Jellyfish | eAmazings

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Jellyfish

Tuesday, 26 May 2009 00:00

Jellyfish are not actually fish, some people consider the term jellyfish a misnomer, and American public aquariums have popularized use of the terms jellies or sea jellies instead. Others find the word jellyfish to be equally useful and picturesque. The word jellyfish is used to denote several different kinds of cnidarians, all of which have a basic umbrella sort of shape, including scyphozoans, staurozoans (stalked jellyfish), hydrozoans, and cubozoans (box jellyfish). Jellyfish don't have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems. They digest using the gastrodermal lining of the gastrovascular cavity, where nutrients are absorbed. They do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. They have limited control over movement, but can use their hydrostatic skeleton to accomplish movement through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are passive much of the time. Jellyfish are composed of more than 90% water; most of their umbrella mass is a gelatinous material - the jelly - called mesoglea which is surrounded by two layers of epithelial cells which form the exumbrella (top surface) and subumbrella (bottom surface) of the bell, or body. Jellyfish do not have a brain or central nervous system, but rather have a loose network of nerves, located in the epidermis, which is called a "nerve net." A jellyfish detects various stimuli including the touch of other animals via this nerve net, which then transmits impulses both throughout the nerve net and around a circular nerve ring, through the rhopalial lappet, located at the rim of the jellyfish body, to other nerve cells. Some jellyfish also have ocelli: light-sensitive organs that do not form images but which can detect light, and are used to determine up from down, responding to sunlight shining on the water's surface.

Most jellyfish undergo two distinct life history stages (body forms) during their life cycle. The first is the polypoid stage, when the animal takes the form of a small stalk with feeding tentacles; this polyp may be sessile, living on the bottom or on similar substrata such as floats or boat-bottoms, or it may be free-floating or attached to tiny bits of free-living plankton or even (rarely) fish. Polyps generally have a mouth surrounded by tentacles that face upwards, like miniatures of the closely-related anthozoan polyps (sea anemones and corals), also of the phylum Cnidaria. Jellyfish polyps may be solitary or colonial, and some bud asexually by various means, making more polyps. Most are very small, measured in millimeters or a fraction of an inch tall. In the second stage, the tiny polyps asexually produce jellyfish, each of which is also known as a medusa. Tiny jellyfish (usually only a millimeter or two across) pull away from the polyp by swimming, and then grow and feed in the plankton. Medusae have a radially symmetric, umbrella-shaped body called a bell, which is usually supplied with marginal tentacles - fringe-like protrusions from the border of the bell that are used to capture prey. (Medusa is also the word for jellyfish in Modern Greek, Finnish, Portuguese, Romanian, Hebrew, Serbian, Croatian, Spanish, French, Italian, Hungarian, Polish, Czech, Slovak, Russian and Bulgarian.) A few species of jellyfish do not have the polyp portion of the life cycle, but go from jellyfish to the next generation of jellyfish through direct development of the fertilized eggs.

Jellyfish are dioecious; that is, they are either male or female. In most cases, to reproduce, both males and females release sperm and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms. In a few species, the sperm swim into the mouth of the female, allowing the fertilization of the ova within the female's body. Moon jellies use a different process, in which the eggs become lodged in pits on the oral arms, which form a temporary brood chamber to accommodate fertilization and early development. After fertilization and initial growth, a larval form, called the planula, develops from the egg. The planula is a small larva covered with cilia. It settles onto a firm surface and develops into a polyp. The polyp is cup-shaped with tentacles surrounding a single orifice, resembling a tiny sea anemone. After an interval of growth, the polyp begins reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or new, immature jellies called ephyrae may be formed. A few jellyfish species are also capable of producing new medusae by budding directly from the medusan stage; such budding has been described from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae (each species bud only from one location). Fission of medusae (splitting in half) has been described for a few of species of hydromedusae. Some of the most common and important jellyfish predators are other species of jellyfish, some of which are specialists in eating jellies. Other predators of jellyfish include tuna, shark, swordfish, and at least one species of Pacific salmon, as well as sea turtles. Sea birds sometimes pick symbiotic crustaceans from the bells of jellyfish near the surface of the sea, inevitably feeding also on the jellyfish hosts of these amphipods or young crabs and shrimp.