Jellyfish: The invasion has begun

Jellyfish are one of those sea creatures that people are starting to recognise and witness more often as they dominate our oceans. They are very alien to look at and seem harmless, yet we know better than to reach out and touch one in the water. In fact, they are strange creatures that most of us know very little about. This article explores some of their mystery and attempts to reveal more about these fascinating creatures.

We just know them as Jellyfish, but their scientific name is phylum Cnidaria, which is derived from the Greek word “cnides” which means ‘nettle’ and they are of the Scyphozoa classification, which means ‘cup’ because of their shape. The Scyphozoa classification is only one of several scientific groups. These include Cubozoa, Straurozoa and Hyrdozoa, all with slight differences.

There are 2,000 known species of Jellyfish that have been found already, but some scientists believe there could be up to 300,000. Jellyfish aren’t related to Octopuses and Squids, despite the tentacles, but they are related to sea corals and anemones.

Jellyfish fact file

Diet: Jellyfish are carnivorous and will eat plankton, eggs, and the larvae of other creatures, small fish, crustaceans and other Jellyfish.

Lifespan: This isn’t known with any certainty. Some only live for a few days. For others, lifespan is estimated at one to several years. One species is considered virtually immortal because it can change itself back from the mature stage to the juvenile stage at will.

Habitat: All over the world. They are found on the surface of the seas and in the depths of the oceans. Some of the smaller species can also inhabit freshwater rivers.

Predators: Natural predators of jellyfish are sharks, swordfish, tuna, sea turtles, penguins and sea anemones. Jellyfish often get caught in fishermen’s nets, resulting (sometimes) in them eating each other.

What are they made of?

Despite the fact that they have fish as part of their name, they aren’t fish. They are invertebrates, which means they don’t have spines or other bones. They also don’t have brains, blood, or hearts. Although they do have a nervous system which is pretty basic, it enables them to react to stimuli in their environment.

Their bodies are made of a gelatinous substance and this includes three layers or caps. Epidermis is the outer layer, Mesoglea is the middle and gastrodermis is the inner. Some are totally transparent, but others have vibrant colours. Still, others are bioluminescent, which means they can produce their own light. They have one hole running through the centre of their bodies which ingests food and expels waste.

This mouth also helps them with locomotion. By expelling water from it they can propel themselves along. This action also aids them in gathering food. The movement of their bodies, which are bell-shaped causes a suction effect that draws in other marine creatures that they can feed on.

How old are Jellyfish?

It is said that jellyfish actually predate the dinosaurs. Scientific investigation can’t accurately pinpoint when jellyfish started to evolve because of the lack of fossil evidence. Because of the composition of their bodies, without bones, their bodies rot and leave nothing behind to carbon date. The few fossil remains that have been discovered up until now are estimated to be between 500-1,000 million years old.

What are their sizes?

The smallest are a mere 1 millimetre or less than a 1/16 of an inch in height and diameter, with tentacles that are a bit longer. Others can grow up to 2 metres or 7 ft. in height and diameter. The Lion’s mane jellyfish, which on average is 1 metre or 3ft. 3 inches in height and diameter has a tentacle length of over 36 metres or 200 ft. The Giant Normura jellyfish that is found off the coasts of China, Japan, and Korea can weigh on average between 150 Kilos or 330 lbs. Some have been known to weigh up to 440 lbs.

(if you fancy procrastinating – check out the below video, which shows a massive jellyfish near the UK!!!!

How they reproduce

For many, it is a complicated business. There are both sexual and asexual ways of reproducing, depending on the jellyfish. Generally, there are 4 parts that contain 14 stages. Below is one example of a reproductive cycle:

  • 1-3 Stages – After the larvae are expelled from the adult, it searches for a secure site to attach itself to
  • 4-8 Stages – The polyp grows
  • 9-11 Stages – The polyp strobilates, which is a form of asexual reproduction
  • 12-14 Stages – The medusa grows into an adult. Medusa means that they are now fully-fledged jellyfish

The rise in Jellyfish population

Overfishing by humans has resulted in a rise in the jellyfish population. Apart from their target catch, fishing boats are also netting and killing off the natural predators of the jellyfish. Once that happens, the jellyfish thrive.

Jellyfish reproduction also depends on factors such as sunlight and water temperature, among others. With global warming and the rise in sea temperatures, the conditions are more favourable for jellyfish reproduction than before because more jellyfish survive in these warmer waters.

In some coastal areas, human activity is causing dead zones. Natural food resources are dying off and this affects the marine population, but it doesn’t affect the jellyfish. This can result in ‘blooms’ of jellyfish populations in which millions of them are born and mature at the same time. Once the jellyfish become the top predators in an area it is almost impossible to replace them with other marine life.

Jellyfish: Why you should be worried

Within the cells attached to Jellyfish tentacles are what are called ‘nematocysts’, which on contact inject venom into the predator or prey of the jellyfish. The venom can result in just a mild discomfort or extreme pain and death. Only certain species of jellyfish carry venom that is highly toxic to humans. It depends on which jellyfish stings you and we will be highlighting some of the most dangerous jellyfish shortly

Attacks by jellyfish

Jellyfish do not attack humans. They are passive hunters that sting anything that touches their tentacles. In this respect, prey and predator are indistinguishable. When humans are stung by Jellyfish it is because they have inadvertently come in contact with a tentacle. A group of Jellyfish is termed a ‘smack’ and once you as a swimmer are among them, it is difficult to avoid the tentacles making it seem like an attack. Incidents of Jellyfish stings have risen over the years because of the rise in the number of Jellyfish, not because of aggression.

The 3 deadliest jellyfish

The Box Jellyfish – of which there are 43 breeds – is not only the most venomous jellyfish but the most venomous of all sea creatures. Death can occur within 5 minutes of being stung. The Box jellyfish is considered more dangerous than the deadly Black Widow spider! Since records began in 1883, there have been at least 69 deaths attributed to the Box Jellyfish. These jellyfish inhabit the shallow shores of South East Asia and the Australian coastline.

The Irukandji is the next deadliest and almost all of its stings are fatal. This tiny Jellyfish only measures 25 millimetres long by 5 wide or 1/5 of an inch. They only have 4 tentacles and each can be a yard long. They are almost impossible to see. The Irukandji’s habitat is the northern and eastern coast of Australia but has been extending further south over the years. It seems that this Jellyfish has started to migrate, they have been found in the waters off Thailand, Japan and the Florida coast.

The Portuguese-man-of-war is one that everyone has heard of, however, its sting is not always fatal if the victim gets medical attention quickly. Oddly enough it isn’t considered a true Jellyfish. It is part of the Cnidaria family because it has tentacles, but scientifically it is called a siphonophore. This means that the man-of-war is actually a colony of tiny creatures called Zooids, not one complete organism. Unlike others, this creature cannot propel itself; it just drifts with the currents or is blown along by the wind. It can be found in the Atlantic, Pacific, and Indian oceans.


Unfortunately, there isn’t a great deal of scientific knowledge about Jellyfish compared to what is known about many other marine species. They are beautiful and fascinating, but some are obviously deadly. New species are being discovered all the time, so the study of them will be a long and important undertaking as their momentum for dominance continues.

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