Jellyfish are fascinating sea creatures, especially known for their long stinging tentacles and their almost ethereal appearance, they are not to be considered fish.
These animals belong to the phylum cnidaria (plankton) family which includes over 10,000 species that are very different from each other in shape and colour.
One of the characteristics of the Cnidarians family is the presence of different shapes: the octopus and the jellyfish.
The two forms are closely related: octopus and jellyfish derive from each other and very often the same species presents both forms at different stages of its life.
The famous corals are nothing but large colonies of polyps, capable of developing an external calcareous skeleton, which forms the basis of the immense tropical coral reefs.
Official name “jellyfish” has been used since 1796.
Physiology of jellyfish
Their body is formed by a sort of bag with an opening that acts as a mouth (but also to expel undigested food), surrounded by tentacles that are their arms to catch prey.
On the tentacles of the jellyfish are small organs called nematocysts, which contain a filament that “snaps” when it comes into contact with a foreign body.
It is this filament that injects the toxic and stinging substances, but do you know that this liquid is composed of three proteins? They cause an anaesthetic, allergenic and paralysing effect, with more or less serious consequences depending on the danger of the jellyfish.
The “stomach”, called celenteron, also prolongs in part in the tentacles.
Their diet consists mainly of crustaceans, plants, small fish and also other species of jellyfish.
They are about 98% water, which is why, when someone catches them and leaves them on the beach, they literally melt in the sun.
The jellyfish reproduces sexually, laying eggs from which small larvae are born.
The larva, called planula, fixes on the seabed and first turns into a polyp, which grows feeding on microscopic marine organisms.
However, in jellyfish, the polyp phase is only present in some groups.
In colonial forms, the octopus can also reproduce in asexual form, producing various copies of itself, to the point of creating a small colony of its own kind.
When the octopus has reached maturity, it produces small female and male jellyfish that grow in open water and reproduce sexually.
The life cycle of the jellyfish ends in a couple of years at most.
To attract their victims, some jellyfish have a colourful and flashy appearance, and also use bioluminescence, i.e. the ability to shine that some living beings have developed, thanks to particular chemical reactions.
By attracting their prey with these little tricks, they get them close enough to inject their powerful poison once they have blocked them.
Their main meals are made up of small crabs, shrimps and crustaceans.
Here are some curiosities about jellyfish:
Not everyone knows that jellyfish were among the first multicellular beings to populate planet Earth.
The few jellyfish fossils found date back 650 million years, even before the dinosaurs.
Some species shine in the dark
Some jellyfish are equipped with bioluminescent, light-emitting organs.
This feature helps them in many things, including attracting or distracting their prey.
They can clone themselves
If one of these animals is cut in two, the pieces can regenerate and give birth to new organisms.
Similarly, if a jellyfish is injured, it can clone itself and potentially produce hundreds of small specimens.
There is a potentially immortal species
For some jellyfish there are two vital phases: the stationary phase and the mobile phase.
Generally, these beings begin their lives as stationary polyps and then develop into jellyfish.
However, there is one particular species, the Turritopsis nutricula, which has earned the nickname “immortal jellyfish” for its ability to return to the stationary phase if environmental conditions are too unfavourable.
The Turritopsis Dohrnii is able to reorganise its cells and return to the octopus stage, which is the original stage of jellyfish; potentially the process could take place indefinitely.
Turritopsis Dohrnii has its umbrella smaller than a coin and long filamentous tentacles.
Not all species have tentacles
Deepstaria scyphomedusa does not need tentacles to trap prey.
There are some giant jellyfish whose scientific name is Drymonema larsoni which feeds on other jellyfish.
Observed for the first time in the Gulf of Mexico, these specimens were believed to belong to a species called Drymonema dalmatinum.
The species was renamed Drymonema larsoni by the scientist Ron Larson, the first to study these jellyfish in the Caribbean.
Another jellyfish of remarkable size is the lion’s mane, a species that could rival a basketball player in height. It can in fact reach two metres in length.
This jellyfish, with the scientific name of Cyanea capillata, is up to two metres wide, and its tentacles reach 30 metres in length.
It lives in the waters of the Pacific Ocean, the Baltic Sea and the North Sea.
They are called Irukandji, with a diameter of only 2-3 cm, may seem harmless.
Its venom is so powerful that it is lethal even with just one bite.
Among the most dangerous animals
Cubomedusae, as the cubozoi class of jellyfish are usually called, are one of the most dangerous animals in the world.
Native to South-East Asia and Australia, they do not get carried away by the currents, but instead choose their prey (preferably fish) and catch it.
They are also called “sea wasps” because of their powerful sting.
Curiously enough, they are a prey in turn, for sea turtles that are immune to their venom.
The cubomedusa has 24 visual sensors, of which 2 eyes are able to see colours, and is one of the few sea creatures to have a 360° view.
They have no brain
It is said that jellyfish do not have a brain.
Although they do not have a real control centre (brain), they have nerve cells all over the body.
They are very ancient beings, among the first multi-cellulars to have been generated and therefore have a very simple structure that detects changes in the environment and coordinates the animal’s responses, based on the impulses received.
They are capable of shutting down nuclear reactors
Over the last 10 years, have been responsible for shutting down several nucleated reactors that often have the bases of cooling systems in ocean waters.
For example, the plant in Oskarshamn, Sweden, which was shut down for a few days for safety reasons because it was clogged with jellyfish.
Some species resemble bags of rubbish
These particular species are known as the enigmatic Deepstaria, and are usually found in the Arctic seas.
Jellyfish love to be in a group
What really makes an impression, however, more than the size, is the concentration that these animals can reach: in good times some species can fill a small arm of the sea with millions of individuals and disappear a few weeks later, usually after a mass reproduction.
One of the most spectacular and curious gathering of jellyfish can be seen in the small salt lagoon of Eil Malik, an island in the Palau group, in the tropical Pacific Ocean, where in the afternoon huge numbers of harmless jellyfish of the Mastigias genera concentrate in a single arm of the lake to enjoy the rays of the sun that allow the photosynthesis of the symbiotic algae that live in their tissues.
Swarms of jellyfish are called blooms.
For references: Wikipedia