FaunaHub

Jellyfish

MarineCnidarianInvertebrate

A translucent moon jellyfish drifting in dark water.

Moon jelly (Aurelia aurita).

Image: André Karwath aka Aka, CC BY-SA 2.5, via Wikimedia Commons.

Overview

Jellyfish (often called "jellies" by scientists) are soft-bodied marine animals in the phylum Cnidaria, the same group as corals and sea anemones. This page is a group-level overview; the moon jelly (Aurelia aurita) is shown as a familiar reference. Jellyfish have a gelatinous bell-shaped body and trailing tentacles armed with specialised stinging cells. They are not fish and have no brain, bones, or heart.

Habitat & Range

Jellyfish are found in oceans worldwide, from warm surface waters to the cold deep sea, and a few live in fresh or brackish water. Many drift with currents in open water, while some live near coasts and can form large seasonal aggregations known as "blooms". Habitat and depth vary widely across the many jellyfish species.

Diet

Most jellyfish are carnivorous, capturing small prey such as plankton, fish eggs, larvae, and tiny crustaceans with their stinging tentacles before moving the food to the central mouth. Some larger species take bigger prey. A number of jellyfish also host symbiotic algae in their tissues that provide additional energy from sunlight. Diet varies by species.

Behavior

Jellyfish swim weakly by rhythmically pulsing the bell, but largely drift with ocean currents. Their stinging cells, called nematocysts, fire automatically on contact to subdue prey and deter predators. Many jellyfish have complex life cycles that alternate between a swimming "medusa" (the familiar form) and an attached "polyp" stage. Some species form enormous blooms under favourable conditions.

Human Interaction & Conservation

Jellyfish are an important part of marine food webs and are prey for animals such as sea turtles. For people, the main caution is their sting: severity ranges from mild to, in a few species, medically serious, so unknown jellyfish — whether in the water or washed up on a beach — should never be handled. Follow local safety advice. Conservation status for most species is not well established and should not be assumed.

Appearance & Recognition

A typical jellyfish has a translucent, dome- or bell-shaped body made largely of a jelly-like material, with tentacles and frilly oral arms hanging beneath. The moon jelly is recognisable by the four horseshoe-shaped gonads visible through its clear bell. Other jellyfish range from tiny, nearly invisible species to large ones with long, trailing tentacles. Bodies are over 90 percent water, so beached jellyfish quickly collapse.

Similar Animals

Jellyfish are cnidarians, related to corals and sea anemones. They are sometimes confused with other drifting gelatinous animals such as comb jellies (a separate phylum) and siphonophores like the Portuguese man o' war, which is actually a colony of organisms rather than a single jellyfish.

More photos of the jellyfish

A moon jelly with visible horseshoe-shaped gonads, lit from the side.

A moon jelly showing the four horseshoe-shaped gonads.

Image: Luc Viatour, CC BY-SA 3.0, via Wikimedia Commons.

Frequently Asked Questions — Jellyfish

Are jellyfish fish?
No. Despite the name, jellyfish are not fish. They are invertebrates in the phylum Cnidaria, with no backbone, brain, bones, or heart. Many scientists prefer the term "jellies" to avoid the confusion.
Do all jellyfish sting badly?
No. Sting severity varies enormously by species, from barely noticeable to medically serious in a few. Because it is hard to tell species apart and even beached or dead jellyfish can sting, the safe approach is never to touch unknown jellyfish and to follow local safety advice.
What is a jellyfish bloom?
A bloom is a large, sometimes sudden increase in jellyfish numbers in an area, often seasonal. Blooms can be driven by factors such as water temperature, currents, and food availability, and they can affect fisheries, tourism, and other marine life.

Sources and further reading

Authoritative wildlife references used for general educational context. Conservation status should always be verified against current IUCN Red List data. External links open in a new tab.