It may be hard to believe, but the gossamer creatures known as jellyfish are a vital part of the marine food chain, both as predators and prey.
Jellyfish are carnivorous. Depending upon their size, they feed on zooplankton, ctenophores (comb jellies), other jellyfish, crustaceans, or even small fish. Different types of jellyfish may be active or passive hunters.
The active hunters are armed with tentacles coated with structures called "nematocysts." Each nematocyst contains a cnidocyte, a barbed and tethered stinging cell like a microscopic harpoon. Generally, these cnidoctyes deliver a toxin along with the sting. After the nematocysts do their job, the tentacles retract, and deliver the stunned prey to the jellyfish's mouth opening, at the bottom of the bell.
Passive hunters, in contrast, have very little sting in their nematocysts. Instead, their tentacles are extremely sticky. As the jellyfish drift tiny organic particles or micro-organisms from the surrounding water stick to the tentacles, and are then drawn up to the mouth. This is known as suspension feeding.
One type of jellyfish, known as the "Upside-down Jelly," doesn't hunt at all. For food, it depends upon symbiotic zooxanthellae, photosynthetic organisms that live inside the jelly's bell. Each day, the jellyfish moves up into the sunlight so that its tiny symbiants can produce food for themselves and their host.
The actively hunting jellyfish probably have the most direct impact on other creatures in the food chain in terms of competition for food. Large active hunters compete with such "advanced predators" as squid, tuna, and small sharks for their share of small fish and crustaceans.
Of course, jellyfish are often on the receiving end of predation, as well. The creatures are 95% water, so not densely nutritious. Nevertheless, the sheer mass of jellies in a given area makes it worth the while of predators to eat them. Jellyfish can reach a density of hundreds of animals per square meter of seawater. Their predators include sea turtles such as loggerheads and leatherbacks, sunfish, moonfish, swordfish, spadefish, salmon and tuna. Jellyfish are also taken by the most voracious predator in all the oceans-humans. Rhizostomeae jellyfish are a delicacy in China, Korea and Japan, and the yearly catch for human consumption is more than 321,000 tons. (Incidentally, I had the opportunity to try jellyfish once in South Korea. I can report that it is flavorless, a bit crunchy, and springy between the teeth.)
Although we harvest significant quantities of 8 species of jellyfish in the Rhizostomeae family, we catch a lot more jellyfish predators. Through either purposeful fishing or by-catch, humans have severely reduced the numbers of such jellyfish predators as tuna, sunfish, salmon, swordfish, and sea turtles. As a result, jellyfish populations are booming. Huge "blooms" of jellies -anomalously high concentrations of the creatures- have been reported in the Gulf of Mexico, the Adriatic Sea, the Mediterranean, and the North Pacific. In recent years, jellies have massed off the coasts of Britain, Japan, West Africa, and California. The sheer number of jellyfish involved in these cases can have serious consequences for nearby humans; fishing nets come up weighted with nothing but jellies, swimmers are forced to retreat to dry land to avoid the squishy, stingy masses, and a Japanese nuclear facility in Shizuoka found its cooling-water intake valve clogged with jellyfish. The nuclear plant had to slow electricity production while the pipes were flushed out.
Jellyfish are a lynch-pin species in the ocean food chain. They serve as food for a number of adult predatory fish, cephalopods, and turtles. They also thin the ranks of micro-organisms, fish larvae and small crustaceans. When the marine ecosystem is in balance, jellies serve as a moderating influence from their position in the middle of the food chain. Now, however, the food chain is out of whack. Large predators have been removed from the sea at an unsustainable rate. If over-fishing doesn't stop soon, jellies will burst their ecological niche. Then it will be "The Age of the Jellyfish" in all of Earth's oceans!