Jellyfish: Species, Stings and Life at Sea

Every summer, swarms of jellyfish arrive along Israel’s coast and disrupt a swim in the Mediterranean. Nomad jellyfish blooms can disrupt swimming, fishing, and coastal infrastructure.

Before you give up on the sea and head for the nearest swimming pool, take a closer look at this remarkable animal.

A jellyfish is an invertebrate that usually lives in the sea. Its simple, gelatinous body belongs to a distant relative of corals and sea anemones, fellow members of the cnidarian group. It has no head, central brain, heart or ears. Sensory structures vary: box jellyfish, for example, have eyes.

Jellyfish belong to several groups of cnidarians. Fossils preserve recognizable jellyfish from about 505 million years ago. Their forms vary widely. Yet most share a broadly similar outline: a bell, oral arms and hunting tentacles.

The bell is the jellyfish’s smooth, umbrella-shaped body. Pulsing movements propel it through the water. Beneath it lie the mouth and digestive cavity. Digestion is simple: food goes in and waste comes out through the same opening.

The digestive cavity is lined with cells that help break down food. A layer of jelly separates this inner cavity from the animal’s outer surface. In most jellyfish, the hunting tentacles hang from the edge of the bell.

A jellyfish has no central brain, but it does have a simple nervous system called a nerve net, spread through its outer layer, the epidermis. Touch or chemical cues can trigger capsules inside stinging cells to discharge a tiny venom-bearing thread.

Stinging cells serve more than defence. They stun, paralyse and kill prey. Despite their dangerous reputation, jellyfish are relatively passive invertebrates that feed mainly on other invertebrates, protozoans and small fish.

Many jellyfish have simple structures that respond to stimuli such as light, while box jellyfish have more complex eyes. Some cells detect light and darkness, allowing the animal to react to changes without forming a true image. In some jellyfish, other structures help them respond to gravity.

Jellyfish can swim by pulsing their bells, although currents strongly influence where they go. The Portuguese man-of-war is a siphonophore colony, not a true jellyfish. Its gas-filled float catches the wind.

Jellyfish stings can be dangerous to people. The effects vary by species. Some stings cause severe pain or scarring, and certain box jellyfish can cause fatal injuries. The Australian box jellyfish, Chironex fleckeri, is also known as the sea wasp. The Portuguese man-of-war can also deliver painful stings, although it is not a true jellyfish.

Here is a closer look at some of these animals.

Crystal jelly

This fascinating jellyfish gives off a faint glow. Its green fluorescent protein, GFP, has become an important tool for observing cells and biological processes.

Australian box jellyfish

You have probably heard that jellyfish can be dangerous. That is true of some species, but the Australian box jellyfish is particularly feared. Also known as the sea wasp, it can be difficult to see at all: its body is almost transparent, and its long tentacles may reach you before you spot it.

Stauromedusae off Costa Rica

Researchers found pink stalked jellyfish near hydrothermal vents off Costa Rica at depths greater than 2 kilometers. Vent fluid reached 335 degrees Celsius, but the animals lived in cooler surrounding water.

Flower hat jelly

This colourful little jelly looks like a hat topped with a glowing pink flower. It is found in the waters of Brazil and Argentina, and even Japan. Small as it is, its bright colours make it one of the most striking jellies.

Nomura’s jellyfish

It sounds like something from a horror film: one of the world’s largest jellyfish, with a bell up to about 2 meters across and a weight of about 200 kg. It is found mainly off Japan.

Nomad jellyfish

The nomad jellyfish reached the Mediterranean through the Suez Canal and was first reported along Israel’s coast in the 1970s. It has since spread farther across the Mediterranean. Large blooms occur in summer and have also been recorded in winter.

These jellyfish cause significant problems for fishing and tourism. They can damage ship engines and power-station turbines, while their venom causes painful stings and can seriously affect people who are sensitive to it.

Lion’s mane jellyfish

The largest jellyfish known to science favours cold northern waters: the Arctic, North Atlantic and North Pacific. The largest recorded example had a bell 2.3 metres across and tentacles 37 metres long. These jellyfish act as floating islands of life, offering shelter to many animals among their tentacles.

Portuguese man-of-war

One of the most recognisable jellyfish lookalikes takes its name from its resemblance to a sailing warship. Surprisingly, it is not a jellyfish at all. It is a colony of organisms called zooids, each with a specialised role and dependent on the others to survive. A gas-filled float keeps the colony at the surface. Above it, a sail-like crest catches the wind and carries this remarkable creature across the water. It lives mainly in warm seas, with an increasing presence reported in the Mediterranean in recent years.