A List of Popular Types of Jellyfish with Pictures

An Ancient Species

Jellyfish have been drifting through the world's oceans for over 500 million years, which makes them the oldest living creatures on the planet. They were here before dinosaurs and sharks, and they have even survived longer than the dinosaurs. Jellyfish, along with the sea whip, coral, and sea anemone, belong to phylum Cnidaria.

Jellyfish are not actually fish, so it is more accurate to call them "sea jellies" or, more broadly, "zooplankton." They are also known as medusae, because their trailing tentacles reminded ancient Greeks of the snake-haired gorgon, Medusa, from mythology.

They live in different water conditions and at different depths, from the ocean floor to the surface. Over 2,000 species of jellyfish have been identified so far, and scientists believe that as we keep exploring the depths of the ocean, many more will be discovered. On land they look like jelly-like, shapeless blobs, but in the water they are graceful, elusive, and sometimes extremely dangerous.

No Brain, No Blood, and Still Thriving

One of the most surprising jellyfish facts is that they have no blood, no brain, no nervous system, and no respiratory system. Instead, they have a neural net that detects stimuli, plus eye spots that can tell light from darkness.

The box jellyfish is a remarkable exception. It has extraordinary eyes complete with a sophisticated lens, cornea, iris, and retina, yet scientists still can't fully explain how it processes what those eyes see. Other jellyfish use chemosensory pits to identify food and prey. It's intriguing how these animals track their food, find their way through the water, and protect themselves, all without a brain.

Size & Body: 95% Water

A jellyfish has no skeleton. Its body is made of a gelatinous substance that is 95% water, meaning only 5% of the jellyfish is solid matter. The bell-shaped body is built from three layers: the outer epidermis, the inner gastrodermis, and the middle layer called the mesoglea.

The fringe-like protrusions found around the edge of the bell are called tentacles. Jellyfish also have 4–8 oral arms growing from the central part of the bell.

Jellyfish Are Delicate Creatures

Because they are made mostly of water, these sea creatures are extremely delicate and perish when exposed to sunlight. Without water to support their body structure, they collapse and drop dead once taken out of the ocean.

Some types of jellyfish, such as the box jellyfish, swim to the bottom of the ocean during the day and only resurface when the sun goes down. Rough handling can also kill them. Aquatic surroundings are vital for jellyfish, a lack of water around them causes them to collapse and die.

Tentacles and Nematocysts

Jellyfish have tentacles lined with myriads of nematocysts, capsule-like structures filled with toxins. Each nematocyst has sensitive, hair-like structures that detect the presence of prey. Once prey is detected, the tentacles release their toxins, which then paralyze the prey. Besides catching food, jellyfish use these same tentacles for self-defense.

Jellyfish also have a perfectly symmetrical body, with all the body parts radiating from a central axis. This perfect symmetry not only lets the jellyfish detect food from all sides, but also helps it respond to danger from every direction.

What They Eat: One Opening for Everything

Jellyfish have one opening inside the bell-shaped body, situated between the gonads. This single hole acts as the mouth while feeding and as the anus while excreting. They feed on sea creatures like crustaceans, larvae, small fish, and other zooplankton.

They trap marine organisms with their tentacles while floating in the water. The nasty sting from the tentacles renders the prey paralyzed or dead. Jellyfish then digest their food quickly inside a sac-like gastrovascular cavity, which keeps them light enough to stay afloat.

How They Swim: Masters of Drift

Jellyfish drift along the water currents and depend on wind, tides, and ocean currents for horizontal movement, they can't really steer sideways. They are, however, capable of vertical movement. A circular muscle runs around the bell structure.

When a jellyfish contracts this circular muscle, water is drawn into the bell-shaped structure. It then ejects the water, causing a propelling force. Once it moves forward, water fills the bell again. This repeated contracting of the body and production of a propelling force pushes the jellyfish forward, making its slow vertical journeys possible.

How They Reproduce

Jellyfish are dioecious, meaning the species has both male and female individuals, each with horseshoe-shaped gonads. They reproduce by mass spawning: the female lays her eggs in the water, and the male, sensing the signals given by the female, releases his sperm nearby. The water currents drift the sperm toward the eggs, and fertilization takes place. Scores of polyps are born on the seabed, and after a while, miniature jellyfish (called medusae) are seen developing.

Jellyfish in Huge Numbers

The oceans are filled with scores of different types of jellyfish. In fact, ecologists are urging people to eat them instead of fish: while the numbers of edible fish are dropping because of over-fishing, the numbers of jellyfish are multiplying at the wink of an eye. Jellyfish feed on tiny fish larvae, which further reduces the fish population.

Jellyfish also cause a lot of nuisance to ships and carriers in the ocean. They generally live and move in large swarms, billions strong!, and are often seen hampering the movement of vessels. The water inlets of the vessels get clogged up and must be cleared of masses of dead jellyfish on a regular basis to keep the ships running. Eating jellyfish would not harm biodiversity; instead, it would help maintain the ecological balance.

Are Jellyfish Edible?

There are twelve types of edible jellyfish, of which sand jellyfish and cannonball jellyfish are the most popular. Although jellyfish are 95% water, they are nutritious with a high protein content. Their lack of cholesterol, carbohydrate, and saturated fat makes them a popular weight-loss food choice.

The first reaction to the idea of eating jellyfish is usually dread, people often loathe the thought of eating something bland, rubbery, and wet. But those who have acquired a taste for jellyfish find it delectable. Jellyfish sold in stores are venom-free, so there is no danger of being stung.

Jellyfish is relished as a delicacy in Asian countries like China, Japan, Malaysia, Korea, and Thailand. While jellyfish do not have any particular flavor of their own, the dishes are made tasty by adding various spices and sauces. Shredded jellyfish is a common delicacy at Chinese weddings, and dried jellyfish often appears in the famous Japanese dish, sushi. Jellyfish burgers are also new dishes found in some restaurants, and Chinese and Thai supermarkets keep fresh (salad-form) and salted jellyfish for people to buy.

Jellyfish Stings

Most jellyfish stings are non-poisonous, although they can leave a person with searing pain, and an itchy red rash or allergic reaction may follow. The most painful sting is that of the Portuguese man-of-war (which is not a true jellyfish), and it can sting you even when it's dead, so if you find one washed up on the beach, don't go near it. A sting from this species can also lead to shock, fever, and respiratory and cardiovascular problems. The sea nettle is also known to sting painfully, with symptoms ranging from rashes and muscle cramps to breathing trouble.

While jellyfish do not attack people on purpose, most cases of jellyfish stings happen when people accidentally come into contact with them on the beach or while swimming in the ocean.

The Most Venomous Jellyfish

The box jellyfish is among the most venomous creatures in the world. Several species of box jellyfish are highly venomous, but the largest and most venomous is Chironex fleckeri, which is also called the sea wasp. Found in the northern part of Australian waters, these jellyfish have thousands of sting cells on their tentacles. The sting of the sea wasp is fatal because it has a direct effect on the heart, nerves, and skin, most victims die of shock, cardiac arrest, or drowning within minutes of being stung.

Also found in Australian waters, the Irukandji jellyfish (a species of box jellyfish) is extremely dangerous too. This tiny jellyfish measures only 2.5 cm in length, yet its sting can cause a string of symptoms ranging from headaches to cramping to even heart failure. The symptoms people experience after the sting are known as Irukandji syndrome.

The reason jellyfish numbers are escalating is the increase in water salinity, which has happened because of a decrease in rainfall. An increase in jellyfish numbers indicates an upset marine ecosystem, and that is something we need to consider very seriously.