World’s Only Immortal: Facts about Immortal Jellyfish

To date, there are only one species that has been called ‘biologically immortal’, the small jellyfish, scientifically known as Turritopsis dohrnii. As we call it fish, there has no similarity with a fish except its water-dwelling habitat. This immortal species was first observed in the Mediterranean Sea in 1883. But its special ability was not discovered until the 1990s. Let’s uncover some amazing facts about Immortal Jellyfish.

Life Cycle of Immortal Jellyfish

Most people are only familiar with full-grown jellyfish, the translucent bell-like creatures that occasionally wash up on sandy beaches. That bell-like stage is a medusa of a jellyfish. Jellyfish have a complex and interesting life cycle with several developmental stages. The life of a jellyfish begins with a fertilized egg, which grows into a larval stage called a planula.

Facts about Immortal Jellyfish - the lifecycle

Planula has an oval shape and covered by numerous cilia. After a quick swim if it isn’t eaten up by predators, soon drops down to settle on a solid substrate like a rock, boat’s wreckage, reef, or ocean floor and develops into a polyp: a tube-shaped body with a mouth and a foot like structure at opposite ends. Stalked polyp remains stuck in place for some time, which may grow into a little colony of polyps that share feeding tubes with each other.

In a suitable environment, depending on the jellyfish species some of the polyps will form an outgrowth called a ‘bud’, or it may produce separate segments stacked on top of one another, that can break away to form immature jellyfish known as ephyra larvae. By this means of asexual reproduction, hundreds of genetically identical ephyra larvae can be produced. These larvae are very small and possess fewer tentacles than adults. Ephyra larva transforms into full-grown male or female medusae which are capable of sexual reproduction, after which they usually die.

An exceptional case of immortality will occur, only if the medusa undergoes an adverse situation.

Exceptional Transformation: One of the Facts about Immortal Jellyfish

For most other jellyfish, the medusa stage is the end of the lifeline. Though, many species like Podocoryne carnae, Cladonema sp., Perarella schneideri , etc., medusae can revert to polyp structures only at the beginning of their development, completely losing this ability before they are liberated. But in the case of Turritopsis dohrnii, if the medusae faces some kind of environmental stresses like lack of food, physical damage, changes in water salinity or temperature, or other crises arise it can revert back to a tiny bolb of tissue, which then changes back into the sexually immature polyp phase of life.

All of the existing cells transform into a younger state. It is a bit surprising like a butterfly turning back into a caterpillar or a frog becoming a tadpole again! Cells of Turritopsis dohrnii are often completely transformed in the process. Muscle cells can become nerve cells or even sperm or eggs.

Regression and Transformation Pathways

Turritopsis dohrnii undergoes two different pathways depending on their age of maturity.

Pathway – 1

When transforming medusae are still immature, they regressed to a ball of tissue (a cyst) covered by perisarc (chitinous integument). If water temperature or other environmental factors are suitable, the cyst develops into a stolon within 3 days. But in adverse conditions, cysts can rest for up to 3 months or longer, without losing their ability to produce stolons. This transformation process has the following steps:

  • Tentacle contraction
  • Size reduction
  • Loss of swimming capability
  • Closure of velar opening
  • Settlement
  • Perisarc production
  • Cyst formation
  • Stolon production
  • Polyp formation

Pathway – 2

Sexually mature medusae attach to a stiff surface by the apex (exumbrellar side) of the bell and can undergo two alternative pathways. Either the bell is everted or regressed without eversion and go through the following pathway:

  • Contraction of the bell around the proximal base of the manubrium
  • Gradual enlargement of the manubrium
  • Progressive mesogloeal resorption
  • Settlement
  • Spawning
  • Perisarc production
  • Stolon production
  • Polyp formation

In both cases, polyps are produced about 2 days after stolons have developed. Then the polyps repeat the next stages of the life cycle.

Facts about Immortal Jellyfish: The Conclusive Remarks

Turritopsis dohrnii is a unique jellyfish and its reverse transformation potential is unparallel within the vast range of living organisms on earth, and this character makes them immortal. This is the first known case of metazoan being capable of reverting completely to a hydroid (maybe colonial) stage after having achieved sexual maturity in a solitary stage, even after senescence (biological aging) has occurred to their cells.

Turritopsis dohrnii isn’t truly immortal. They can still be consumed by predators or killed by other means. And most of the time they die the following senescence in absence of any stress factors.

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Ms. Subhra Das is a biology teacher in high school by profession. Besides she is a passionate science writer and nature lover.

As a teacher, she never restricts herself within the four walls of the classroom, rather she loves exploring the crude science behind the natural facts that include human and animal health, critical diseases, typical characteristics of wildlife, and mother nature.

Ms. Subhra Das is also a passionate traveler and explorer; she always tries to uncover natural flora and fauna at every destination she travels.

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