Is a jellyfish a fish?

In this blog post, we will answer the question “Is a jellyfish a fish?”. We will discuss jellyfish biology and its ideal conditions to be kept in tanks.

Is a jellyfish a fish? 

No, Jellyfish is not a fish. The jellyfish is a cnidarian, a member of the corals’ family. 

Do you have any idea why the jellyfish gets its name? 

The popular name ‘jellyfish’ originated due to its body being made up of 95% water. This water creates a gelatinous aspect and plays an important role in the animal’s physiology. The jellyfish body is similar to an umbrella with tentacles.


The jellyfish is classified in the Hydrozoa class, which belongs to the Cnidaria phylum. In this class, two main forms of organisms are found, polyps, which are generally sessile (coral), and jellyfish, which correspond to free-living forms and swim in water using water propulsion movements. 

Both animals have radial symmetry and are formed by two layers of tissue, the epidermis, the outermost layer, and the gastrodermis, the innermost layer that lines the gastrovascular cavity, where the food consumed is digested. It is possible in some cases to observe the content that will be digested in the gastric cavity.

Between these layers, there is a gelatinous mass characteristic of the group, the mesoglea. This substance acts as the ‘connective tissue’ of the cnidarians, filling spaces between the dermis, in addition, influencing the density of the jellyfish. The mesoglea facilitates their submergence or emergence in the water column. That is why the name refers to Jellyfish because this tissue looks like jelly.

The jellyfish’s medusoid shape divides its body into three main parts: 

–       umbrella: the upper part of the body whose shape resembles that of an open umbrella, where the gastrovascular cavity is located; 

–       oral tentacles: which surround the mouth; 

–       ‘stinging’ tentacles, where cnidocytes are to capture prey and protect against predators.

Why do jellyfish burn?

The cnidocytes are the structures that gave their name to the phylum Cnidaria. Cnidaria derives from the Greek ‘knidos’ and means stinging or burning. 

These cells are capsule-shaped and have a structure called cnidocyle, which works as a trigger. When the cnidocyle is stimulated, the nematocyst is projected out of the capsule and attaches itself to the prey’s body, releasing a stinging substance capable of paralysing and even killing the prey.

In humans, this fact often results in the famous jellyfish burn, causing only pain. But there are species, like Chironex fleckeri, commonly known as the sea wasp, that can cause more severe damage to humans. This jellyfish species can be found in the Pacific waters, and it is a small body with huge tentacles up to 3 meters in length. Each of these tentacles has millions of nematocysts and its contact with humans can result in paralysis, cardiac arrest, and even death, within minutes.

Is it possible to raise jellyfish in a tank?

It is possible to raise them as pets, as long as you provide the ideal environment for them to survive. For example, it is advisable not to keep jellyfish with other aquatic animals unless your tank has plenty of space to give all the animals enough space, so they can swim without bumping into each other. That is why the vast majority of people buy a separate aquarium for their jellyfish.

What is the ideal jellyfish species to have in a tank?

There are several species of jellyfish, but not all of them can be raised in a tank. Even though you have several options available to choose from, one of the most popular jellyfish that are raised in tanks is the famous Moon Jellyfish (Aurelia aurita).

The moon jellyfish live in tropical climates and is possible to be raised in small spaces. Also, they do not need a very strong water flow to swim. Thus, they are better suited to tanks than most other species.

In addition, the moon jellyfish do not usually grow large, they reach no more than 30 centimetres in length and can reproduce successfully in captivity if under the right environmental conditions.

What are the basic aquarium requirements for a jellyfish?

The vast majority of tanks have a rectangular shape, but this type of shape is not ideal for a jellyfish. They can get stuck in the corners, without being able to find a way out, as they cannot see. For this reason, the best option for a jellyfish is a round tank, as this ends up avoiding this type of problem.

Another thing you need to keep in mind when buying a jellyfish tank is to check the water flow and filtration system. Jellyfish can get trapped in pumps or filtering equipment, so careful mounting of the tank is extremely important.

In a jellyfish tank, the water flow is very meaningful. They use it to move and float around. They also depend on the water flow to find food and get rid of waste.

What is the ideal temperature for a jellyfish aquarium?

Jellyfish prefer cooler temperatures compared to other aquatic animals, which is another reason they need to have their aquariums.

The ideal temperature for raising a jellyfish in a tank needs to be between 51.8 and 62.6 degrees Fahrenheit.

What kind of food do jellyfish consume?

Jellyfish can consume many food types. Thus, you will be required to have a good variety of food supply for these animals. In the ocean, they usually feed on plankton and shrimps. You will only be able to find this type of food in stores specialized in marine fishkeeping.

One of the most common mistakes many jellyfish keepers make is not knowing exactly how much food the jellyfish consume. 

What is the right type of lighting for the jellyfish?

Jellyfish need intense light to live healthily. These bright lights also allow algae to form, which will later help with feeding the jellyfish.

A good idea is to place lights on top of the aquarium that will light directly the entire environment.

How long does a jellyfish live?

The life expectancy of a jellyfish is around 1 year depending on the environmental conditions. Some species, such as Catostylus mosaicus can live for 9 months, while Chrysaora quinquecirrha can live for several years.

Finally, is it safe to raise a jellyfish at home?

Jellyfish bites are quite common and are best to avoid. However, the Moon Jellyfish has a sting that does not usually hurt too much, but you will feel it. 

Creating a jellyfish in a tank is not one of the easiest tasks. They are demanding and require very specific water conditions to succeed. Also, finding food for these animals can be a very difficult task.


This post responded, “Is jellyfish a fish?”. We learnt about jellyfish biology and its ideal conditions to be kept in tanks.

Frequently Asked Questions (FAQs): Is a jellyfish a fish?


What are the effects of touching a jellyfish?

In some rare cases, eruptions may appear. Such effects are allergic reactions to the poison acting on the skin. The most serious symptoms are difficulty breathing and swallowing, chest pain, headache, cramps, nausea and vomiting.

Can a jellyfish kill?

The jellyfish of the cubozoan group is considered the most lethal jellyfish in the world. The amount of poison in an adult specimen is enough to kill an adult human.

When a jellyfish dies does it turn into just water?

Jellyfish are capable of transforming their living cells into a newer state. Let’s say it dies, but resurrects younger. The scientific process is called trans-differentiation. This process occurs when an already differentiated cell transgresses that differentiation and becomes another type of cell.

Is it possible to have a jellyfish at home?

It is possible to raise them as pets, as long as you provide the ideal environment for them to survive.

What to do to avoid jellyfish burn?

Prevention. Many countries usually signal areas with a high incidence of jellyfish, using visual signs and warnings. One way to protect yourself is therefore to avoid entering the water in these areas.

What are cnidarians?

The cnidarians are diblastic animals, with very simple body and that have two forms of life: polyps and jellyfish.


National Oceanic and Atmospheric Administration (NOOA). What are jellyfish made of?

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