JellyFish Facts

True Jellyfish

True Jellyfish picture

True Jellyfish

True jellyfish, also called medusa, are all those jellyfish that belong to the phylum of Cnidaria. There are approximately two hundred species of true jellyfish, and all these species belong to the class Scyphozoa. Some examples of jellyfish that are classes as true jellyfish would be sea nettles, moon jellies, Lion's Mane jellyfish, Mediterranean Jellyfish, Blue Jellyfish and several other species that are lesser known.

The life of a true jellyfish is split into two stages. During the juvenile stage they are found in the polyp form, while in the dominant adult life stage they are found in the form of a medusa. Once they reach their adult stage, they are able to feed on a large variety of fish and crustaceans, which they get a hold of using their stinging cells or nematocysts. Nematocysts are connected to the tentacles of the jellyfish and then flow downwards from the umbrella dome.

The bodies of true jellyfish have four part symmetry and within their bodies is an internal gelatinous material referred to as mesoglea. There are no hard, durable body parts, as they have no skeleton, no head and no specialized organs or body systems for excretion or respiration. Most true jellyfish are made up of as much as ninety nine percent water. As a result fossils of true jellyfish are very rarely found.

A difference that can be seen between Scyphozoans and other Cnidarians is that they lack the circular membrane umbrella, called a velum that propels jellyfish through water. Because they don't have this locomotive device, they instead move by relaxing and contracting the muscles found in their simple umbrella. These periodic contractions and relaxations help it move through the water so it can catch prey or escape predators.

Similar to other Cnidarians, food and waste both pass in and out of the bodies of true jellyfish through the same opening. They have no digestive tract. The simple digestive cavity of these jellyfish has cilia which help move gases, dissolved food particles and water around. Smaller jellyfish feed on food particles that they find floating around the sea, while larger forms predate on fish or swimming vertebrates. The larger species accomplish this through use of their stinging cells to paralyse their prey with the venom released by their nematocysts.

The stinging cells are found on the jellyfish's oral arms, not on the peripheral tentacles. Many true jellyfish are harmless to humans, but some stinging jellyfish can cause some injury to swimmers. Even tentacles that have broken off the bodies of jellyfish can cause harm to humans. Some tentacles simply cause numbness that goes away in time and isn't harmful at all. There are also some species of fish that are immune to stings by true jellyfish, and even travel along with the jellyfish in hopes of food for themselves as well as for protection.

A large proportion of true jellyfish are free-swimming, pelagic forms found in the open ocean. There are small species that are planktonic, and one order of these jellyfish that are sessile and remain on the ocean floor. All true jellyfish are marine creatures, although a few species can be found in fresh water.

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Written by and Sudarsana Sinha.

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