14 Jellyfish Facts About the Ocean's Graceful Drifters

Jellyfish are some of the oldest living beings on Earth. In fact, they have been drifting through the world’s oceans for over 500 million years now. With their intriguing biology, unusual behavior, and ecological significance, read on for some often surprising jellyfish facts. 

Though jellyfish are known for their painful sting and ethereal beauty, they possess various fascinating abilities and features. For instance, some species can glow in the dark, while others carry potent venom that puts them at the top of the list of the world's most venomous marine creatures. 

Despite lacking a brain, heart, or bones, these unique animals showcase their adaptability and hunting prowess through their intricate nerve net and specialized stinging cells.

Get ready to explore the incredible realm of jellyfish as we delve into some of the most fascinating facts about jellyfish. 

14 Facts About Jellyfish Exploring These Unique Marine Dwellers

1. Jellyfish are invertebrates related to corals and sea anemones.

Jellyfish are invertebrates under the phylum Cnidaria. This diverse group of aquatic animals also includes corals and anemones. As invertebrates, these creatures lack a backbone or spinal column. Their unique structure showcases radial symmetry, with body parts symmetrically arranged around a central axis.

Despite their simple structure, these creatures have a gastrovascular cavity that functions as both a stomach and a circulatory system. While they share the same phylum, jellyfish, corals, and sea anemones have evolved to fill distinct ecological roles and exhibit unique adaptations. 

For instance, jellyfish are free-swimming predators, corals form colonies that build vast reefs, and anemones attach themselves to hard surfaces, remaining stationary.

Related: Why are Coral Reefs Dying?

2. They're also called sea jellies, medusae, and jellies.

The variety of names for these captivating invertebrates showcases their unique features. "Sea jellies" highlight their gelatinous, water-based composition, setting them apart from fish. Researchers and marine biologists often prefer this name, as it accurately indicates that jellyfish are not fish but cnidarians⁴.

Jellyfish are also known as "medusae," a term derived from Greek mythology. This is because these sea creatures' tentacles resemble the venomous snakes that make up the hair of the Gorgon, Medusa.

3. They are composed of 90% water and have a bell-shaped body and trailing tentacles.

Jellyfish live in every major oceanic area of the world, residing in temperatures both warm and cold, and depths from the shallow coastal waters to the dark abyssal depths of the ocean floor.

Comprising over 90% water, jellyfish showcase a unique, bell-shaped body that allows them to glide through ocean currents easily. By contracting and expanding the medusa, jellyfish propel themselves efficiently through the water. This movement and their inherent buoyancy ensure they can drift seamlessly in search of food and away from potential threats¹.

The jellyfish tentacles, which trail from their bell, serve multiple purposes. They use them to attract prey (such as small fish and plankton) and for self-defense. 

Additionally, these appendages help them sense their surroundings, responding to stimuli from any direction. Although tentacle length varies among species, their adaptability and versatility have played a crucial role in the jellyfish's ability to thrive in diverse ocean environments.

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

Groups of jellyfish are called "bloom," "swarm," and "smack," and each term carries a distinct meaning. A "bloom" typically refers to a large aggregation of jellyfish brought together by environmental factors such as a seasonal nutrient increase or water temperature².

On the other hand, a "swarm" implies a more coordinated movement, with jellyfish often traveling together in response to changes in their surroundings or to boost their hunting success.

Jellyfish blooms, swarms, or smacks can vary significantly depending on species, population density, and local conditions. Some species of jellyfish tend to form groups for breeding purposes, while others gather in numbers for protection or to optimize their foraging efficiency. 

These groupings can range from just a few individuals to thousands, even millions in some instances, creating awe-inspiring underwater spectacles.

5. They're over 500 million years old and are one of Earth's oldest creatures.

Did you know that jellyfish have been around longer than dinosaurs? They emerged more than 250 million years before the first dinosaurs, and jellyfish have existed for over 500 million years. 

They first appeared during the Cambrian Period, when marine life rapidly diversified. This period, known as the Cambrian Explosion, occurred around 541 million years ago and brought about a range of invertebrates, including the predecessors of modern-day jellyfish.

Despite being one of the oldest creatures, most species tend to have short lifespans, with the majority only surviving for 1-3 years. Interestingly, the Irukandji Jellyfish, the smallest species, can live up to 5 years in the wild. However, the largest species, Lion's Mane, typically only lives for one year. Additionally, some smaller species only live for a few hours.

6. Some jellyfish glow in the dark due to bioluminescence.

Most jellyfish (more than half) are bioluminescent, meaning they can produce light. Certain species of jellyfish, like comb jellies and lion's mane jellyfish, possess bioluminescent organs. Bioluminescence occurs through a biochemical reaction involving the interaction of luciferin, a light-emitting molecule, with an enzyme known as luciferase.

During the process, energy is released as light, usually in the blue or green range. These colors are ideal for underwater environments because they can travel deep into the water and are noticeable to other marine creatures.

Jellyfish use their bioluminescence for essential purposes such as scaring away predators, attracting prey, and communicating. When threatened, jellyfish can create a sudden burst of light to confuse or scare off predators and allow for an escape. 

However, the light can also lure prey toward the jellyfish's dangerous tentacles. Additionally, bioluminescent signals can help jellyfish coordinate group behaviors and attract potential mates³.

7. They have no brain, heart, or bones.

Unlike other animals, jellyfish do not have a central nervous system. Instead, they have a nerve net of interconnected neurons spread throughout their bell and tentacles. This unique system allows them to detect light, chemicals, and vibrations.

Interestingly, jellyfish also lack a heart, bones, and other complex organs. Their thin, gelatinous bodies allow for the direct diffusion of nutrients and oxygen. Furthermore, the mesoglea—a gel-like substance filling most of their body—provides support and buoyancy without requiring a rigid skeletal structure.

8. Jellyfish have specialized cells called cnidocytes that contain stinging nematocysts.

Have you wondered why jellyfish sting? Cnidarians (jellyfish, corals, and anemones) possess tiny stinging cells called Cnidocytes. These Cnidocytes contain nematocysts, which are the organelles responsible for jellyfish stings. 

Cnidocytes are found mainly on a jellyfish's tentacles, although they can appear on other body parts. Stings occur when physical or chemical stimuli trigger the jellyfish. These nematocysts rapidly discharge a coiled, thread-like structure to inject venom or capture prey.

The venom's potency varies among species, from mild to highly lethal. For example, the moon jellyfish's nematocysts carry venom that is relatively harmless to humans. In contrast, venom from a box jelly and sea nettle is considered dangerous as it can cause death. 

Interestingly, the firing mechanism of nematocysts is one of the fastest cellular processes in nature, taking only a few milliseconds to complete. Researchers are exploring potential medical applications of nematocyst toxins, including pain relief and cancer treatment options. 

9. Their diet includes plankton, small fish, and other jellyfish.

Most jellies can eat anything drifting along the ocean’s current but prefer zooplankton and phytoplankton. To catch and subdue its prey, a jellyfish stings. The stinging cells in their tentacles release toxins that paralyze or kill their prey, making it easy for the jellyfish to consume them. Depending on the species, jellyfish eat small fish, crustaceans, tiny plants, and sometimes other jellyfish depending on the species. 

Jellyfish have a fascinating way of ingesting and digesting food, despite having a simple body structure and no conventional digestive system. When they catch prey, it goes into their mouth, located in the center of their body. 

The jellyfish digest their food in the internal space called the gastrovascular cavity. Nutrients are then broken down and distributed throughout the jellyfish's body via radial canals. 

What's even more interesting is that some species of jellyfish form symbiotic relationships with photosynthetic algae. This means that they can get extra nourishment from the byproducts of photosynthesis.

10. Jellyfish reproduce both sexually and asexually.

Jellyfish boast a remarkable dual mode of reproduction, combining both sexual and asexual methods. This dual reproductive strategy enables jellies to expand rapidly, adapt to fluctuating environmental conditions, and flourish across diverse marine ecosystems. 

During the medusa stage of their life cycle, adult jellyfish engage in sexual reproduction by releasing sperm and eggs into the water. This external egg fertilization leads to the formation of fertilized eggs, which develop into tiny, free-swimming planula larvae. These larvae then settle onto a hard surface and transform into sessile polyps, marking the beginning of the asexual phase of reproduction.

Asexual reproduction occurs during the polyp stage when the anchored polyp produces genetically identical offspring through a process called budding. As these buds mature, they turn into ephyrae or baby jellyfish.

Ephyrae have short tentacles and a small, disc-shaped body. However, these juvenile jellyfish face multiple challenges, such as finding food and avoiding predators. Ephyrae primarily feed on tiny organisms drifting in the water, but they must also watch out for larger marine animals that see them as easy prey due to their weak, stinging cells. 

Over time, temperature, salinity, and food availability influence the ephyrae's growth. Their tentacles gradually lengthen, and their bodies resemble the iconic bell shape. Once they mature, these once-tiny creatures become mesmerizing adult jellyfish.

11. One jellyfish species is considered to be immortal.

One of the most astounding jellyfish facts is the immortality of one particular species. Turritopsis dohrnii is called the immortal jellyfish. It can revert from the adult or medusa stage to the juvenile polyp stage. This amazing process is called Transdifferentiation.

Factors like stress, injury, or an unfavorable environment can trigger this transformation. Undergoing Cellular Transdifferentiation enables the immortal jellyfish to reset its life cycle, thus avoiding the natural aging process and allowing them to cheat death. However, it's important to note that these species still face threats such as diseases, predation, and environmental factors that could cause mortality.

As the organisms cells essentially renew, its unique life cycle has captured the interest of scientists worldwide as it could potentially offer valuable insights into the fields of aging research, cellular regeneration, and regenerative medicine.

12. The box jellyfish is the most venomous marine creature.

Did you know that the box jellyfish, specifically the Australian species Chironex fleckeri, is considered the most venomous marine creature on Earth? They are primarily found in the Indo-Pacific region and can frequently be found along Australia's northern coast. 

Their venom contains a powerful combination of toxins that can affect the heart, nervous system, and skin cells. It carries enough venom to kill more than 60 people. As active hunters, box jelly actively pursues small fish and invertebrates. Their advanced swimming skills and well-developed eyes give them an edge in hunting prey. 

The box jellyfish is a formidable predator with tentacles reaching up to 10 feet long, each containing around 5,000 stinging cells called nematocysts; the box jellyfish is a formidable predator. When these stinging tentacles come into contact with a victim, the nematocysts release venom, causing intense pain that can persist for weeks. In more severe cases, a box jellyfish sting may result in cardiac arrest, paralysis, or even death. 

It’s important to note that jellyfish don’t purposely attack humans, and not all jellyfish sting. Most stings occur when individuals inadvertently come into contact with a jellyfish. However, if you get stung by a jellyfish, you can use salt and vinegar to counteract the venom. It’s also essential to ask for immediate medical attention since the sting from box jellies is life-threatening.

13. Jellyfish blooms are linked to climate change and overfishing.

The increase in jellyfish populations in some marine regions, known as jellyfish blooms, worries scientists and coastal communities. These blooms can harm fish populations, damage fishing nets, and even cause nuclear power plants to shut down due to clogged cooling water intakes. Nomura’s Jellyfish, in particular, is a great threat to Japanese, Chinese, and Korean fisheries. 

These phenomena owe much of their occurrence to two interconnected factors: climate change and overfishing. As global warming causes ocean temperatures to rise, warmer waters create a perfect breeding ground for jellyfish reproduction. 

This increase in temperature also leads to a decrease in dissolved oxygen, which negatively affects many marine species but not jellyfish. As a result, they gain a competitive edge, thriving in environments where other species struggle.

Overfishing also exacerbates the issue of jellyfish blooms. By removing natural predators and competitors, such as sea turtles and certain fish species, overfishing clears the way for the populations to grow. 

As fish populations decrease, jellyfish benefit from a more extensive food supply, further supporting their growth and proliferation. Additionally, overfishing disrupts the balance of marine ecosystems, ultimately creating conditions that favor rapid increases in jellyfish populations.

Related: Environmental Impact of Fishing.

14. Over 2,000 species of jellyfish exist, from tiny to massive.

Last but certainly not least, from our top jellyfish facts, there are over 2,000 known species of jellyfish. Species vary greatly in size, from the tiny Irukandji jellyfish, barely visible to the naked eye, to the largest giant jellyfish, the lion's mane, which boasts a bell diameter of up to 7.5 feet and long tentacles extending over 120 feet.

Exhibiting a multitude of colors and shapes, some jellyfish are transparent and nearly invisible, while others captivate with vibrant hues of red, blue, or purple. These colors can serve various purposes, such as camouflage or warning potential predators of their venomous nature.

Read more: 21 Different Types of Jellyfish From Stingers To Eerie Floaters, and to further explore the animal kingdom, check out some of the other animals that start with J.