When you think of a jellyfish, you are probably picturing a translucent, bell-shaped animal with long flowing tentacles. While iconic, that's only a short stage of a jelly's life - the "medusa" phase of a true scyphozoan jellyfish before it dies. Jellyfish live far longer, more interesting lives than you may have imagined!
Let's take a look at the incredible life cycle of jellyfish; for consistency, we're going to be using images of Aurelia moon jellies as much as possible:
Life begins - the egg
The life story of a jelly begins just like ours - with a male and a female looking for a chance to mate. At dusk or dawn, adult jellies, known as medusae, gather in large numbers to spawn. This means that they release huge amounts of sperm and unfertilised eggs into the ocean around them. These spawning events continue throughout the lives of the adult jellies and are triggered by their proximity to other jellies, light and food abundance. The larger an adult jelly can grow in this time, the more offspring it may be able to have.
The fertilised eggs are usually left to drift freely in the ocean currents, but some jellies make more of an effort to protect their eggs. For example, moon jellies allow the fertilised eggs to attach to their oral arms and protect them until they are ready to survive the next phase of their lives...
Jelly babies - planula larva
The egg grows into a small larva called a planula. This planula can swim freely and resembles a microscopic flatworm, covered in tiny hairs called cilia. These cilia beat rhythmically and allow the planula to swim about. The planula continues to grow and when it is sufficiently large it tries to find a suitable solid surface to attach itself to.
In most cases, the planula finds a piece of solid seabed to attach to, with different species preferring different types of terrain. In some cases, they may even attach themselves to other animals, particularly the bodies of other adult jellyfish. Once a perfect spot has been found, the planula enters the next phase...
The long childhood - polyps
The attached planula develops into its next life stage - the polyp. Polyps can be thought of as small stalked animals with one end fixed to the ground, and the other end extending into the water with a ring of tentacles surrounding its mouth/anus. In this phase, the jelly has a fully developed digestive system and is able to catch prey and feed itself efficiently.
This jellyfish polyp very closely resembles other marine animals - hydroids, anemones and corals. In fact, these animals are very close relatives of true jellyfish and would have followed the same lifecycle up to this point. The difference is that a jellyfish will develop into new stages beyond the polyp, while an animal like an anemone will essentially become an "adult polyp".
Depending on the species, the jelly can remain in this polyp stage for years. This allows the jelly to wait for perfect conditions to mature when conditions are suitable or large numbers of their species to spawn before moving onto their final adult form.
Budding teenagers - scyphistoma
When conditions are ideal, the polyp begins to reproduce asexually, by cloning itself. The polyp elongates and forms segments which will eventually bud off to form independent animals or even more polyps to speed up this process. This process is known as strobilation, and the strobilating polyp is called a scyphistoma.
The process of strobilation begins with the polyp's tentacles being reabsorbed into its body. The polyp's body becomes narrower and clearly visible segments begin to form.
Eventually, the grooves between these segments become deep enough for the segments to separate entirely, taking a fully independent set of muscles, nerves and digestive system with it...
Finding themselves - ephyra
The tiny, newly-budded segment of polyp becomes a free-living organism known as an ephyra, which is the precursor of the adult jelly. At this stage, the ephyra is no more than a few millimetres across, but as it swims away, it feeds and grows. The ephyra does not have the closed bell-shape and stinging tentacles of an adult jellyfish, and has to rely on the undeveloped lobes of its bell to push food towards its mouth.
What's important to keep in mind is that, although this jellyfish has entered a new stage of its life, it is just one of many clones. It is entirely possible that the original polyp and copies of that polyp are still alive and may even outlive this new jelly.
Frisky, hungry adults - medusa
As the ephyra grows, its bell takes on the characteristic shape and its tentacles and oral arms will grow into the forms unique to its species. This adult form is called the medusa.
Although the initial adult is small, it is already a mature animal and capable of reproducing - but there are a lot of incentives to grow as large as possible as quickly as possible, for example:
- Jellies, like moon jellies or compass jellies, gather in huge numbers to spawn. To be reproductively successful, a jelly needs to produce more sperm or eggs than its rivals - and that means being a bigger jelly.
- Deep-sea dwelling jellies are fewer and far between, so their incentive for growing large quickly is to deter predators to that they can survive long enough to reproduce.
- Some jellies, like South Africa's pink meanie, rely on very specific conditions (in the case of the pink meanie, it needs other types of jellies to bloom in large numbers). Because those conditions might be unusual or short-lived, the jelly needs to grow from polyp to medusa as fast as possible to take advantage of them.
As this point, you might be wondering "If the jelly clones itself and then goes off to reproduce sexually, how does it avoid mating with itself?" Good question - to avoid this almost all jellyfish species have discrete sexes, i.e. the original egg and all its future clones will be either male or female. Hermaphroditism only occurs in rare species - and is usually a survival tactic for jellies that never occur in large enough numbers to be guaranteed finding a mate in any given generation.
Death (but not really)
The purpose of the medusa is only to live long enough to reproduce, and typically jellyfish do not live very long in this phase. Many of the jellies we get at the Aquarium live for no more than a few months before essentially reaching their "old age". As you have hopefully come to realise by this point, the medusa does not represent the full life of the jelly, just the end of it.
Jellies are exceptionally good at identifying the perfect conditions to reproduce in, so they do not need unnecessarily long lives. If the conditions are good for it to become an adult medusa, it will likely be able to spawn the next generation quickly and efficiently too.
You may have heard of jelly fish blooms occuring as a result of overfishing or climate change - this is an example of how effectively and quickly jellies can reproduce when conditions are ideal for them.
It's possible that you may have seen stories of an "immortal" jellyfish, one that can theoretically live forever because its adult medusa can revert back to be polyp stage. This is true - but it's not the only jelly that has beaten death. Remember, when the adult medusae of most jellies are dying of old age, clones of their polyps are still thriving on the seafloor. Through cloning, most jellies can theoretically live exceptionally long lives.
So, what did we learn? Jellies are awesome biological cloning machines, masters of efficiency, possibly immortal, and undeniably awesome.
Be sure to visit the jellyfish in our Jelly Gallery on your next Two Oceans Aquarium visit.