Distribution: Jellyfish are found in every ocean, from the surface to the deep sea.
Description: Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish.
Ecology: 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.
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. 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 pass through 2 distinct life history phases (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;
Jellyfish polyps may be solitary or colonial, and some bud asexually by various means, making more polyps. Most are very small, measured in millimeters. 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. Jellyfish are dioecious; that is, they are either male or female. In most cases, to reproduce, both males and females release sperm or eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms.
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. Jellyfish lifespans typically range from a few hours with most of the large coastal jellyfish living about 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter. They feed continuously and grow to adult size fairly rapidly. After reaching adult size (which varies by species), jellyfish spawn daily if there is enough food in the ecosystem. In most jellyfish species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn.
Interesting facts: Interesting facts Jellyfish are, by the nature of their life cycles, "bloomy". Their presence in the ocean is usually seasonal, responding to the availability of prey, which is seasonal in most places, increasing with temperature and sunshine in the spring and summer. Ocean currents tend to congregate jellyfish into large swarms or "blooms", consisting of hundreds or thousands of individuals. The formation of these blooms is a complex process that depends on ocean currents, nutrients, temperature and ambient oxygen concentrations. Increased nutrients in the water, ascribed to agricultural runoff, have been cited as an antecedent to the proliferation of jellyfish. Ecosystems in which there are high levels of nutrients provide nourishment for the small organisms on which jellyfish feed. In waters where there is eutrophication, low oxygen levels often result, favoring jellyfish as they thrive in less oxygen-rich water than fish can tolerate. The fact that jellyfish are increasing is a symptom of something happening in the ecosystem.
Blue Blubber Jellyfish (Catostylus mosaicus) from the Baltimore National Aquarium.
Venomous species from the coastal waters of eastern and northern Austrailia.