Jellyfish

Jellyfish belong to the taxonomic phylum Cnidaria and the class Scyphozoa. The name Cnidaria comes from the Greek word "cnidos," which means stinging nettle. Casually touching many Cnidarians will make it clear how they got their name when their nematocysts eject barbed threads tipped with poison.

Many thousands of Cnidarian species live in the world's oceans, from the tropics to the poles, from the surface to the bottom. Some even burrow. A smaller number of species are found in rivers and fresh water lakes.

There are four major groups of Cnidarians: Anthozoa, which includes true corals, anemones, and sea pens; Cubozoa, the amazing box jellies with complex eyes and potent toxins; Hydrozoa, the most diverse group with siphonophores, hydroids, fire corals, and many medusae; and Scyphozoa, the true jellyfish.

They are invertebrate marine animals, shaped with a spherical "head" with tentacles suspending from the "head", where 90% of the jellyfish's internal structures are contained. Jellyfish can be found in every ocean in the world, and most bodies of water. It's been estimated that there are over 200 different species of jellyfish, with more species being found all the time.

Anatomy


Although jellyfish are typically regarded as simple animals, they accomplish the same feats as many more complex animals enabling them to survive, grow, and reproduce. Jellyfish can, for example, prey on vertebrates, distribute food and oxygen to their tissues, remove metabolic wastes, sense changes in light and orientation, and use the sun to migrate. Jellyfish don't have eyes like we know them; some medusae have ocelli which are light-sensitive spots on the rim of the bell. Jellyfish have sensory organs called rhopalia, which form a row of small round structures along the rim of the bell. The rhopalia include sensory organs called statocysts that help maintain the jellyfish's balance. When a jellyfish tips too far to one side, the statocyst will stimulate nerve endings that cause muscles to contract, turning the jellyfish right side up. The Oral Arms, often thought to be tentacles, form a temporary brood chamber to accommodate fertilization. Jellyfish do not have a specialized digestive system, excretory system, respiratory system, and circulatory system. They are able to digest with the help of the gastrodermis that lines the gastrovascular cavity where nutrients from their food is absorbed. They do not need a respiratory system since their skin is thin enough that air can diffuse in and out of their body. They do not have a brain, a heart, a central nervous system, a skeletal system and also no bones and no blood. Jellyfish move using a hydrostatic skeleton that controls the water pouch in their body to manipulate their movements.

A jellyfish can regenerate small amounts of tissues that have been damaged.

Reproduction


Most Jellyfish are asexual, with some exceptions, because some are sexual and release huge amounts of sperm into the water which consummate with female Jellyfish's eggs which are released in huge amounts as well. Most Jellyfish have both female and male reproductive systems. A Jellyfish's reproductive cycle is shown in the upper photograph, but to elaborate:

Egg
The fertilized eggs begin their development either inside the female or in brood pouches located on the oral arms. After the fertilized eggs have undergone embryonic development, they leave the mouth or brood pouch as free-swimming planula larva. The planula larva is a short-lived stage in the jellyfish's life cycle.

Polyp
The planula larva eventually settles to the bottom where it attaches itself to a hard surface and transforms into a polyp. This polyp stage in the jellyfish life cycle is a sessile stage.

Polyp colony
The polyp develops into a strobilation polyp colony that is linked together by feeding tubes. The polyp colony, like the original polyp, is sessile.

Ephyra and medusa
The polyp colony reproduce asexually to bud off tiny free-swimming jellyfish (called Ephyra). The Ephyra grow in size and become the adult (Medusa) form of jellyfish.

Ecology
Most Jellyfish primarily drift throughout the water and feed on Zooplankton and micro-organisms. Jellyfish have varying numbers of tentacles equipped with Cnidoblasts. A Cnidoblast is a cell in the epidermis of coelenterates in which a nematocyst is developed. Each Cnidoblast has a trigger, the cnidocil, which is usually activated by physical contact but sometimes by chemical stimuli (such as minute quantities of animal juices in the water), causing the cell to burst open and extrude its contents. Inside each cnidoblast is a coiled thread, a nematocyst, which explodes out of the cell. A Nematocyst is a capsule within specialized cells of certain coelenterates, such as jellyfish, containing a barbed, threadlike tube that delivers a paralyzing sting when propelled into attackers and prey. Only those nematocysts located in the stimulated area are discharged. The mechanism for release is probably intensified water pressure in the cell, brought about by a sudden increase in the permeability of the cell membrane. Water rushes in and the pressure increases until cell pops open and the nematocyst is forced out, turning inside out as a sock does when pulled off one's foot. Thus the barb or coiled thread that is inside the nematocyst as it lies in the cnidoblast becomes the tip and comes into contact with its prey.

Evolution and Jellyfish
Due to being an invertebrates, jellyfish are seldom fossilized. However, some fossilized jellyfish have been found as far inland as Wisconsin!