Jellyfish have one of the most, if not the most fascinating lifecycle systems in the animal kingdom in that they produce both sexually and asexually; and they produce in two different forms or stages of their lives. Few marine animals are as mysterious as these translucent gelatin-like creatures.
Classified as scyphozoa, with relatives such as hydrozoa, and cubozoa under the phylum cnidaria (the "c" is silent), "true" jellyfish, which are not really fish because they have no backbones, go through lifecycle stages in which they possess various body forms, with two forms, polyps and medusae, serving as their major forms.
Jellyfish are invertebrates with elementary nervous systems, otherwise known as nerve nets. Because jellyfish lack basic sensory organs and have no brain, nerve nets along with the aid of rhopalia (small sensory structures) assist the jellies in the detection of light, odor, and other stimuli, which of course aids in the jellies in finding food, and detecting enemies.
Jellyfish come in an assortment of sizes, shapes and colors. Most are bell-shaped and glassy or semi-transparent. Although some jellyfish may reach up to seven feet, their sizes usually range from under an inch to just over a foot across the bell. The tentacles, on the other hand, have been known to reach one hundred or more feet.
For the most part, jellyfish are rather fragile. They normally consist of less than five percent organic material. Mesoglea, or middle jelly, refers to the elastic jellylike substance located between the jellyfish's epidermis and its gastrodermis. The mesoglea produces most of a jellyfish's jelly.
Despite their lack of density, jellyfish are quite resilient as far as living environments go. They can be found in all major oceanic areas in the world, a few bodies of freshwater, and can withstand a wide range of water temperatures and salinities. Although most jellyfish are found in shallow waters along the coast, there are plenty who prefer to inhabit water depths as deep as twelve thousand feet!
Although jellyfish are mostly harmless, stings from two jellies of the cubozoa class, the box jellyfish (Chironex fleckeri), and the Irukandji - that has the deadliest most potent venom of all known species, can lead to anaphylaxis and even death; therefore, if stung, it is recommended that victims receive immediate first-aid attention.
Since jellyfish reproduction abilities vary, their lifecycles vary, and all jellyfish species don't reproduce the same way.
Although jellyfish are dioecious, meaning they consist of males and females, as stated earlier, their lifecycles take them through stages of both sexual and asexual reproduction. The medusa, the dominant group in the scyphozoa class, normally reproduces in the sexual stage while the polyps, known as scyphistomae, reproduce during the asexual stage.
During the sexual stage, male jellyfish spew sperm from their mouths into water columns via their coelentera (body cavities). From this point most female jellies take the sperm into their mouths enabling their eggs to become fertilized. Eggs from female moon jellies, however, become dislodged in female oral arms in what is known as brood pouches and are fertilized there. In either case, embryonic development occurs and eventually, tiny swimming plankton larvae called planulae emerge. These planulae are held in brood pouches, on a parent's tentacles, or in the male's coelentera until they swim to rocks or other hard surfaces in shady areas of ocean floors. The planulae stage of a jellyfish's lifecycle is very short-lived.
Attaching themselves to hard surfaces planulae enter a sessile stage, meaning they become stationary as opposed to free-swimming. Eventually, planulae develop into polyps. Polyps asexually reproduce forming sessile stalks during a process called strobilation. A sessile stalk is essentially a stack of interconnected polyps one atop the other that resembles a pile of capsized sow bugs.
In the complete process of strobilation, polyps asexually reproduce forming colonies from which tiny free-swimming jellyfish called "ephyra" bud off. Once separated from the colony, the free-swimming ephyra, in a short few weeks, grow into adult jellyfish. Jellyfish life-spans run from two and a half to approximately six months.
After sessile stalks have exhausted their ability to produce ephyra, they either die or regenerate themselves. Regenerating themselves allows them to again undergo the process of strobilation as asexual reproducers thus continuing to play significant roles in the jellyfishes' lifecycle.