Jellyfish range from the size of a thimble or the eraser tip of pencil to approximately eight feet in diameter with tentacles that reach 200 feet. That is as long as two blue whales.
Each jellyfish tentacle is armed with thousands of cells called cnidoblasts. Inside the cnidoblasts are nematocysts, each of which contains a coiled stinging thread. When a fish or other object becomes tangled in the tentacles, the pressure inside the nematocysts causes the venomous threads to uncoil like a spring-loaded harpoon.
The jellyfish’s main defense mechanisms are its stings and its transparent body, which makes it easy for it to hide.
In an episode of Friends, one of the characters applies urine to a jellyfish sting. However, neuroscientists say urine is ineffective.
Despite popular belief, urine is not an effective treatment for jellyfish stings
Many neurosurgeons say that popular remedies for stings—such as vinegar, urine, or meat tenderizers—do not help stings. Instead, the best thing to do is get out of the water immediately, and wash the affected with salt water (NOT fresh water). Salt water will deactivate the stinging cells, while fresh or tap water will reactivate them. The best way to remove the cells is with something such as a credit card.
Jellyfish have a short tube that hangs down from its body. The tube acts as both its mouth and its digestive tube. In some jellyfish, the central tube is surrounded by frilly pieces that look like curly ribbons in the water. These are called oral arms or mouth arms.
Jellyfish do not have brains, hearts, ears, heads, feet, legs, or bones. Their skin is so thin that they can breathe through it.
While jellyfish do not have a brain, they have an elementary nervous system with receptors that detect light, vibrations, and chemicals in the water. These abilities, along with the sense of gravity, allow the jellyfish to orient and guide itself in the water.
Jellyfish are invertebrates, which means they are animals without a skeleton. Approximately 95% of their body is water.
Jellyfish are usually seen in shallow coastal water; however, scientists have discovered a few species that live at depths of 30,000 feet (9,000 meters). While most jellyfish prefer warm water, some live in subarctic temperatures.
A jellyfish tentacle can still sting even if it is separated from the jellyfish’s body.
The smallest jellyfish in the world is the creeping jellyfish. It has bell disks from 0.5 mm to a few mm in diameter. It reproduces asexually by splitting in half. Another contender for the smallest jellyfish is the extremely toxic Australian Irukandji, which is only the size of a fingernail.
The largest jellyfish in the world is the Nomura’s jellyfish. Other candidates for the largest jellyfish are the Lion’s mane jellyfish and the Stygiomedusa gigantea.
The giant jellyfish called Stygiomedusa gigantea has been seen only 17 times in the last 110 years.
The man-of-war is not a true jellyfish; true jellyfish usually have four oral arms
The Portuguese man-of-war looks like a jellyfish, but it is not a true jellyfish. In fact, it is not even a single animal. It is a siphonophore, which is an animal made up of a colony of organisms that work together. Specifically, the man-of-war consists of four separate polyps. The top polyp is a gas-filled bladder that reminded people of the sails on a once common ship called a man-of-war.
Jellyfish produce both asexually and sexually. They are usually male or female, though hermaphroditic species have also been found.
Jellyfish live from a few hours to many months. One species has been reported to live as long as 30 years. Jellyfish in aquariums typically live longer than those in the wild. Because jellyfish are so fragile, it’s easier for people to capture jellyfish in the polyp stage when they are the least vulnerable.
Most jellyfish are passive carnivores. They feed on plankton, crustaceans, other jellyfish, fish eggs, and small fish. They eat and void through the same hole in the middle of the bell.
Jellyfish have several predators, including other jellyfish, sharks, tuna, swordfish, sea turtles, and one species of Pacific salmon.
One species of jellyfish, the Turritopsis nutricula, is also known as the “immortal jellyfish” because it can transform from the mature medusa state back to the polyp state, thereby essentially avoiding death. It accomplishes this through the cell development process called transdifferentiation.
The type of jellyfish most seen on the shores of North America and Europe is the Moon jellyfish. This type of jellyfish is typically blue or pink and is found in waters approximately 20 feet (6 meters) deep. Its sting is usually mild, but can leave an itchy, red rash.
Even a dead jellyfish can sting.
Jellyfish have lived in the waters of the world for more than 650 million years, long before the dinosaurs, making them the oldest multi-organ animal.
Portuguese men-of-war can grow to be 165 feet (50 meters) long, although 30 feet is more typical. On a baseball diamond, they would reach from home plate to first base and more than halfway to second. Men-of-war can sometimes be seen in groups of 1,000 or more floating in warm ocean waters. To avoid threats, they can deflate their air bladder and briefly sink under the water.
The upside-down jellyfish does not float in the water. Instead, it anchors itself to the ocean floor, with its short arms and tentacles reaching up. It looks like a bowl of plants rather than a typical jellyfish.
Crabs have been known to hitchhike on jellyfish
Crabs have sometimes been seen hitching a ride on a jellyfish. The hard, outer shell of the crab protects it from the jellyfish’s sting. The crab also usually catches a little of the food the jellyfish traps.
There are more than 2,000 different types of jellyfish. Approximately 70 can hurt people, with the box jellyfish (Chironex fleckeri) being the most dangerous. Scientists believe that there could be as many as 300,000 different species of jellyfish yet to be discovered.
Jellyfish are not fish. They are actually plankton from the phylum Cnidaria (Greek for “stinging nettle”) and the class Scyphoza (from the Greek “cup”). Some aquariums are trying to popularize the terms “jellies” or “sea jellies” instead of jellyfish.
Jellyfish move in essentially two ways. They take water in their bell and then squirt it out behind them, which creates a jet of water that moves them forward. They also drift on water currents.
The body of a jellyfish has six parts. The inner layer is the gastrodermis, which lines the gastrovascular cavity. The middle layer is a thick substance called the mesoglea. The outer layer is the epidermis. It also has an orifice and tentacles.
Researchers suggest that the shipping industry has artificially distributed jellyfish into nonnative habitats where they then colonize. Ships transport jellyfish around the world in two ways: 1) juvenile jellyfish (polyps) attach to ship hulls and travel with them and 2) ships take on ballast water in originating harbors and then dump their ballast water (along with jellyfish and other organisms) in new waters. Billions of gallons of ballast water are transported annually around the globe.
Jellyfish are aggressive colonizers. Eight years after comb jellyfish were introduced into the Black Sea in 1982, they totaled about 900 million tons. Jellyfish have caused $350 million in losses to the Black Sea’s fishing and tourism industries.
Environmental stress may increase jellyfish swarms. Jellyfish are one of the very few creatures that can adapt to ocean dead zones, or zones where there is little oxygen and lots of pollution. There are over 400 marine dead zones in the world.
A group of jellyfish is called a bloom, swarm, or smack.
Environmental stressors may increase the size of a jellyfish bloom
Jellyfish blooms near Japan can have over 500 million jellyfish, with each jellyfish the size of a refrigerator.
Jellyfish have been responsible for temporarily shutting down a nuclear power plant in Japan after becoming stuck in its cooling system. The Japanese called these jellyfish echizen kurage or, in English, Nomura’s jellyfish.
Jellyfish account for more than 10 times the weight of the annual fish catch around the world.
Approximately 150 million people annually are exposed to jellyfish around the globe. Around 200,000 people are stung each year in Florida and 500,000 around the Chesapeake Bay.
Many scientists believe that environmental stressors—including climate change, pollution, overharvesting of fish, and dams—have led to the proliferation of jellyfish.
Jellyfish are harvested for collagen, which is used in a variety ways, including treating rheumatoid arthritis.
In 2007, jellyfish blooms decimated Northern Ireland’s salmon farm killing more than 100,000 fish.
Jellyfish belong to the Phylum Cnidaria. All animals in this group are radially symmetrical. Other animals in this phylum include hydras, sea anemones, and corals.
In some countries, jellyfish are a delicacy
In several parts of the world, jellyfish are considered a delicacy. For example, in Malaysia, people call them “music to the teeth.” Approximately several hundred metric tons of jellyfish a year are eaten at $15 a pound, making it a multimillion-dollar business. The most commonly eaten jellyfish is the Cannonball jellyfish.
An American tourist died in 2002 after being stung by the deadly Irukandji jellyfish off the waters of Australia’s Great Barrier Reef. This jellyfish is only the size of a thumbnail but can have three-feet-long tentacles. Most jellyfish have stingers on their tentacles, but the Irukandji jellyfish has stingers on its bell piece as well.
An adult jellyfish is named a “medusa” (plural “medusae”), after the Greek monster Medusa that had snake for hair.
In the 2008 movie Seven Pounds, the character Ben Smith, an IRS agent played by Will Smith, commits suicide by sharing his bathtub with a deadly jellyfish. The title refers to the “seven ponds” of flesh Will Smith gives to other people to make up for killing seven innocent people in a car accident.h
Jellyfish digest their food quickly. It would be difficult to float if they had to carry around large amounts of food.
The venom in a single box jellyfish can kill 60 people.
Jellyfish kill more people than sharks do each year.
Different jellyfish have babies in different ways. Some jellyfish shoot eggs out of their mouth to be fertilized outside the body; others carry the eggs in their mouth until it develops enough to live on its own.
Most jellyfish have two basic life stages. In the first stage, they are called polyps and grow by making buds, like plants. The polyp then buds off a young jellyfish called an ephyra. After a few weeks, the ephyra becomes an adult jellyfish or medusa.
A dead jellyfish can still sting
Jellyfish are made up of more than 95% water. If they are removed from the water, they collapse and die.
The box jellyfish is unique because it has 24 eyes, four parallel brains, and 60 anal regions. Two of its eyes can see color. It is also one of the animals in the world that has a 360-degree view of its surroundings Box jellyfish are not considered “true” jellyfish of the Scyphozoan class.
Jellyfish that stay near the sunlight at the water’s surface tend to be colorless. Jellyfish that swim deeper are often red, purple, green, yellow, and sometimes they may even have stripes.
A military robotic jellyfish has been created to go on search-and-rescue and survey missions. The silicone “Robojelly” uses hydrogen and oxygen for fuel as it swims, so its only “exhaust” is heat and water. Its muscles are made from shape-memory alloy, a metal invented by NASA that “remembers” its original shape.
The top of the jellyfish that looks like the top of a mushroom is called the bell.
Scientists place true jellyfish and box jellyfish into two different groups. True jellyfish are scyphozoans, meaning “bowl animals.” Box jellyfish are cubozoans, meaning “cube animals.” A true jellyfish’s bell is rounded, like a bowl. A box jellyfish bell has sides and rounded corners—shaped similar to a box.
Scientists have developed antivenom for box jellyfish stings. In Australia, where box jellyfish live, ambulances and hospitals keep the life-saving venom on hand.
The Lion’s mane jellyfish is one of the largest of the true jellyfish and can have tentacles up to 120 feet (36.6 m) in length. Its bell can measure up to 7 feet, 6 inches (2.3 m) across. It lives in oceans around the world and feeds on small fish, shrimp, and other small jellyfish. Its sting is painful, but not deadly for humans.
The deadliest and most venomous sea creature in the ocean is the box jellyfish, which is also known as the “Marine Stinger” and the “Sea Wasp.”
The venom in a single box jellyfish can kill 60 people
On average, one person per year is killed in Australia by a box jellyfish. Box jellyfish may kill as many as 100 other people per year in other parts of the world, such as the Philippines, Indonesia, and Thailand.
Both true jellyfish and box jellyfish use their stinging tentacles to catch shrimp and small fish for food. However, box jellyfish are more active hunters, mainly because they have eyes, unlike true jellyfish. They are also better swimmers. A box jellyfish can swim at speeds of up to 4 mph, which is faster than most people can swim.
The uncoiling of the jellyfish’s small stingers is one of the fastest actions in nature. Stingers shoot out even faster than a bullet from a gun.
A box jellyfish sting can kill a person by stopping his or her heart in about two minutes.