The Jellyfish: Nearly All Water and Sting, But Neither Jelly Nor Fish
FlaglerLive | October 6, 2012
One of the least known animals in the ocean is the jellyfish. Comprised of 95 percent water and 5 percent a gelatinous material, jellyfish are found in all oceans and have more than 200 species. Believed to have existed on earth for over 650 million years, jellyfish predate dinosaurs. As their longevity on earth indicates, jellyfish are hardy survivors. They exist in all oceans, from the arctic to the tropics and from bays and estuaries to offshore and deep-ocean waters.
Jellyfish are not very sophisticated in their body development. They do not have a brain, heart, or eyes (with only one exception). They do not have bones or blood. Basically, they are packets of water and a combination of one percent nitrogen and carbon and three percent salt encased in two layers of tissue. The casing which contains this bell-shaped mass has an inner layer (endoderm) that lines its gut and an outer layer (ectoderm) that surrounds the jelly
Jellyfish have a simple nerve system to allow it to respond to food sources or danger from any direction. This system triggers responses to external stimuli, thereby allowing the jellyfish to trap food, realign its position in the water, and move slightly within their ocean world.
Jellyfish, along with corals and anemones, belong to the phylum Cnidaria (pronounced “Nih DAR e uh”). The “cnid” in Cnidaria refers to nettles, stinging barbs, called nematocysts. These microscopic weapons line the tentacles of the jellyfish and fire tiny venom-filled harpoons into organisms that brush against them. Then, appendages hanging from the bell near the mouth bring the startled prey to the mouth. In the rudimentary digestive system of the jellyfish, the mouth serves both for ingesting food and expelling waste. Thin canals transport nutrients throughout the body.
Locomotion for jellyfish is a fairly tame act. They have limited control over movement, so their mobility is partly a matter of passive drifting on waves and currents. However, they can regulate vertical movement to some extent, employing a kind of jet propulsion. The tissue on the underside of the umbrella contracts, resulting in the water being pushed out of the hollow bell in one direction, thus propelling the jelly in the opposite direction.
Ecologically, jellyfish are an important link in the marine food web. While they are not typically an element of the human diet, they are a source of food for numerous fish species, as well as for marine birds, sea turtles, and other jellyfish. While only a few societies favor eating jellyfish, there is another area of human interaction – stinging upon contact in the ocean.
When some jellyfish come in contact with humans, their released nettles may cause a burning sensation on the skin of humans. Most jellyfish stings are not very painful to humans, but may cause an allergic reaction in a small number of people.
In recent years, there has been considerable discussion among scientists about whether jellyfish are appearing in greater numbers around the world. Some say that climate change, ocean temperature variations, reductions in alpha predators, invasive species, and human interactions (pollution, runoff) have allowed massive blooms of jellyfish worldwide.
This is a difficult question to answer accurately because no one has tried to examine jellyfish blooms from all over the world before. When conditions are good (for example, the temperature is just right and there is plenty of food), it is normal for jellyfish to grow fast and reach large numbers. This just is part of the natural cycle of many species of jellyfish.
Locally, we have seen a number of jellyfish blooms along our shores this summer. One bloom in particular arrived in June. It was the moon jellyfish and there were thousands of them in the water and on the beach. Four visiting grandchildren at my house were intrigued by them and, despite my advice, insisted on picking them up and staring at them, without any stings.
Scientists are researching the data, so whether there are increasing masses of jellyfish destined to take over the oceans remains an unknown for now. As the answers are learned, I’ll report them here. In the meantime, be well and do something special for nature this week.