How Long Do Jellyfish Live? Polyps, Medusae and Immortality

How long do adult jellyfish live?

The swimming, bell-shaped jellyfish that people see is called the medusa. In most species the medusa grows fast, breeds and dies within a single season. Life spans vary widely between groups.

In aquariums, where there are no predators and food arrives on schedule, some medusae live longer than they would in the sea. Even so, few jellyfish of any kind survive more than a year or two as adults.

The jellyfish life cycle: two very different stages

To understand jellyfish life spans, it helps to know that most true jellyfish have two body forms. The medusa is the familiar drifting bell that reproduces sexually. The polyp is a tiny, stalked form, a few millimeters tall, that sits on rocks, shells or piers and looks a little like a miniature sea anemone.

The moon jellyfish is a good example. Adults spawn, and the larvae settle and grow into polyps. The polyps bud off more polyps, and when the water temperature changes in winter or spring, each polyp divides into a stack of discs. These discs break away as baby jellyfish called ephyrae, which grow into new adults over a few months.

How long do jellyfish polyps live?

Polyps are the long-lived part of the jellyfish story. A polyp colony can stay in the same place for years, releasing a new batch of jellyfish each year. Polyps can survive months without food, and some form tough resting cysts that wait out bad conditions.

Because polyps keep making copies of themselves, a colony is in some ways a single individual that lives on for a long time, even though each medusa it produces lives only a season. Little is known about how long polyps survive in the wild, because the hidden places where many species settle on the seabed have never been found.

How long do moon jellyfish live?

In the wild, an adult moon jellyfish usually lives for several months to about a year. Ephyrae released in winter or spring grow through the spring and early summer, and the adults breed in summer and autumn. After weeks of spawning, many adults shrink, become damaged and fall prey to disease, warm late-summer water and storms. A medusa older than about two years is exceptional.

The polyps, meanwhile, can survive for years. Scientists have also seen some Aurelia medusae transform back into polyps in the laboratory, which suggests the ability to reverse the life cycle is not limited to the immortal jellyfish.

The immortal jellyfish: Turritopsis dohrnii

Turritopsis dohrnii is a tiny hydrozoan jellyfish with a bell no more than about 4.5 mm (0.2 in) across and a bright red stomach. It was named in 1883 after Anton Dohrn, the founder of the marine research station in Naples, Italy. Its fame came in the 1990s, when researchers working in Italy showed that its medusae could turn back into polyps.

When the medusa is starved, injured, suddenly chilled or simply old, it can sink, shrink into a blob of tissue and reorganize itself into a polyp, which then grows a new colony and releases new medusae. Its cells change from one type into another, a process called transdifferentiation. In principle the cycle can repeat again and again, so the species is often called biologically immortal.

This does not mean the animal cannot die. In the sea, most immortal jellyfish are eaten or killed by disease long before they get the chance to reverse their life cycle, and the reversal has mainly been studied in the laboratory. It is also tiny and harmless to people, despite its dramatic nickname.

What kills jellyfish in the wild?

Most jellyfish do not die of old age. Their main causes of death include:

• Predators, such as leatherback sea turtles, ocean sunfish, seabirds, fish and other jellyfish.
• Exhaustion after breeding, especially in seasonal species that spawn many times in summer and autumn.
• Storms and stranding, when waves tear fragile bells or winds push swarms onto beaches.
• Starvation and disease, which hit adults hard when plankton runs short late in the season.

The small larvae and polyps face their own dangers. Many are swallowed by filter-feeding animals such as mussels and sea squirts before they ever grow into jellyfish.

Jellyfish and other “immortal” animals

The immortal jellyfish is often compared with other animals said to live forever. Freshwater hydras, small relatives of jellyfish, showed no sign of aging in a four-year laboratory study, and lobsters keep growing throughout life, although they still die (see Do lobsters live forever?). None of these animals is truly immune to death, and for record ages, sponges, corals, clams and the Greenland shark are far more impressive (see longest-living animals).

Jellyfish as a group are also very old: their ancestors were swimming in the seas more than 500 million years ago. Individual jellyfish, though, are usually among the shortest-lived animals in the ocean.