Most jellyfish live less than one year, and some some of the smallest may live only a few days. Each species has a natural life cycle in which the jellyfish form is only part of the life cycle (see video clip showing different life cycle stages).
Jellyfish Lifespan
Most jellyfish live one to three years, but certain species can live from days to decades. Jellyfish have complex life cycles, beginning with fertilized larvae floating in the ocean. The larvae sink and attach to the seabed or coral reefs, from there they develop into 'polyps'.
Of those that do exist, the oldest-known jellyfish fossils, found in Utah, date to 505 million years ago and have enough detail to show clear relationships with some modern species of jellyfish.
The most widely kept jellyfish in smaller aquariums are moon jellyfish or common jellyfish, which can live around 12 to 15 months. Typically a jellyfish kept in ideal conditions, which can be quite difficult to achieve, will live for 6-12 months.
How long does the immortal jellyfish live? Potentially forever. Which gets more impressive considering these creatures have been floating through the oceans long before the dinosaurs went extinct (66 million years ago) – it's biologically possible for a single immortal jellyfish to have been alive for this entire time.
The hydrozoan Turritopsis dohrnii, an animal about 4.5 millimetres wide and tall (likely making it smaller than the nail on your little finger), can actually reverse its life cycle. It has been dubbed the immortal jellyfish.
So how do they function without a brain or central nervous system? They have a basic set of nerves at the base of their tentacles which can detect touch, temperature, salinity etc. Since they don't have a brain, they depend on automatic reflexes in response to these stimuli! Catching prey is also a matter of chance.
Jellyfish reproduce both sexually and asexually. One generation (the medusa) reproduces sexually and the next generation (the polyp) reproduces asexually.
They discovered that the jellyfish go through periods of inactivity at night, only pulsing about 39 times per minute, compared to about 58 times per minute during the day.
Under favourable conditions they will do this once a day, usually synchronised to dawn or dusk. The fertilised eggs hatch into tiny free-swimming flatworms called planulae, which either develop directly into adult jellyfish or settle on rocks to form an intermediate polyp stage.
Although many species of jellyfish have some capacity to reverse aging and revert to a larval state, most of them lose this ability once they reach sexual maturity. However, Turritopsis dohrnii appears to be the only known species able to repeatedly revert back into a larval stage even after sexual reproduction.
Lacking brains, blood, or even hearts, jellyfish are pretty simple critters. They are composed of three layers: an outer layer, called the epidermis; a middle layer made of a thick, elastic, jelly-like substance called mesoglea; and an inner layer, called the gastrodermis.
Jellyfish predate dinosaurs by hundreds of millions of years. Jellyfish don't have bones, so fossils are hard to come by. Even so, scientists have uncovered evidence these creatures have been living in our Ocean for at least 500 million years!
Jellyfish may now be kept as pets in your own house thanks to the creation of specialized circular aquariums. Pet jellyfish can live for several years if they are properly cared for and fed.
When the medusa the immortal jellyfish (Turritopsis dohrnii) dies, it sinks to the ocean floor and begins to decay. Amazingly, its cells then reaggregate, not into a new medusa, but into polyps, and from these polyps emerge new jellyfish. The jellyfish has skipped to an earlier life stage to begin again.
Do they have brains? No, jellyfish have no single centralized brain. Instead, they have radially distributed nervous systems that are adapted to their unique body plan.
Jellyfish “do it” in so many ways! They never get bored. The male and female medusa of some species (there are thousands of species) shed eggs and sperm in similar localities. The eggs get fertilized and develop into swimming larvae, which transform into polyps.
They may not have a brain, but jellyfish do get stressed out when handled roughly, scientists find.
Can jellyfish feel pain? Jellyfish don't feel pain in the same way that humans would. They do not possess a brain, heart, bones or a respiratory system. They are 95% water and contain only a basic network of neurons that allow them to sense their environment.
Cannonball jellyfish have a ball-shaped bell bordered with brown or purple pigment and short protruding oral arms.
Some jellyfish can lay as many as 45,000 eggs in a single night. And there's some jellyfish whose survival strategy almost sounds like science fiction.
Jellyfish have symbiotic relationships with living things of all sizes, from fish and shrimp that feed off them or off the pieces of food left between their tentacles, to single-celled photosynthesizing organisms that take shelter inside the cytoplasm of the jellyfish's cells.
Though jellyfish do not have a brain, they are incredibly smart and adaptable. For more than 500 million years, they've been bobbing around almost all the world's oceans, both close to the water surface as well as in depths of up 700 meters. The jellyfish is the world's oldest animal.
If you see a collection of jellyfish, keep everyone away. “If you see jellyfish on the beach, realize they can hurt you if you touch them,” Conroy said, noting that even dead jellyfish have venom in their tentacles that can sting on contact.