Jellyfish are gelatinous aquatic animals belonging to the subphylum Medusozoa. These free-swimming invertebrates evolved around 500 million years ago and have been roaming the oceans around the world, in both warm water and cold water. They are easily identified by their trailing tentacles and umbrella-shaped bells.

Scientific Classification


Scientific Classification


Types of Jellyfish

List of Common Types of Jellyfish Species

The subphylum Medusozoa consists of four major classes, including Scyphozoa (or large jellyfish), Hydrozoa (or small jellyfish), Cubozoa (or box jellyfish), and Staurozoa (or stalked jellyfish). There are more than 1200-1800 species of jellyfish divided into these four classes, out of which Scyphozoa has about 200 species, Staurozoa contains roughly 50 species, Hydrozoa includes 1000-1500 species, and Cubozoa consists of approximately 20 species. Here are some of the commonly found types of jellyfish species:

  • Pacific Sea Nettle
  • Lion’s Mane Jellyfish
  • Portuguese Man o’ War Jellyfish
  • Carukia Barnesi
  • Moon Jellyfish
  • Chironex fleckeri Box Jellyfish
  • Many-ribbed Jellyfish
  • Blue Button
  • Mushroom Cap Jellyfish
  • Velella
  • Nomad Jellyfish
  • Barrel Jellyfish
  • White-spotted Jellyfish
  • Mauve Stinger
  • Cigar Jellyfish
  • Stinging Cauliflower Jellyfish
  • Compass Jellyfish
  • Crystal Jellyfish
  • Black Sea Nettle
  • Flower Hat Jellyfish
  • Fried Egg Jellyfish
  • Atolla Jellyfish
  • Flame Jellyfish
  • Immortal Jellyfish


Physical Description and Appearance

Size: Their size varies depending on the species, the smallest being the creeping jellyfish of the genera Eleutheria and Staurocladia, having bell disks with a diameter of 0.5 mm (0.02 in). The lion’s mane jellyfish is probably the largest species with tentacles extending up to a length of 36.5 m (120 ft). Another giant species is the Nomura’s jellyfish, which can have a bell diameter of 2 m (6 ft 7 in).

Jellyfish Size

Weight: An adult box jellyfish can weigh up to 2 kg (4.4 lb) while the giant Nomura’s jellyfish weighs about 200 kg (440 lb).

Color: Some are clear, while others are purple, blue, yellow, and pink. There are bioluminescent jellyfish, too, meaning they can create light.

Bell: It is an umbrella-shaped hollow structure containing a mass of jelly-like tissue, called mesoglea, which functions as its hydrostatic skeleton. The mesoglea comprises water, collagen, fibrous proteins, and mobile amebocytes that can engulf bacteria and debris.

Manubrium: It is a tube-like structure located on the underside of the bell. It hangs down from the middle, along with the mouth, which opens into a gastrovascular cavity used for digestion and absorption of nutrients.

Eyes: Several species of jellyfish have light-sensitive organs called ocelli or simple eye. Others, such as the box jellyfish, have an advanced vision with each individual possessing 24 eyes.


Jellyfish are found in both seawater and freshwater throughout the world. Examples of freshwater species include cosmopolitan hydrozoan jellyfish and golden jellyfish. Some species like the lion’s mane jellyfish are capable of surviving in cold Arctic water, while others such as the tropical box jellyfish live in warm waters near the tropics.

What Kind of Habitats do They Live in

Although most jellyfish species float close to the ocean’s surface, some species, including the upside-down jellyfish that lie on the bottom of lagoons, and stalked-jellyfish that remain attached to seaweed or rocks on the bottom.

Jellyfish Habitat

How Long do They Live

Not much is known about the lifespans of jellyfish species that are found on the ocean floor. However, the moon jellyfish found in coastal waters have an average lifespan of 12-18 months. With proper care, captive moon jellyfish can survive for about 20 years. The flame jellyfish, found in warm temperate waters, have a life expectancy of 3-12 months.

What do They Eat

They are typically parasitic or predatory animals, and their diet includes planktonic organisms (algae, bacteria, protozoa, and archaea), crustaceans, fish eggs, larvae, small fish, and other jellyfish. Some species, like the Aglaura hemistoma, are omnivores that feed on a mixture of microscopic plants and free-floating protozoa and crustaceans.


  • After attaining sexual maturity, jellyfish spawn (release sperms and eggs) on a regular basis if an adequate supply of food is available. Most jellyfish species spawn at the same time of day, usually at dawn or dusk.
  • Jellyfish save a lot of energy while swimming. They swim through radial expansion and contraction of their bell-shaped bodies, a mechanism known as passive energy recapture. It allows the invertebrate to swim 30% farther on each cycle.
  • They form blooms or large masses under favorable environmental conditions involving sunshine, temperature, ocean currents, nutrients, reduced predation, oxygen concentration, and availability of prey.
  • Jellyfish have the ability to detect ocean currents, swimming against them, and assembling in blooms.
  • Like most invertebrates, they too exhibit a sleep state and remain inactive at night, typically pulsing around 39 times in a minute, which is significantly less as compared to the regular pulsating frequency of 58 per minute.
Jellyfish Tentacles


  • Jellyfish have nematocysts, which are barbed, threadlike tubes used for delivering a paralyzing sting. They use this as a defensive mechanism for protecting themselves against threats. Contact with their tentacles triggers thousands of nematocysts to inject venom into the body of a predator.
  • Their long tentacles are adapted for reaching out and capturing prey and then pushing food items toward their mouths.
  • The bell-shaped bodies of jellyfish have specialized muscles that are adapted for contraction, allowing the animal to propel forward. At the end of the contraction, their body recoils elastically, causing it to stop.
  • Although they do not possess any respiratory system, the thin membrane (epidermis) of jellyfish allows for the diffusion of oxygen.

How do They Reproduce and Mate

Adult jellyfish, in most cases, discharge sperms and eggs in the surrounding water where the fertilized eggs mature into larvae. In some species, the sperms travel into the mouth of the female jellyfish and fertilize the eggs within the body. These fertilized eggs develop into planula larvae, which become cylindrical vase-shaped polyps. These inconspicuous polyps pass through the immature ephyra stage and finally transform into matured medusae.

What does The Baby Jellyfish Look Like

The planula is a small, free-swimming, flattened, and ciliated larval form that develops into a polyp, containing a small stalk with a mouth having upward-facing tentacles. These polyps resemble corals and sea anemones.

Baby Jellyfish


1. Do jellyfish have brains?

No, jellyfish do not have a central nervous system or brain. However, they possess a basic set of nerves at the lower end of their tentacles. The nerves help detect temperature, salinity, and touch.

2. What predators may eat jellyfish?

Swordfish, tunas, sharks, sea turtles, and some salmon species usually prey upon jellyfish. Large species of jellyfish may also feed on the smaller ones.

3. Do jellyfish have eyes?

No, a jellyfish does not have any eyes or ears.

4. Can you eat jellyfish?

Some jellyfish species, including cannonball jellyfish, Nomura’s jellyfish, and jelly blubber, are suitable for consumption. These jellyfish are harvested and consumed as seafood in several Asian countries, such as China, Korea, Japan, Thailand, Vietnam, Malaysia, the Philippines, and Indonesia.

5. How to treat a jellyfish sting?

First, get out of water. Then, rinse the affected area with vinegar for about 30 seconds to stop the itching. You may use an oral antihistamine or mild hydrocortisone cream to relieve discomfort and swelling.

6. What is a group of jellyfish called?

A group of jellyfish is called a ‘swarm’ or ‘bloom’.

7. Do jellyfish have hearts?

No, they do not have a heart.

8. Do all jellyfish sting?

Although all jellyfish sting, not all of them are venomous. The box jellyfish and the Portuguese man o’ war are the two most poisonous jellyfish species on earth today.

Interesting Facts

  • Jellyfish act as hosts of a variety of parasites, including the endoparasitic helminths and digenean trematodes that infect fish.
  • These aquatic invertebrates are harvested not just for a source of food but also for their collagen, which is a protein used for the treatment of rheumatoid arthritis.
  • A large swarm of jellyfish can choke cooling equipment and disable power stations. A cascading blackout occurred in the Philippines in 1999, while the Diablo Canyon Power Plant was damaged in 2008.
  • The Roman naturalist, Pliny the Elder, described in his book that the slime obtained from jellyfish emitted light when rubbed against a walking stick.