Jellyfish Life Cycle

A Jellyfish comes in many shapes colours and sizes. They are found in most of the ocean in the world. If there is saltwater whether it is warm tropical seas or frigid polar water jellyfish are there. Jellyfish belong to a large group of mammals called Cnidarians. Cnidarians are the most common type of sea animals with more than 10000 species. The smallest jellyfish reaches the size of the thumbnail. Whereas the largest jellyfish may reach up the size of 7 feet or 2.3 meters across with tentacles 121 feet or 37 meters long. Jellyfish are called Invertebrates which implies they do not have a backbone. The jellyfish don’t have bones at all they don’t have eyes, ears, heart, or brain either. Unlike fish, jellyfish don’t have gills instead they observe oxygen through their skin. Jellyfish use a simple type of jet propulsion to get around, they open their body like an umbrella then close it. By contracting their body the water inside is forced out and the jellyfish moves forward. Constantly moving is the only thing keeping jellyfish afloat, but even with this effort, they can be easily pushed around by strong winds or ocean currents which is what leads some jellyfish to be washed up on beaches. The top of the jellyfish is called the Bell, their jelly-like bodies are almost made entirely of water about 95%. Some jellyfish have light sensors called eyespots that can sense sunlight and help the jellyfish know which way is up. Some jellyfish are Bioluminescent, which means they can produce light from their bodies kind of like a firefly. Although know one knows why they light up, the scientist believes it to be a defence mechanism to startle the predator away or perhaps to attract another predator to eat their attacker. Jellyfish are largely transparent, which means you can see through them. This makes it difficult to see them in water, which hides them from predators that like to eat them. The largest jellyfish were seen to have a bell diameter of 2.3 meters long and tentacles 27 meters long. This will be like a quarter length of the football field. Some of the jellyfish species change colour, this is triggered by a colour changing pigment, this effect is called bioluminescence. This is believed to be a defence mechanism to startle or confuse its predators. A jellyfish seen in a group is called Bloom. The number in which they are seen is assumed to be in millions. The life cycle of a jellyfish has many phase, they are as follows.

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Adult jellyfish consist of both male and female, both of them have reproductive organs which helps them to sexually reproduce which is called the Gonads. When both male and female jellyfish reach their maturity and are ready to reproduce, the male jellyfish would release sperm through an opening in their body called the Mouth. This mouth is usually found on the bottom of the bell. Coming to female jellyfish there seem to be many variations, some of the female jellyfish species carry their eggs in Brood Pouches which usually lies beneath the arm of the female jellyfish. These eggs in the brood pouches get fertilised when the female jellyfish swims through the male lade sperms. In case of other species of the female jellyfish, they carry their eggs in their mouth. The process is initiated when a male sperm goes inside the female jellyfish and the eggs are fertilised. Once the fertilization process is completed, they leave the stomach and get themself attached to the female jellyfish arms. In the case of the adult female jellyfish which is called the Medusa, the fertilization of the eggs takes place internally.

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This is the second stage in their life cycle where their size is very tiny. The eggs after fertilization continue their development inside the female jellyfish. The division of the cell takes place several times inside the egg forming an embryo, this is called Embryonic Development. Once the development is complete the egg hatches and comes out in the form of a tiny ova, flatten creatures called a Planula. This comes out from the female jellyfish mouth. Here it separates itself from its parent to float freely in the ocean. It will be very tiny as a plankton and will not be easily able to spot with our eyes. These will not be having any tentacles at this state, they just keep on floating. The direction in which they will be floating will not be defined until they reach a place where they can hold on to something.  A mature zygote goes through several cell divisions to form a Planula larva. This larva is free-swimming which will set out on its own. A thin hair-like cilia covers planula completely, these cilia beat rapidly there-by helping it to flow in the ocean current for a brief period of time until it is set for the next stage of its life. After floating on the surface of the water this tiny Planula if survived drops from the surface of the water and tries to settle on the rock or any other solid surface which is suitable to attach itself.


Once the planula finds something sturdy to anchor itself and undergo transformation into a Polyp. The name that has been given to this Polyp is Scyphistoma. This will be cylindrical in shape and will look something like a stalk. At the top of Polyp where the mouth is present consists of a large number of small tentacles. Coming to the bottom of the Polyp, it displays a disk-like feature. The Polyp will be able to retain its form for a period of several years. During this stage in their life, it will be feeding on small marine animals by drawing them using its tentacles towards its mouth. During this stage of Polyp, the mouth and the tentacles are facing towards the top.      

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The Polyp does not require any additional kind of fertilization for it to initiate budding, as they will be performing asexual reproduction. This happens when the Polyp becomes bigger and mature. This is initiated by Polyp, by starting to grow branches on its sides. As these branches grow to become larger, they would detach themselves and drift down to besides the original Polyp to attach itself to the surface. This creates the same copy of the original Polyp. Both of them now begin to create more Buds. This intern leads to a much larger colony of Polyp covering the ocean floor.  Gradually as time passes the Polyp forms a hybrid colony, where each and every Polyp is attached to one another by feeding tubes. When these polyps attain a certain age or grow into a size it moves on to the next stage of development. The state in which the Polyp develops further is called the Ephyra. This is no fixed time for its development, it may go up to several years.


This is the next stage in the jellyfish life cycle, where-in the Polyp colony matures. The stalk part in the Polyp starts developing grooves in a horizontal manner. This process in which Polyp develops grooves is known to be Strobilation. The Polyp will look like a stack of saucers when the grooves get deeper and deeper. The upper portion of the Polyp is the once that matures faster and the later buds of Ephyra. This is what is known as a tiny immature jellyfish that is seen in the ocean.


In this stage of a life cycle, a jellyfish which is fully grown in size is called a Medusa. Medusa is an adult jellyfish, this is the shortest part of the jellyfish life cycle, as they live only for a short period of time. The structure of Medusa resembles that of an umbrella, with tentacles hanging on to its lower position of the body. Once a jellyfish is grown full into an adult, we can say it is ready to reproduce. A jellyfish gets a significant amount of food it spawns very frequently. We observe 2 major cell layers in Medusa, the internal layer is called Gastrodermis and the outer layer is called the Epidermis. The jellyfish gut is lined by the Gastrodermis. The food is ingested through the gut and this is where the reproductive cells are taken in and released. As there are no lungs, stomach or intestine in a jellyfish, the nutrients, oxygen directly flow in and out of the body through the Gastrodermis. The Epidermis has cells which are loosely set in the network called the Nerve Net. This is the most basic nervous system that you will see in any of the multicellular animals. The layer seems between the epidermis and the gastrodermis is called mesoglea. This layer consists of most of the jellyfish body. As the jellyfish body consists of 95% water, we can say that mesoglea is mostly made up of water. The jellyfish consists of structural proteins, nerve cells, and muscle cells along with the water, all of this contributes to making the internal skeleton. The sensory structure in jellyfish is called Rhopalia, these contain receptors which help in deducting light, movement and chemicals. The most fascinating thing to see is how a jellyfish intercepts an image that has been created by the eye, as the jellyfish do not have any brains. Scientists believe the nerve ring that we see in the jellyfish play a major role in performing the brain duties. This nerve ring contains a cluster of nerves in the shape of a ring, which we will be able to observe in the adult jellyfish. 

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FAQ (Frequently Asked Questions)

1. How many groups of jellyfish have been divided?

   Ans: Jellyfish are been divided into 4 different groups, they are as follows

  1. Scyphozoa- This is the most common form which is formed abundantly in the sea. There are around 200 species which are found for this type. This type of jellyfish spends most of its lifespan in Medusa state, these are big and colourful. 

  2. Hydrozoa- These are not found so easily in the sea. IT stays a very little time in the Medusa state, but the Polyp form results in the formation of a large number of colonies. 

  3. Cubozoa- They are commonly known as the box jellyfish due to their bells resembling the box. There are some species of this jellyfish which are very venomous. These species have complex retina and eyes with a well developed nervous system compared to others. These types of jellyfish exhibit elaborate courtship behaviour.

  4. Sturozoa- These are usually found attached to the rocks or seaweeds, rather than floating on the water. They're mostly found in the cold water places and usually look like a trumpet.


2. Why are jellyfish called energy-efficient animals and what does a jellyfish eat as food?

Ans: Jellyfish are made up of a smooth bag-like body, with tentacles armed with tiny sting cells. They usually sting their prey before eating it. Studies indicate that they have a dual purple system which helps them to travel great distances with less effort. Hence they are called energy-efficient.

The jellyfish can quickly digest their food, which consists of fish, shrimp, crab and tiny plants. The jellyfish have the ability to clone themselves. They do not have bones, heart or brain, they comprise 95 % of water. Jellyfish consists of structural proteins, nerve cells, and muscle cells along with the water. The jellyfish moves in water by squirting a jet of water from its mouth, they seem to propel forward.

3. How long does a jellyfish believe to have lived and which is the deadliest in the species and Why is one of the jellyfish species called the immortal jellyfish?

Ans: They are believed to have lived on earth for over a period of 650 million years, long before the dinosaurs which makes them the oldest multi-organ animal. 

The box jellyfish is said to be the most venomous and the deadliest jellyfish. It is said that its venom can stop a person's heartbeat within 2 minutes.

A particular species in the jellyfish is capable of reverting back to its earlier life stage at any point in time. This jellyfish creature can be killed, but will not die of old age, hence the name is given as the immortal jellyfish.