How Jellyfish Work

Jellyfish are probably some of the most unusual and mysterious creatures that you’ll ever encounter. With their gelatinous bodies and dangling tentacles, they look more like something from a horror movie than a real animal. But if you can get past the weirdness — and the fact that getting too close to one results in a nasty sting — you’ll discover that jellyfish are pretty fascinating. They’ve been around for more than 650 million years, and there are thousands of different species, with more species discovered all of the time.

In this article, we’ll learn all about these mysterious animals and find out what to do if you do happen to get in the way of a stinging jellyfish tentacle.

Marine Life Image Gallery

Jellyfish live mainly in the ocean, but they aren’t actually fish — they’re plankton. These plants and animals either float in the water or possess such limited swimming powers that currents control their horizontal movements. Some plankton are microscopic, single-celled organisms, while others are several feet long. Jellyfish can range in size from less than an inch to nearly 7 feet long, with tentacles up to 100 feet long.

Jellyfish are also members of the phylum Cnidaria, (from the Greek word for “stinging nettle”) and the class Scyphozoa (from the Greek word for “cup,” referring to the jellyfish’s body shape). All cnidarians have a mouth in the center of their bodies, surrounded by tentacles. The jellyfish’s cnidarian relatives include corals, sea anemones and the Portuguese man-o’-war.

Jellyfish are about 98 percent water. If a jellyfish washes up on the beach, it will mostly disappear as the water evaporates. Most are transparent and bell-shaped. Their bodies have radial symmetry,which means that the body parts extend from a central point like the spokes on a wheel. If you cut a jellyfish in half at any point, you’ll always get equal halves. Jellyfish have very simple bodies — they don’t have bones, a brain or a heart. To see light, detect smells and orient themselves, they have rudimentary sensory nerves at the base of their tentacles.

A jellyfish’s body generally comprises six basic parts:

• The epidermis, which protects the inner organs
• The gastrodermis, which is the inner layer
• The mesoglea, or middle jelly, between the epidermis and gastrodermis
• The gastrovascular cavity, which functions as a gullet, stomach, and intestine all in one
• An orifice that functions as both the mouth and anus
• Tentacles that line the edge of the body

An adult jellyfish is a medusa (plural: medusae), named after Medusa, the mythological creature with snakes for hair who could turn humans to stone with a glance. After the male releases its sperm through its orifice into the water, the sperm swim into the female’s orifice and fertilize the eggs.

Several dozen jellyfish larvae can hatch at once. They eventually float out on the currents and look for a solid surface on which to attach, such as a rock. When they attach they become polyps — hollow cylinders with a mouth and tentacles at the top. The polyps later bud into young jellyfish called ephyrae. After a few weeks, the jellyfish float away and grow into mature medusae. A medusa can live for about three to six months.

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 nematocystscontaining 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.

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

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.  Any signs of an allergic reaction (shortness of breath,hives, wheezing) warrant immediate medical attention.

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

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 Chinaand 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.

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

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 crampsand difficulty breathing.

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

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.

Image courtesy Florida Keys National Marine Sanctuary A moon jellyfish

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.

Portuguese Man-o’-war   Image courtesy NOAA A Portuguese man-o’-war isn’t a jellyfish, although it resembles one. They’re actuallysiphonophores — floating colonies that include four individual animals, each with its own role (for example, stinging, feeding, movement and reproduction). Named for the 18th century ship that it resembles, the Portuguese man-o’-war is a member of the same phylum,Cnidaria, as the jellyfish. Its sting can be very painful and can lead to symptoms such as chills, fever, nausea, vomiting and shock. In some cases, the stings are fatal.