Jellyfish: The smart stinging creatures drifting through our oceans

Jellyfish are invertebrate animals, meaning they do not possess a backbone, and they belong to the phylum Cnidaria. Their relatives include familiar marine creatures such as brightly colored corals and sea anemones.

Jellyfish have soft, gelatinous bodies with a characteristic bell-like shape. From the lower part of the bell hang numerous tentacles. The underside also contains the mouth and four oral arms, which are thicker and more substantial than the surrounding tentacles.

What Do Jellyfish Look Like?

Jellyfish display an extraordinary variety of forms, colors, and body sizes. For most species, the bell measures somewhere between two and 40 centimeters across. However, certain species can exceed two meters in diameter, while their narrow trailing tentacles can extend even farther.

Jellyfish have radial symmetry. This means that if their bodies were divided through the center, the resulting sections would have a similar arrangement, much like the segments of an orange.

These cnidarians are particularly famous for their ability to sting, although the strength and effects of the sting differ considerably between species.

Some jellyfish possess another remarkable ability: bioluminescence. By producing their own light, these animals can create spectacular displays that make them seem almost otherworldly.

How Do Jellyfish Use Bioluminescence?

The atolla jellyfish, Atolla wyvillei, provides an excellent example of how bioluminescence can be used as a defense mechanism.

When threatened, this jellyfish produces rotating patterns of light that function somewhat like an alarm signal. The display can attract larger predators that may attack whatever is threatening the jellyfish. This gives the atolla jellyfish an opportunity to escape while its attacker is distracted.

Do Jellyfish Have Brains?

Jellyfish do not have bones, brains, hearts, teeth, or blood.

Instead, they possess a network of nerves known as a nerve net. This simple nervous system enables them to respond to information from their surroundings, including changes in light, temperature, and the chemical composition of the surrounding water.

How Many Jellyfish Species Are There?

Jellyfish are also commonly referred to as sea jellies. The true jellyfish groups belong to Medusozoa, which is divided into four major classes: Hydrozoa, Scyphozoa, Cubozoa, and Staurozoa.

Comb jellies are sometimes included under the broader term “sea jellies,” but they are not cnidarians. Instead, comb jellies belong to a separate group called Ctenophora.

Scyphozoa

There are roughly 200 scientifically described species of scyphozoans. Most are free-swimming animals with the familiar bell-shaped body known as a medusa.

These are among the jellyfish that most closely match the traditional image people have of a jellyfish drifting through the sea.

Cubozoa

Cubozoans are more commonly called box jellyfish. They can be distinguished from scyphozoan jellyfish by their unusual cube-shaped medusae. Their tentacles are positioned around the four lower corners of the body.

Approximately 45 species of box jellyfish are known, and this group includes some of the most dangerously venomous animals on Earth.

Box jellyfish are also more active predators than scyphozoan jellyfish. The largest species, Chironex fleckeri, has been recorded swimming at approximately five centimeters per second.

Another unusual feature of box jellyfish is their vision. Instead of lacking complex eyes entirely, they possess clusters of sophisticated eyes positioned around each corner of their cube-shaped body.

Hydrozoa

Hydrozoa contains approximately 3,600 species. These animals are generally smaller than scyphozoan jellyfish, and in many species the polyp stage is more obvious than the relatively small adult jellyfish form.

Not every hydrozoan, however, is considered a jellyfish. The Portuguese man o' war, for example, belongs to Hydrozoa but is classified as a siphonophore.

Siphonophores are unusual colonies made up of numerous highly specialized individual animals that live and function together. Their bodies contain combinations of structures resembling both polyps and jellyfish.

Physalia physalis, commonly called the Portuguese man o' war or bluebottle, is a colony composed of four types of highly modified individuals. Together, these specialized members form an effective and striking marine predator.

Staurozoa

Staurozoans are commonly known as stalked jellyfish. Their appearance and lifestyle can sometimes make them seem more closely related to sea anemones or corals than to typical free-floating jellyfish.

Unlike most jellyfish, stalked jellyfish do not drift freely through the ocean. Instead, they attach themselves to seaweed, rocks, and other underwater surfaces.

There are approximately 50 known species of stalked jellyfish. Most occur in shallow or intertidal environments, although some have been discovered at depths exceeding 3,000 meters.

How Do Jellyfish Move?

Jellyfish often depend on ocean currents to carry them through the water. When necessary, however, they can use their oral arms and body movements to pulse through the surrounding water.

They are not particularly powerful swimmers, which means that strong currents and storms can move them considerable distances. This is one reason large numbers of jellyfish may sometimes be washed onto beaches after severe weather.

How Long Do Jellyfish Live?

The lifespan of a jellyfish varies depending on the species. Many jellyfish survive for only several weeks, while some species are capable of living for a year or even longer.

What Do Jellyfish Eat?

Jellyfish capture food by stinging their prey. Their oral arms then help move the captured food toward the mouth and into the digestive cavity.

Interestingly, the jellyfish's mouth also serves as its anus, meaning that food enters and waste leaves through the same opening.

Their diet can include small fish, fish larvae, shrimp, krill, copepods, amphipods, and microscopic plant material such as algae. Some jellyfish will even consume other jellyfish.

Jellyfish can also take advantage of food remains left behind by larger predators. They may feed on scraps produced by animals higher in the food chain, including sharks and turtles.

What Animals Eat Jellyfish?

Jellyfish are an important food source for a variety of marine animals. Leatherback sea turtles are well-known jellyfish predators, while ocean sunfish and grey triggerfish also consume them.

Some seabirds, including fulmars, feed on jellyfish as well. Other predators include whale sharks, certain species of crabs, and some whales.

Humans also consume certain jellyfish species. In parts of East Asia, particularly Japan and China, selected species are regarded as delicacies.

In China, people have been catching and eating certain jellyfish as food for approximately 1,700 years.

Jellyfish could potentially become a more common food source in the future because overfishing has reduced the availability of some traditional fish species.

However, people's willingness to eat jellyfish depends on many factors, including age, gender, travel experience, and familiarity with the food. Food neophobia, or reluctance to consume unfamiliar foods, can also discourage people from adding jellyfish to their diets. This behavior has been observed among consumers in Italy, Canada, and Latin America.

How Do Jellyfish Sting?

Jellyfish possess specialized stinging cells called nematocytes. Thousands of these cells can be found along their tentacles.

Inside each stinging cell is a microscopic harpoon-like structure containing venom. When a jellyfish captures prey or needs to defend itself, these structures can rapidly fire outward, penetrate the target's skin, and deliver toxins.

Jellyfish stings can produce significant burning pain, which is why people often instinctively reach for ice. However, research has indicated that applying heat or immersing the affected area in water that is as hot as can be comfortably tolerated may provide better relief in some cases.

Jellyfish Stings and Their Effects

Not every jellyfish species is capable of causing a harmful sting in humans, and the severity of a sting varies greatly between species.

Some marine animals have even developed ways to take advantage of jellyfish stinging cells for their own protection. Certain fish sometimes shelter among jellyfish tentacles, feeding on leftover pieces of food while gaining protection from larger predators.

How Do Jellyfish Reproduce?

Jellyfish undergo substantial changes in body form during their life cycle.

Adult jellyfish release eggs and sperm into the surrounding seawater, where fertilization takes place. The fertilized egg develops into a free-swimming larva.

Eventually, the larva transforms into a polyp. The polyp attaches itself to a solid surface, such as a rock, and develops there.

As the polyp matures, it eventually produces young jellyfish that detach and become free-swimming animals, beginning the next stage of the jellyfish life cycle.

Fascinating Jellyfish Species

Jellyfish represent an incredibly diverse group of marine animals, with many species displaying unusual forms and lifestyles.

Lion's Mane Jellyfish

The lion's mane jellyfish, Cyanea capillata, gets its name from its numerous flowing tentacles, which resemble the mane surrounding a lion's head.

It is recognized as the largest known jellyfish species and occurs in the Arctic, North Pacific, and North Atlantic oceans. It is also frequently encountered in waters around the United Kingdom and Ireland.

Its tentacles can extend for more than 30 meters, making them longer than a blue whale.

Upside-down Jellyfish

Upside-down jellyfish belong to the genus Cassiopea. Their unusual behavior gives them their common name: instead of swimming in the typical orientation, they frequently rest upside down on the seafloor with their tentacles directed upward.

These jellyfish maintain a symbiotic partnership with algae living within their tissues. By positioning themselves in shallow tropical water with their undersides exposed to sunlight, they allow the algae to carry out photosynthesis.

The products generated through photosynthesis provide Cassiopea with much of the nutrition it needs.

Immortal Jellyfish

The remarkable hydrozoan Turritopsis dohrnii is often described as biologically immortal because of its extraordinary ability to reverse its life cycle.

Rather than simply continuing to age in its mature form, it can transform back into an earlier developmental stage through a process known as transdifferentiation. Under suitable conditions, it can then develop once again into a mature jellyfish.

Fried Egg Jellyfish

The fried egg jellyfish, Cotylorhiza tuberculata, receives its common name from its unusual appearance. Its rounded, yellowish body resembles the yolk of a fried egg.

This species is mainly found in the Aegean, Adriatic, and Mediterranean seas. It has relatively short oral arms and primarily feeds on zooplankton and other jellyfish.