Jellyfish are intriguing beings from their life cycles to the way that they carry out basic life functions. Jellyfish are classified in the pylum Cnidaria. Also grouped into this category are sea anenomes, hydras and corals to name a few. There are over 9,000 species that make up this pylum; they are divided into three classes: Anthozoa, Hydrozoa and Scyphozoa. Jellyfish are members of the latter. They have a plethora of unique features and characteristics, making them one of the most insteresting sea-inhabiting species.
As mentioned above, jellyfish are Cnidarians. The defining traits of creatures in this pylum are: radial symmetry, a sac-like digestive tract, a nerve net, stinging tentacles and two cell layers. The flexible appendages known as tentacles are evenly arranged around a central mouth, thus making the jellyfish radially symmetrical. Since they are invertebrates, jellyfish do not possess a backbone or spinal column (Hence the simile, "As spineless as a jellyfish").
Jellyfish exhibit what is known as a nerve net, which is located in the epidermis. This simple nervous system is a network comprised of nerve and receptor cells. Contrary to humans, impulses in jellyfish do not travel along determined pathways. Therefore, when any part of the jellyfish is stimulated, the entire being responds to that stimulation, not just a specific part of it.
Like all members of the Scyphozoa class, jellyfish have an umbrella-like structure called a medusa. The medusa consists of two membranes: the gastrodermis ( the inner membrane) and the epidermis (the outer membrane). Located between the two membranes is the mesoglea, a low density substance which facilitates floating. Due to their non-hydrodynamic structures, jellyfish are not skilled swimmers. Floating is achieved through contractions of the medusa in a cyclical fashion, allowing the creature to pulsate through the water.
Respiration is made easier by the thinness of the membranes. Oxygen is diffused directly into the cells of the jellyfish from the water. Carbon dioxide is diffused in the opposite direction, traveling through the membranes and out into the sea. Respiration (the exchange of oxygen and carbon dioxide) takes place over the whole surface of the jellyfish.
Located inside the tentacles are cnidoblasts. These are stinging cells which contain coiled threads with a spike at the end; these coiled threads are called nematocysts. Nematocysts can be released in response to both mechanical and chemical stimuli. Nematocysts cannot be re-used however, new ones are produced to replace those that were discharged. The stinging tentacles are used to subdue prey. They are also used as a defense mechanism for jellyfish to protect themselves against predators. The nematocyst discharges into the prey, incapacitating it via a paralyzing toxin in the barb. The jellyfish then contracts its tentacles to bring its prey into its mouth which is located in the center of the medusa. Extracellular digestion takes place in a sac-like tract. Here the food is broken down by digestive enzymes. Waste products are ejected from the mouth. Jellyfish have what is called a two-way digestive tract, meaning that food is ingested from the same cavity that waste is expelled from. Stinging prey with tentacles is only one of the two ways that the jellyfish can obtain food. The second is by catching plankton on its medusa via a mucus-like substance. Cells containing cilia then push the food into the mouth.
The jellyfish population is comprised of both male and female sexes. Testes and ovaries form a four leaf clover shaped patten in the medusa. The male jellyfish produces sperm which swims out of the mouth of the male and into the mouth of the female. As you can see, the mouth is a very versatile part of the jellyfish. The eggs are fertilized in the ovary and released into the water where they undergo embryonic development. Some species release their gametes into the water and external fertilization takes place. Before becoming an adult jellyfish, the zygote undergoes various stages of deveopment.
This brings us to the lifecycle of the jellyfish. During the first embryonic stage, the zygote divides into two halves. Mitosis continues until the morula (a solid ball of cells) is formed. Subsequent to the morula phase, a blastula is formed. A blastula is a hollow ball of cells. Once the external cells of the blastula become ciliated, a planula (a swimming larva) is formed. The planula usually adheres itself to a solid substrate and becomes a polyp. The polyp consists of a digestive tract, tentacles and a mouth. Immature jellyfish (ephyra) then break off from the polyp. This is a form of asexual reproduction called budding during which an extension of the parent seperates and becomes a new individual.
Caution should be exercised when swimming near jellyfish. Most stings are painful, but otherwise harmless. Some are more dangerous and can be fatal. Stings inflicted by the sea wasp and the box jelly have proved to be deadly. It is recommended that you exit the water immediately if stung. Serious stings can cause anaphylaxis, a potentially fatal allergic reaction. Remaining in the water while experiencing anaphylactic shock can result in drowning.