to the mouth of the female jellyfish. This procedure allows for the fertilization of the ova. Most of these fish lodge the eggs in their oral cavities, forming a brood chamber for fertilization.
- The tentacles of a jellyfish are an important defense mechanism. Each tentacle is covered with stinging cells, known as cnidocytes.
- Jellyfish do not have any water motion, or are not hydrodynamic. This hampers their swimming speeds. It is necessary for them to create water currents which reaches their tentacles. They make this possible by opening and closing their bell shaped bodies in a rhythm.
- Jellyfish swim by contracting and expanding their bodies. They do not have scales or shells. If exposed to the hot sun, they disappear, leaving only a circle of film.
- Jellyfish have a defense mechanism of oral arms or tentacles which are covered with organelles called nematocysts. These nematocysts are paired with a capsule which contains a coiled filament that stings. The filament unwinds and launches into the target, thereby injecting toxins upon contact by foreign bodies.
- Jellyfish are generally not dangerous to humans. However, some can be very toxic, and may be fatal. Recently, two deaths attributed to these fish were reported in Australia. A sting from these fish is extremely painful and can also cause various allergies in humans.