It is safe to say that anyone who is familiar with these waters, such as tour guides, has at one time or another come into contact with jellyfish. These invertebrates, members of the phylum Cnidaria, are what cause pica pica ("itchy itchy"), that infamous rash-like condition that results from contact with thousands of unseen juvenile jellyfish. Because of the sting that jellyfish can inflict on humans, they are often seen as an annoyance, invaluable to the sea. In actuality, there is a lot to be discovered about these tropical water creatures.
Unlike what their name suggests, jellyfish are not fish, but invertebrates (lacking a backbone). They have no heart, no brain, no true eyes and are composed of 97% water. Jellyfish have two layers of cells, an outer layer (ectoderm) and an inner layer (endoderm) filled with a jelly-like substance called mesoglea. These creatures can range in size from a quarter of an inch to up to eight feet! Perhaps the most identifying feature of jellyfish are their tentacles, responsible for stinging and immobilizing prey. It is no surprise then, that adult jellyfish are given the name "medusae" after the mythological Medusa, a dangerous snake-haired woman who could paralyze humans by looking at them, turning them into stone. Naturally, jellyfish cannot turn their prey into stone, but they can inflict a nasty sting. Their tentacles are lined with stinging cells called cnidocytes that contain tiny harpoons called nematocysts. When a prey, such as plankton or larval fish, brushes up against the jellyfish, the nematocysts are shot into it, paralyzing the prey. Although the nematocysts usually leave humans with a relatively harmless stinging rash, there are reports of much more serious injuries. In Australia lives the sea wasp, a species of jellyfish that is capable of killing a human within three minutes of stinging!
Contrary to popular belief, jellyfish do not intentionally sting humans. When something comes into contact with a jellyfish, such as humans or potential prey, the tentacles involuntarily react with an explosion of thousands of stinging cells. After prey is paralyzed, the oral arms of the jellyfish capture the prey and the jellyfish begins to digest the prey by secreting enzymes onto the prey, while sweeping partially digested food into its mouth. When food is in short supply, jellyfish have an interesting survival technique of shrinking in size, thus requiring less food. It has been reported that jellyfish will also resort to eating other jellyfish in times of need. The primary predators of jellyfish, however, are sea turtles, ocean sunfish, and blue rockfish. Humans are also known to eat them; in Asian countries jellyfish are considered a delicacy when prepared salted and dried.
To reproduce, jellyfish use both asexual and sexual reproduction. When the jellyfish young or larvae begin life, they attach to a solid surface and grow into a polyp. These polyps then perform reproduction by cloning themselves, eventually forming into free-swimming jellyfish. These young jellyfish immediately learn to ride currents, using propulsion to move up and down in the water column. Depending on the species, the lifespan of jellyfish ranges from a few weeks to many years. These creatures are regular inhabitants of this area and play an important role in the ocean's food web. As much as they are sometimes seen as an annoyance to humans, we need to remember that jellyfish are an important food source for many different animals. Impacts such as global warming or pollution tend to have a negative impact on the delicate balance of the ocean's ecosystem, thus affecting the fate of many animals, including jellyfish.
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