Jellyfish are carnivores -- they eat other animals. Smaller jellyfish eat algae and other tiny plankton called zooplankton. Larger jellyfish eat crustaceans and other bigger aquatic animals. They don't seek out people to attack -- their nervous system is too simple to do that. Their sting is both a defense mechanism and a way to capture their prey.

Each jellyfish tentacle is covered with thousands of cells called cnidoblasts, which house nematocysts containing stinging threads. When a jellyfish encounters another object, pressure inside the nematocyst causes the threads to uncoil. The stinging cells spring out at the unwitting victim like tiny darts, firing venom into it. The venom is a neurotoxin designed to paralyze jellyfish prey. Although a jellyfish can kill a small aquatic animal, its sting is not usually fatal to humans. It tends to cause pain, skin rashes, fever and muscle cramps. The degree of pain and reaction to a jellyfish sting can depend on the species -- larger jellyfish have larger cnidoblasts that can penetrate deeper into the skin, and some jellyfish have stronger venom than others.



" " A jellyfish washed up on a beach. Image courtesy Captain Albert E. Theberge/ NOAA

When you're at the beach, watch out for jellyfish both on the water and on the sand. Even a tentacle that has been separated from its jellyfish can sting. If you do get stung, first remove any tentacles clinging to the skin. Don't wash the area with fresh water -- it could release more venom into your body. Instead, clean it with rubbing alcohol, ammonia, vinegar or urine (yes, you read right). You can also apply meat tenderizer or a mixture of baking soda and water. Any signs of an allergic reaction (shortness of breath, hives, wheezing) warrant immediate medical attention.

" " Jellyfish salad, an appetizer common in Chinese and other Asian cuisines Public domain

Jellyfish have excellent protection against predators: their stinging tentacles are a strong deterrent, and their transparent bodies help them hide. A few animals, such as loggerhead turtles, sunfish and spadefish, eat jellyfish. Some young fish actually live on or even in jellyfish. They hide out in the tentacles to avoid being eaten by predators until they mature. And some people -- especially in China and Japan -- also eat jellyfish, considering them a delicacy.

Aside from their occasional stings, jellyfish are not generally a nuisance. But in recent years, certain parts of the world -- namely Japan, Australia, and Europe -- have seen a problematic increase in jellyfish populations. Scientists believe the increase in jellyfish numbers may have to do with additional nutrients in the water, climate change or fishing along the coastlines. Dramatic population increases are called blooms. Some researchers are concerned that the increased numbers of jellyfish could compete for food resources with fish and other marine animals, and eventually bump out native local species. In large numbers, jellyfish also wreak havoc with local fishing industries by tearing holes in fishing nets and disrupting other fish populations.

" " A smack, or small group, of jellyfish Image courtesy Kevin Connors / MorgueFile

Jellyfish do best in their natural environment, but many aquariums have jellyfish tanks. People who capture and raise them in captivity must be very careful not to damage their fragile bodies. It's easier to collect jellyfish in the polyp stage, when they are less vulnerable. Ideally, they should be in a tank free from any sharp corners or obstacles on which they could hurt themselves. In addition, the water needs to have some flow to it because jellyfish primarily move with currents.

Here are just a few of the many different types of jellyfish:

Box jellyfish

This jellyfish looks like a square with its four sides -- hence the name "box." This subclass of 16 jellyfish species includes the sea wasp. Box jellyfish tend to gravitate toward the mouths of rivers and creeks, and their sting is very painful. People who have unwittingly been stung can experience intense muscle cramps and difficulty breathing.

" " Atolla wyvillei, a deep-sea jellyfish Image courtesy E.Widder/ NOAA Ocean Explorer

Deep-sea jellyfish

The name of this type of jellyfish says it all. Deep-sea jellyfish live in very deep waters, as far as 23,000 feet below the ocean's surface. They are usually dark-colored -- brown, violet or black.

Irukandji jellyfish

Irukandji are a type of box jellyfish found in Australia. Although they're small (about the size of a human thumbnail), their venom is extremely toxic. This type of jellyfish has cnidoblasts on its body as well as its tentacles. The Irukandji's sting is so painful and causes such severe symptoms that scientists have given them a name: Irjukadji syndrome. Symptoms include high blood pressure, vomiting, headaches, extreme cramping and pain, and a burning sensation. Irukadji syndrome can last up to two weeks, and there's no antidote. Doctors have found that magnesium infusions can bring some relief, but the syndrome can be fatal.

Moon jellyfish

This is the type of jellyfish most commonly seen on the shores of North America and Europe. This pink or blue jellyfish usually lives in waters about 20 feet deep. Its sting is mild, leaving a red, itchy rash.