Most free-floating jellyfish have an average lifespan of one to three years. Others may only live for a few months, while some can theoretically live forever. Juvenile polyps live on the seafloor attached to hard surfaces and may live for several years before becoming adults.
Jellyfish are part of the phylum Cnidaria. Cnidarians are a group of animals with stinging cells called “nematocysts.” There are over 10,000 cnidarians in the world’s oceans, but only about 4,000 are jellyfish.
Simple creatures, jellyfish do not live very long lives.
Jellyfish have two life stages: polyps and medusae. Juvenile polyps can live for several years while reproducing asexually. A medusa is the gelatinous adult form that we recognize as jellyfish. They must reproduce sexually and rarely live for more than a year in this stage.
Keep reading to find out the fascinating life cycle of jellyfish.
What Are Jellyfish?
Jellyfish are gelatinous free-floating animals that live across our world’s oceans. They are ancient animals, having roamed the oceans as many as 500 million years ago.
Jellyfish are a type of hydrozoa with a simple body plan. Their bodies comprise three layers: the epidermis, gastrodermis, and mesoglea.
The epidermis makes up the outside of their bodies. A layer of cells across the epidermis contains a “nerve net.” The nerve net is a basic network of nerves that allows jellyfish to interact with the world around them.
The gastrodermis is the inside layer that acts as the stomach, digestive tract, and anus. Jellyfish have a single opening from which they eat and expel waste. They also use this singular hole for reproduction. Jellyfish do not have stomachs or lungs. Instead, oxygen permeates through the cell walls.
The majority of a jellyfish’s body comprises the mesoglea. The mesoglea is a gelatinous material that sits between the epidermis and gastrodermis.
The mesoglea contains primarily water; a jellyfish’s body comprises about 95% water. The rest of the mesoglea consists of proteins, nerve cells, and muscle cells.
Another unique characteristic of jellyfish is their “nematocysts,” or stinging cells. They use their nematocysts to capture prey and ward off potential predators. The nematocysts contain barbs called “cnidocytes.”
When an animal touches a jellyfish, the cnidocytes shoot into its skin. Then, the cnidocytes release paralyzing venom that stuns or kills their prey. Most jellyfish are not venomous enough to cause much harm to humans.
There are four different types of jellies:
- Scyphozoa — about 200 species of “true” jellyfish
- Hydrozoa — 3,700 species — not “true” jellyfish
- Cubozoa — 36 known species — many produce venom and are more developed than Scyphozoans
- Staurozoa — about 50 species — stalked jellyfish that attach to rocks or seaweed
How Long Can Jellyfish Live?
The exact lifespan depends on the type of jellyfish. Life expectancy of adult jellyfish is short. Once they become adults, or medusa, most only live for a few months.
As juveniles, or polyps, jellyfish can live for several years. Some species can even live for several decades. Polyps remain in their juvenile stage for much longer.
How Do Jellyfish Reproduce?
Jellyfish have the special ability to reproduce both sexually and asexually. As juvenile polyps, they reproduce asexually, producing identical clones of themselves. As adult medusae, they reproduce sexually.
Polyps reproduce by budding. Budding refers to the polyp dividing itself in half. Doing so produces another polyp that is an identical clone of itself. Rather than producing more polyps, some polyps bud off adult medusa.
Adult medusae reproduce by spawning. Males and females release massive amounts of sperm or eggs into the ocean. Many times, entire jellyfish populations spawn at the same time.
Adult jellyfish are either male or female. There are almost no hermaphrodites. In most species, fertilization takes place right in the water. In other species, the sperm swims into the female’s mouth to fertilize the eggs she holds there.
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Jellyfish Life Cycle
We know jellyfish for their gelatinous bodies and long, flowing tentacles. Yet, jellyfish only spend a short amount of their lives in this stage. They spend the rest of their lives as polyps, or juvenile jellyfish. Polyps look nothing like their adult counterparts.
The life of a jellyfish begins as an egg. Fertilized eggs hatch into free-floating planulae larvae. They float about the surface waters for a time, then settle on the seafloor. They attach one end of themselves to a hard surface like a coral reef.
After attaching, the larvae develop into stalked polyps. They feed from small food that drifts past them. As the polyp continues to grow, it develops buds.
The buds develop into immature jellyfish called ephyra larvae. Although the polyp produces buds, the polyp can remain a polyp, producing buds for months or years to come.
Ephyra larvae look a lot like adult jellyfish but are smaller. After some time, the ephyra larvae develop into adult jellyfish called medusa.
Common Jellyfish Species & Their Lifespans
Although all jellyfish have similar lifespans, they vary between species. Here are some of the most common and popular jellyfish species and their lifespans.
1. Moon Jelly (Aurelia aurita)
Jellyfish Lifespan: Up to 25 years (polyp) & Up to 1 year (adult)
The moon jellyfish lives in the open ocean and coastal waters. They inhabit tropical to temperate waters in the North Atlantic Ocean. It is most common along the coast and in upwelling areas where there is more access to prey.
Moon jellies received their name because their bells are shallow. They also have short tentacles that make them look like the moon. They are not strong swimmers. So, beach-goers often find them washed up on beaches after strong tides or storms.
Unlike most animals, moon jellies seem to do better in areas affected by humans. The pollution and overfishing that we cause reduce the number of predators in the area. It also increases the moon jelly’s access to prey.
2. Lion’s Mane Jellyfish (Cyanea capillata)
Jellyfish Lifespan: Up to 1 year
The lion’s mane jellyfish is the largest species of jellyfish in the world. It uses its large, powerful tentacles to capture prey.
Their tentacles can grow up to the astounding size of 120 ft (36.6 m) in length. Some of these jellyfish are almost as large as the blue whale — the largest animal in the world. Yet, most only grow between 1.5-6.5 ft (0.45-2 m) in length. Despite their size, these jellyfish pose little danger to humans.
These jellyfish live in the cold open ocean of the Pacific and Atlantic Oceans, and the North Sea.
3. Crystal Jellyfish (Aequorea victoria)
Jellyfish Lifespan: Up to 2 years
Crystal jellyfish live in the North Atlantic waters off the coast of North America. They are completely colorless, making them appear crystalline. Their glassy bell holds about 150 delicate tentacles.
Not only are they completely transparent during the day, but they glow at night. They exhibit greenish-blue bioluminescence on spots around the edges of their bell. They have over 100 minuscule light-producing organs on the outer shell of their bell.
These jellies can also widen their mouths to eat other jellyfish as much as half the size of themselves.
4. Crown Jellyfish (Cephea cephea)
Jellyfish Lifespan: About 3-6 months
Crown jellyfish, or cauliflower jellyfish, have wart-like protrusions from their bell. They exhibit about 30 of these protrusions from the top center of their large bell. Researchers aren’t sure what these appendages are for.
They are huge jellyfish with bells that reach 1.5-1.9 ft (0.45-0.58 m) in diameter.
They usually live in the waters of the mid-Pacific and Indo-Pacific. Some also live in the Atlantic Ocean off the coast of West Africa.
Cauliflower jellyfish are one of the most venomous jellyfish in the ocean. Yet, they are not very dangerous to humans. They sting and cause pain like any jellyfish, but a sting is not fatal.
5. Australian Box Jellyfish (Chironex Fleckeri)
Jellyfish Lifespan: Up to 9-12 months (adult)
The most dangerous jellyfish species in the world include the box jellyfish. They live in the Indo-Pacific region of northern Australia. Sometimes, you will find this jellyfish inland in freshwater rivers and mangrove channels. Either way, you’ll be quite unlucky if you come across one.
The Australian jellyfish is the most venomous marine animal in the world. It is also the largest box jellyfish. Their bodies can reach lengths of up to 10 ft (3 m) long with bells about 1 ft (0.3 m) in diameter.
A sting from these jellyfish is always unintentional since they can’t eat humans. Still, their venom has killed over 60 people in the last 100 years. It seems that children and young adults are the most at risk.
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Pet Jellyfish Lifespan
Jellyfish kept as pets have a similar average lifespan as their wild counterparts. They may live somewhat longer because there is no threat of predation. Still, captivity will not extend their lives by much. A few species may live up to two to three years in captivity.
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Are Jellyfish Immortal?
Most jellyfish have short lives. Most only live for a few months when they become adults. Still, one species, Turritopsis dohrnii, may live forever.
The immortal jellyfish has the incredible ability to revert to its polyp stage. It becomes a polyp again when it feels threatened or becomes injured. They also revert if water conditions become harmful or there is insufficient food.
By returning to its polyp stage, it’s theoretically starting its life all over again. These jellyfish can revert to their juvenile stage as often as they need. Once they reach the adult stage and are about to die, they hypothetically “go back in time.”
Still, immortal jellyfish can die. They are still susceptible to predators and other dangers. They will die like any jellyfish if they don’t have time to revert to their polyp stage.
Are Jellyfish Related To Octopus?
Jellyfish and octopuses look similar with their large, squishy bodies and long tentacles. It makes sense for people to think the two animals are of the same family, but they’re not. Jellyfish are part of the phylum Cnidaria. Octopuses are part of the phylum Mollusca.
For instance, the tentacles of an octopus are dense and meaty. A jellyfish’s tentacles are gelatinous and are more like hollow straws.
Plus, octopuses are highly intelligent creatures — one of the smartest in the world. Jellyfish don’t have a brain and aren’t very conscious of what’s going on around them. Yet, they have a nerve net that lets them interact with the world around them.
Which Jellyfish Are Dangerous To Humans?
There are over 400 species of jellyfish worldwide. Only about 50 of them present a real danger to humans.
Of those 50 jellyfish, only a few species are likely to kill a person. Most jellyfish produce powerful stings that are painful to humans but are not fatal.
The Cubozoans are the species we know as box jellyfish. They comprise about 50 species of jellyfish with exceptionally powerful stings.
The sea wasp jellyfish is likely the deadliest jellyfish on the planet. It can kill up to 60 people with venom from a single nematocyst. The Irukandji jellyfish is also deadly. It produces venom 100 times stronger than a cobra produces.
Most fatal jellyfish stings cause cardiac arrest, paralysis, collapse, and difficulty breathing. About 100 people die each year from box jellyfish stings.