Jellyfish

Though they are not usually aggressive, these creatures are famous for their ability to sting, allowing them to defend themselves against any sort of danger.

These animals use their tentacles for hunting. However, they do not have any bones, heart, or most other organs. Interestingly, their bodies are mostly made up of water.

Jellyfish lifespans vary by species; some live only a few months, while others, such as the moon jellyfish, can live for several years.

Do Jellyfish Have Brains?

As beautiful as these sea creatures are, they don’t have a brain. Instead, the body is made up of a complex nervous system, which is where all of its motor functions and sensory activity occur. Through the neurons in this system, the body tells the muscles when to contract, which is how they swim.

Incredible Jellyfish Facts

• No brains, heart, or eyes: These animals are mostly made up of water. They have no brains, hearts, or eyes. They also have no bones, and their body is primarily controlled by the nervous system.
• Ancient, prehistoric beings: Jellyfish are known to have been around for millions of years, even before dinosaurs.
• Bioluminescent: These animals are bioluminescent, which means that they can produce their own light.
• Quick digestion: The digestive process doesn’t take very long when jellyfish eat. This quick process ensures that they can remain afloat in water.
• Worldwide delicacies: Jellyfish are not only loved by the predators that feed on them, but also by the human population across the globe.

Evolution

Jellyfish are believed to have evolved from the phylum Cnidaria, a group that includes anemones and corals. Jellyfish were likely the first swimmers powered by muscles in the ocean. They originated during the Precambrian Era when ecological and geological changes were taking place, leading to an explosion of animal life in the later Cambrian Era.

The more accepted theory is that jellyfish transformed from polyps that grew on the ocean floor to swimming medusae with stinging tentacles. Their shape-shifting abilities likely helped them survive multiple mass extinctions over more than 500 million years.

A second explanation from some scientists is that Cnidaria originally had a medusa life stage, differentiating them from sea anemones and corals.

Classification and Scientific Name

These animals are classified in the class Scyphozoa and belong to the kingdom Animalia and phylum Cnidaria. Scyphozoa comes from two Greek words — skuphos and zōion. While skuphos means “drinking cup,” the word zōion means “animal.” The name is an interpretation to mean that this animal contains water. The phylum Cnidaria is interesting as well, since it derives from the modern Latin word knidē, meaning “nettle.”

As part of their classification, these animals belong to the subphylum Medusozoa and the class Scyphozoa. Medusozoa comes from the Ancient Greek Μέδουσα, which comes from the word for “rule over” (μέδω).

Species

Jellyfish form a large family of planktonic creatures, and recent studies show that there are at least 4,000 that have already been discovered. Considering the vastness of the oceans, scientists believe that this number is barely a fraction of what is actually in the sea.

Even with all of these species around the world, only 70 are considered to be a threat to humans. Some of those dangerous species include the Malo kingi and the Chironex fleckeri, which are both in the box jellyfish family. The venom is potent and painful enough to kill.

Some types are even kept as pets, primarily for their inability to sting their owner. The most common jellyfish to keep as a pet is the moon jellyfish, which lives for about 15 months. Other species of jellyfish can live much, much longer.

A group of jellyfish is called a swarm, smack, or bloom.

Immortal Jellyfish

Turritopsis dohrnii — also called the Immortal jellyfish — is a small and transparent jellyfish that can, interestingly, reverse to earlier stages of its life. This transition can take the animal back to the state that the jellyfish was in as a fertilized egg when it settles on the seafloor in old age.

Classifications of Cnidaria

There are numerous species of jellyfish that fall into four classes of Cnidaria. We’ve left out the fifth, Anthozoa, which includes sea anemones and corals:

• Scyphozoa: There are 200 marine Scyphozoa, known as “true jellyfish.” This description refers to jellyfish with a cup shape. These jellyfish swim freely. Some of the individual species in this group include: Moon Jelly, Barrel jellyfish, Cannonball jellyfish, Cassiopea andromeda, Aurelia coerulea, Helmet jellyfish, and Lion’s Mane jellyfish.
• Hydrozoa: Hydrozoa is a classification that houses 700 unique jellyfish species. They are small, either transparent or pigmented, and mostly found seasonally in coastal habitats from late spring to early fall, as well as in freshwater. Some examples are the freshwater jelly (Craspedacusta sowerbyi), freshwater polyps (Hydra), Obelia, Portuguese man o’ war (Physalia physalis), chondrophores (Porpitidae), “air fern” (Sertularia argentea), and pink-hearted hydroids (Tubularia). 
• Cubozoa: There are 50 species of Cubozoa, or box jellyfish, characterized by their box-like shape when viewed from overhead. They sport well-developed eyes and either four evenly spaced tentacles or tentacles in bunches. The stings from multiple types of box jellyfish are extremely painful. Examples include: Australian sea wasp (Chironex fleckeri), Carukia barnesi, and Malo kingi.
• Staurozoa: Staurozoa jellyfish are known as “stalked jellyfish.” They have trumpet-shaped bodies, and grow upside-down when compared to, so that their tentacles project upwards, with the stalk situated in the center of their umbrella. Some species include. Haliclystus antarcticus, Manania handi, Lucernaria quadricornis, and Haliclystus octoradiatus.

Appearance

Some of these animals are transparent, while others exist in bright colors like yellow, blue, and pink. These animals are bioluminescent, which means that they produce their own light.

Their bodies may look complex due to the way they look, but they are quite simple. Jellyfish have smooth bodies with tentacles that contain tiny cells that can be used when they use their stinging capabilities.

They have no bones, brains, hearts, or eyes. Their mouths are found in the center of their bodies. They are usually about 0.5 to 16 inches and can grow up to 7 feet and usually weigh around 440 pounds.

Jellyfish Tentacles

These animals have tentacles that are equipped with tiny stinging cells that are activated when they launch a sting attack on their prey. These tentacles are used to paralyze and stun the prey that the jellyfish stings. These tentacles are found hanging from the jellyfish’s body.

Though these tentacles are controlled by the nervous system, the sting is rarely fatal. Most box jellyfish have strong enough venom to kill the victim. The purpose of the tentacles is to stop the movement of its prey, though they are also used as a way to defend the animal.

Deadly Jellyfish

Although some species of jellyfish are pretty harmless, as noted above (such as the moon jelly or Aurelia aurita, which is especially common in the United Kingdom), others are far more lethal. The most dangerous jellyfish are generally found in the Indo-Pacific Ocean and also around northern Australia.

The most lethal of them all is the Australian box jellyfish, or Chironex fleckeri. The largest variety of all box jellyfish, it is easily recognizable by its tentacles, which are reminiscent of shoe laces, capable of growing up to 10 feet long, and a pale blue, skull-shaped dome.

Those tentacles, like those of all other venomous jellyfish, are laden with millions of nematocysts, which are responsible for emitting venomous darts when touched. The pain from its venom has been compared to that of a branding iron, and the creature can cause death in a matter of minutes.

However, the Chironex fleckeri is a favorite snack of turtles, who are capable of ignoring its stings thanks to all that thick skin, allowing turtles to enjoy a delicious jellyfish lunch.

Distribution, Population, and Habitat

They are found across the globe and can be found in every ocean on the planet. Many species choose to live in warm tropical waters or cold Arctic waters. They can exist at the bottom of the oceans as well as on water surfaces, making them incredibly versatile.

Though the specific areas may vary, every species must live in saltwater to thrive. As of 1990, there were 900 million tons of jellyfish in the Black Sea alone.

Even with this ability to survive in so many locations, pollution is a big threat to every species. Oil spills and dumping chemicals in the water can easily absorb into their skin, which makes it impossible for them to reproduce. Though the pollution doesn’t always kill them instantly, most species will not live nearly as long once they are exposed.

Predators and Prey

These animals face threats from various sea and land creatures, including sea anemones, swordfish, sea turtles, tuna, and penguins. When the jellyfish get washed up on beaches, they are usually found and eaten by foxes and other birds, and animals. If humans manage to catch them, it isn’t uncommon to cook them as a delicacy.

These animals, in turn, feed on many different things like planktonic eggs, small plants, small fish and larvae, fish eggs, and other small marine animals.

The Sting of the Jellyfish

These animals have tentacles that are armed with tiny sting cells that the animals use on their prey or when they sense danger. They often use their stinging characteristics to defend themselves and use their tentacles to inject venom into other creatures.

The stings can cause pain and irritation and can sometimes also result in whole-body illness. Some stings can even be life-threatening. The most severe sting comes from Australian box jellyfish, which is the deadliest jellyfish in the world. Their venom can be so painful that it often leads to drowning for victims who go into shock.

Reproduction and Lifespan

These animals are known to reproduce both sexually and asexually. While one of the species reproduces sexually, some other types reproduce asexually. However, for both processes, the fertilized eggs settle on the seafloor after developing into a multicelled planula.

The average jellyfish lives three to six months, but some can survive as long as two to three years. J

Jellyfish in Fishing and Cooking

Jellyfish can be caught and eaten. About 25 to 30 species of jellyfish are considered edible and are savored as delicacies, particularly in Asia. These animals are a rich source of protein and fatty acids.

One popular recipe is sesame jellyfish, which pairs the jellyfish with soy sauce, vinegar, sesame oil, and sometimes chili oil.