For your convenience, this information sheet is also available as part of the full Amazing Jellies teachers guide pdf.

Quick facts

Blobs of goo

Sea jellies are not just floating, stinging lumps of slime.

Mistakenly called jellyfish, sea jellies are not fish at all. Jellies have no backbone, no brain, no heart, and are 95% water. Out of the water, they are
shapeless gelatinous blobs often found on the beach after a rough storm. Underwater, however, they are beautiful, graceful swimmers.

Family tree

“Jellies” includes a huge spectrum of gelatinous zooplankton.

Most jellies are stinging cnidarians, but “jellies” also includes salps (more closely related to humans than to cnidaria) and ctenophores. Neither salps nor ctenophores sting. Anemones, corals and sea pens are also cnidarians, but unlike adult jellies, they do not have a free-swimming medusa life stage.

Ancient animals

Sea jellies have lived for millions of years.

Fossils containing impressions of jellies have been found in 650-million-year old rocks off the coasts of England, South Africa and Australia. Today, jellies live in all oceans and in some freshwater rivers and lakes.

Eat or be eaten

Jellies are both predators and prey in the ocean food chain.

They eat as they drift, catching plankton, fish eggs, larval fishes and invertebrates, and small shrimp-like animals. Some jellies also eat adult fishes and other jellies, while others supplement their diets with the products of photosynthesizing algae living in their tissues. As prey, jellies are a tasty meal for other animals, especially sea turtles.


Jellies eat on the go.

Most use their tentacles to capture a meal, and then slowly pass the food toward their bell (or saucer-shaped body). If food is scarce, jellies have the unusual ability to shrink in size, requiring less food. When food is abundant, they can grow in size.

On the move

Jellies either ride the ocean currents or use their pulsing swimming style.

By simply riding the ocean’s currents, a jelly can travel hundreds of miles. Most jellies can move themselves toward the sunlight, a food source, or the current, by using their pulsing swimming style.

Bad reputation

Out of approximately 2,000 jelly species, only about 70 are known to be harmful to humans.

The Portuguese man o’ war is often feared. This jelly is actually a colony of animals working together to create the infamous floating creature. Swimmers can easily avoid the Portuguese man o’ war’s gas-filled float, but they might not see the 60 feet of stinging tentacles that trail behind. The sting from this jelly
can be very painful, but is rarely fatal. The ocean’s most dangerous jelly is the Australian sea wasp. Its sting can be deadly.


The jelly’s familiar bell shape is typically one stage in its complex life cycle.

For example, moon jellies, which are common in Boston Harbor, spend part of their lives as tiny individual polyps attached to the ocean fl oor. The polyp can bud to produce more polyps or transform into a stack of tiny jelly saucers that break off to grow into the bell-shaped adult. The adult jellies then produce eggs that hatch and settle to the bottom to produce more polyps.


A jelly called Aequorea is especially interesting to scientists for its ability to emit light.

This species contains proteins that glow when they come in contact with calcium. Because calcium plays an important role in human cell division, researchers studying cancer are using the glowing properties of Aequorea to successfully map calcium in a dividing cell.

Spaced out

In 1991, some 2,500 baby moon jellies were launched into space on the shuttle Columbia.

These voyagers were then closely monitored to study what effect a weightless environment had on their swimming abilities. Scientists hope to learn how gravity influences development in jellies and higher animals, like humans.

Too big to believe

Sir Arthur Conan Doyle popularized the lion’s mane jelly by making it the killer in one of his Sherlock Holmes mysteries.

The lion’s mane can grow to be one of the largest jellies in the world, reaching up to eight feet in diameter with tentacles up to 200 feet long. The lion’s mane is a coastal jelly usually found near the water’s surface. Lion’s mane jellies can cause severe stings, but there have been rare reports that these stings are fatal.