Jellyfish: Characteristics, types, reproduction habitats and more

Next, you will learn in this article everything related to jellyfish, which are very beautiful creatures that have inhabited the planet for a long time, and in many  cases can be dangerous.

What are Jellyfish?

They are invertebrate aquatic animals of soft and free swimming that resemble a sunshade and / or with the shape of a gelatinous bell, which also have some tentacles.

Jellyfish are classified in the subphylum Medusozoa which is an important part of the Cnidaria phylum (coelenterate) to which also belong corals, Portuguese men of war, sea anemones, sea pens, whips and sea fans.

It’s  good to note that not all Medusozoa species are considered jellyfish. Jellyfish aren´t invertebrate fish, and in some public aquariums they often call them jellies or sea jellies instead.

The Jelly Fish ’ Main Characteristics

All jellyfish share several basic characteristics. As invertebrates  they are, they lack a backbone. Their body has two layers of tissue: an external ectoderm and an internal endoderm. In the middle of these layers is the gelatinous mesoglea which provides volume and support for the animal.

The mesoglea doesn´t  contain any cell and is composed mainly of water, this being the main reason why they are transparent.

The jellyfish´s body for most of their life cycle has the shape of a bell. In the case of box jellyfish the bell has four different sides.

Extending from the body of most jellyfish are long tentacles embedded with nematocysts.

Some species lack  tentacles, among which are the large red jellyfish (Tiburonia big red ), which uses long and fleshy «feeding arms» to grip their prey.

Jellyfish also lack  head, circulatory system and organs for respiration and excretion. Their mouth is in the lower part of their body, and is connected to a central cavity filled with hair-like structures called cilia that help transport food and other materials throughout the body.

Their lower muscles contract and expand the body, like an umbrella, allowing the animal to swim. A network of nerves runs under the body´s coat and coordinates the muscles.

Some true jellyfish have simple eyes around the edge of their body. In the case of box jellifish these eyes are more complex and include lenses, corneas and retinas.

The different jellyfish species show a huge diversity in color and size. Some species are white, while others may be orange, brown, pink, red or blue. The lunar jellyfish are translucent, like all jellyfish.

Some jellyfish are bioluminescent, which means they can shine in the dark. The jellyfish´s bodies generally vary from about 1 to 16 inches (2 to 40 centimeters) in diameter, although some species are much larger, while the tentacles´ number and length vary according to the species.

The Jellyfish’ Bite

The jellyfish bites are relatively common problems for people who swim or dive in the sea and their long tentacles can inject venom using thousands of microscopic spike stings. Such bites vary greatly in severity. In most cases, they cause immediate pain and red marks and skin irritation.

Some bites can cause whole body (systemic) diseases, and in rare cases, jellyfish stings are potentially deadly.

Fiery burning pain, red, brown or violet patches on the skin, itching, swelling and throbbing pain radiating from a leg or arm are the symptoms of a common sea jelly burn.

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Jelly Fish’ Habitat and Distribution

The jellyfish are inhabitants of the tropical seas and the Arctic´s cold waters, where they have been for more than 650 million years.

They are pelagic animals, that is, they live in the open sea, and although they can be propelled with rhythmic movements of their umbrellas, they move basically at the mercy of the sea´s currents.

How Do Jelly Fish Reproduce?

Jellyfish reproduce both sexually and asexually. When reaching the adult size, they reproduce daily if they have enough food.

In most species, spawning is controlled by light, so the entire population appears at approximately the same time of the day, often at dusk or dawn.

Jellyfish present male and female individuals with occasional hermaphrodites. In most cases, adults release sperm and eggs in the surrounding water, where unprotected eggs are fertilized and mature into new organisms.

The sites of flowering vary according to the species; from the bulbs tentacles, the handlebar above the mouth or the gonads of hydromedusae.

jellyfish : typical lyfe cycle

Most jellyfish live from a few weeks to several months, although some species can live a year or more. Jellyfish have a life cycle with two main body´s forms.

A form of free swimming called a jellyfish and a non-swimming form called a polyp. The bell shape in the case of true jellyfish and four sides in the case of box jellyfish is the form that dominates the life of the animal.

The jellyfish reproduces sexually, which means that they  produce gametes, or ovules and sperm. The eggs are fertilized by the sperm and then released into the water, where each one becomes a free-swimming larva called planula.

Each planula eventually adheres to a rock or ocean floor and develops in the form of a polyp similar to a stem. The polyp is an asexual stage and reproduces by sprouting.

The true jellyfish´s  polyps release many buds and each is released as an immature form that then moves away and matures in a jellyfish. In the case of box jellyfish, the polyp can only produce a single jellyfish. The alternation of a sexual phase and an asexual phase in the life cycle of the jellyfish is known as alternating generations.

Here they are

Main Types of Jellyfish

The term jellyfish was created by Carlos Linneus in 1752, alluding to the tentacled head of Medusa in Greek mythology.

This term refers exclusively to the life´s stage without polyps that occurs in many cnidarians, which is characterized by a large pulsating gelatinous bell with long dragging tentacles.

The four main classes of jellyfish are mentioned below:


These jellyfish are usually called true jellyfish, although they are no more authentic jellyfish than the others listed here. This group of Cnidarians encompasses some 200 species, commonly called «jellyfish» or «living waters». Their size is very variable, as it ranges from 2 cm the smallest to 2 m in diameter the largest, with long tentacles up to 40 m in length.

Their name was given in memory to the Greek´s mythological creature Medusa, whose head was covered with snakes and transformed into stone anyone who looked at her. True jellyfish are all pelagic, good swimmers, except a few planktonic and benthic species.


The cubozoans  or box jellyfish are commonly called «sea wasps» because of their dangerous poison. They are a class of the phylum Cnidaria inhabitants of the marine macro plankton. Their name refers to the cubic form of their  body. ( we invite you to read our article box jellyfish, to learn more about this species).


Hydrozoans belong to the Hydrozoa Class of the Cnidaria Phylum, and their name comes from the Greek hydra meaning «similar to an aquatic snake». Among the hydrozoans there are marine and freshwater species and it is thought that it is the most primitive class from which the rest of the Philo classes derived.

Unlike anthozoans whose structure is only in the form of polyps, hydrozoans are shaped like polyps and jellyfish and their gastro vascular cavity is simple without divisions as in the case of anthozoans that is radially septated.

Among the best known hydrozoans are the hydra,; the Portuguese boat and the fire corals,. We invite you to read our article hydras to learn more about this species)


To this class of the Cnidaria phylum, belong approximately 50 species commonly known as stalked jellyfish or stauromedusa.

The term stauromedusa is derived from the Greek “stauro”  for shape of a cross and jellyfish for the gorgona.  Staurozoa are small animals [1-4 cm, which live in marine environments, usually attached to algae, rocks or gravel.

The group has a largely anti-tropical distribution, with most of the species found in shallow  boreal or polar waters, near the coast.

However, some species inhabit the deep sea, including at least one that is associated with mid-ocean ridge habitats adjacent to hydrothermal vents.

In addition, a small number of species have been observed in warmer tropical and subtropical waters of the Atlantic, Indian and Pacific Ocean basins. The 50 species of Staurozoa are currently classified in one order, six families and 14 genera.


This is an order of hydrozoans that generally live as colonies. They are not considered true jellyfish. Their propulsion method is based on the coordinated movement of thousands of cilia which they employ as oars, instead of a pulsating bell, although some species swim by flapping large lobes.

Marine biologists have described 175 different species of siphonophores, which include the most stretched creatures of the animal kingdom.

In fact, some specimens reach 40 meters in length and are considered voracious predators. These gelatinous creatures hunt their victims employing nematocysts, which are a sort of capsules filled with a poison and a harpoon that is fired when being rubbed.

The siphonophores live swimming underwater, excepting a floating species, the Portuguese frigate Physalia physalis, and the Rholadiilae family, which clings to the bottom. Finally it is good to highlight, that these creatures are actually beings made up of multiple units called zooids.


 Salps: (Salpidae)

They are transparent and gelatinous marine organisms that form pelagic colonies like the siphonophores. Salps are chordate , which makes of them more related to humans than to cnidarians and comb jellyfish.

What do Jellyfish Feed On?

The jellyfish are carnivorous and can quickly increase in size and create a large number of individuals when food is plentiful. However, if food is scarce, they can become smaller. These animals, of gelatinous consistency have a very unsophisticated anatomy, although it is very effective.

They feed mainly on zooplankton and  small crustaceans, although some small fish and other jellyfish  may also comprise their diet. It’s uncommon to see the last prey of the jellyfish inside their body before being digested.

here they are again.

Jellyfish ’Main Predators

There are few predators that take advantage of jellyfish and rather they can be considered the main predators of the food chain.

However among the species that take advantage of them we can mention the following: moonfish, gray triggerfish, turtles (especially the leatherback turtle), some seabirds (such as fulmars), whale sharks, some crabs (such as arrow and hermit crabs), and some whales (such as humpbacks).

Jellyfish and Humans.

Sea jellyfish have a wide relationship with humans. Due to their large population it’s  possible to use some species as food, and they are also often used to decorate aquariums. However, with certain species, great care must be taken, since the burning of sea jellyfish can be very painful, and in some cases, fatal.


Some species of jellyfish are suitable for human consumption and are used as a source of food and as an ingredient in several dishes. The edible jellyfish is seafood that is harvested and consumed in several Asian and Southeast Asian countries, and in some Asian countries it’s   considered a delicacy. Edible jellyfish are often processed in a dry product.

You can prepare various types of food and dishes with edible jellyfish, which include salads, sushi, noodles and main dishes. There are several methods of preparation.


The jellyfish are displayed in many public aquariums, in the bottom of their tanks of blue color, illuminating them with side light, which increases the contrast between the animal and the bottom.

In natural conditions, the jellyfish are so transparent that they are almost invisible. The sea jellyfish are not adapted to closed spaces, they depend on the currents to be transported from one place to another, for this reason it is necessary to have certain cares that only people specialized in aquariums should do.

Jellyfish Blooms!

A Jellyfish bloom is a seasonal abundance of their population due to reproduction or growth.  Jellyfish blooms have closed beaches in the Mediterranean, obstructed power stations in Sweden and have stopped fishing in Japan.

Blooms used to occurr every 40 years, but due to the increase of the ocean´s temperatures they occur more frequently nowadays.

In many parts of the ocean, high concentrations of agricultural nutrients make plankton grow explosively.

This, in turn, depletes some areas of oxygen from the ocean, creating “death zones”.

The majority of marine life cannot survive in an environment that is deprived of oxygen, except for the jellyfish. They are commonly found in the northern climates of the Mediterranean, North Queensland and on the coasts of Florida and Japan.

The Japanese Case

In recent years, Japan has experimented what could be called a sign of the future «saturated with jellyfish». The massive blooms of Nomura´s jellyfish  began to suddenly occur each year in Japan. Experts wondered if it was the agricultural runoff from agriculture.

Jellyfish have obstructed fishing nets, have devastated fisheries, have damaged power plants and have killed tourism. The common reasons for the problem of Japan ranged from overfishing, global warming, pollution, low levels of oxygen, toxic zones and the nutrients for zooplankton.

The efforts of observation in marine ecosystems have increased in both the scientific community and the Japanese entrepreneurs, which are struggling to find solutions, since killing   the Jellies would just lead them to release their eggs, which, in turn, would create new spawning areas.

Not all bad

Jellyfish are edible but are not attractive to eat; The Japanese chefs are looking for recipes to present them. On the positive side, there have been advances in medicine as, for example, a better understanding of anaphylaxis, as a luminous marker in cell research and the use of toxins from jellyfish for anticancer compounds.

Jellyfish are also hosts of various other animals. The ecosystem of the open ocean lacks physical refuges, a place that supports other forms of life.

In fact, Portuguese men of war often house juvenile fish or small adults under their hood or between their tentacles, and this hideaway provides a haven for predation and gives them better feeding opportunities. The fecal pellets of jellies fall to the ocean´s bottom and also supply carbon sequestration, and it is likely to be a direct function of the abundance of salps in the oceans.

An economy of jellyfish may be the only way to balance the negative impact of booming populations. A Japanese scientist is even building artificial reefs and storing jellies in them, to be devoured by fish.

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How does the Jellyfish´s Defense System Work?

Their tentacles, with their stinging cells, serve as a defense and as a powerful weapon to capture prey. When they come into contact with their victims, the nematocysts (venom-loaded cells) present in the tentacles, release the harpoons or filaments inside them and, through them, a toxic substance is released that paralyzes the prey. The oral arms help in the capture and ingestion of the captured animal.

What preventive measures should be taken against them?

It can be dangerous for humans to swim too close to a jellyfish, but several organisms have found a good refuge in them. The fry of some fish, such as bogue or amberjack, hide within the protection of their tentacles.

The main preventive measures are detailed below.

    • If there is a large quantity of jellyfish in the coastal waters, the beach should be closed for at least 24 hours, taking precautions even if the jellyfish are abundant at some distance from the coast.
    • If the jellyfish are near the beach, it is best to stay out of the water and away from the breaking waves.
    • If a jellyfish is seen in the water, it is better not to take any risks, even if it is some distance away, because, with the action of the waves, its tentacles can break and the floating fragment cells will remain active. In addition, it is necessary to inform bathers who are not familiar with these organisms that they should not touch them, even if they seem to be dead.
    • Don´t touch jellyfish on the sand, although they seem to be dead, since the stinging cells remain active for a period of time. Even walking along the water’s edge can be dangerous since there may be traces of tentacles in the sand.

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