Jellyfish

Jellyfish at a glance

Need to know

• Jellyfish are ancient animals that have lived in Earth's oceans for over 500 million years.
• Some jellyfish, like the box jellyfish, are among the most venomous animals on Earth.
• Jellyfish are 95% water and have no brains, hearts, or bones.
• Certain species, like the Immortal Jellyfish, can 'age in reverse' by reverting to their juvenile stage.
• Jellyfish play crucial roles in marine food webs and ecosystem balance.
• Massive jellyfish blooms are increasing in frequency and can disrupt fisheries and tourism.

About the Jellyfish

Jellyfish, members of the class Scyphozoa, are gelatinous, free-swimming marine animals recognized by their bell-shaped bodies and trailing tentacles. Ranging in size from tiny to enormous, jellyfish inhabit every ocean on Earth and have survived for more than half a billion years, adapting to shifting global climates and various marine environments. They are not fish, but cnidarians—relatives of sea anemones and corals—whose stinging cells (nematocysts) are used for capturing prey and deterrence.

Jellyfish lack a centralized brain, heart, and specialized respiratory organs, instead relying on a simple nerve net and passive diffusion of oxygen. Their ethereal, translucent beauty masks a fascinating complexity: some are nearly invisible, others brightly colored or bioluminescent, and a few possess venom potent enough to be deadly to humans. Recent studies highlight their increasing populations due to climate change and overfishing, as well as their vital ecological roles and medical potential.

Physical characteristics

Size: From less than 1 cm (Immortal Jellyfish) to over 2 m bell diameter (Lion's Mane Jellyfish). Weight: Typically 0.01–200 kg depending on species. Lifespan: Several months to a few years; some can theoretically avoid natural death.

• Gelatinous, bell-shaped body (medusa)
• Stinging tentacles with nematocysts
• Oral arms used for feeding
• Lack of central nervous system or skeleton
• Some species are bioluminescent or vividly colored

Notable species

• Lion's Mane Jellyfish (Cyanea capillata) — Not Evaluated. Cold, boreal seas near the surface and deeper waters Regions: North Atlantic Ocean, Arctic Ocean, Bering Sea, North Pacific Ocean.
• Moon Jellyfish (Aurelia aurita) — Not Evaluated. Harbors, estuaries, bays, and inshore ocean waters Regions: Atlantic Ocean, Pacific Ocean, Indian Ocean, Mediterranean Sea, Baltic Sea.
• Box Jellyfish (Chironex fleckeri) — Not Evaluated. Coastal tropical waters and shallow bays Regions: Coral Sea, Arafura Sea, Timor Sea, waters off Northern Australia.
• Nomura's Jellyfish (Nemopilema nomurai) — Not Evaluated. Temperate coastal and offshore waters Regions: Yellow Sea, East China Sea, Sea of Japan, waters off Korea.
• Mauve Stinger (Pelagia noctiluca) — Not Evaluated. Open ocean, sometimes near the surface Regions: Mediterranean Sea, Northeast Atlantic Ocean, South Atlantic Ocean, Caribbean Sea.
• Immortal Jellyfish (Turritopsis dohrnii) — Not Evaluated. Warm to temperate marine waters Regions: Mediterranean Sea, Caribbean Sea, East China Sea, North Pacific Ocean.

Habitat and range

Original range: Ancient marine environments; believed to have originated in the world's primordial oceans

Current distribution: Jellyfish are now distributed in all ocean basins, from surface waters to deep sea and from tropical to polar regions. Some species also thrive in brackish or estuarine environments.

Preferred environments: Open ocean (pelagic zone), Coastal waters, Estuaries, Deep-sea environments, Bays and harbors

Regions: Pacific Ocean, Atlantic Ocean, Indian Ocean, Arctic Ocean, Antarctic Ocean, Mediterranean Sea, Caribbean Sea, Bering Sea, Baltic Sea, Sea of Japan, Coral Sea, Yellow Sea, Gulf of Mexico

Where to see Jellyfish

• Monterey Bay, California (Pacific Ocean) — Spring and summer. Moon jellies and sea nettles often bloom near the coast and are visible from piers and aquariums.
• Sea of Japan — July to September. Massive Nomura's jellyfish blooms can be witnessed, especially after typhoons.
• Great Barrier Reef, Coral Sea — November to May (stinger season). Australian box jellyfish are present; use caution as they are highly venomous.
• North Atlantic (e.g., off Nova Scotia) — Summer to early fall. Lion's Mane jellyfish can often be seen near the surface or washed up on beaches.

Behavior and diet

Diet: Jellyfish are carnivores, feeding on plankton, small fish, fish eggs, larvae, crustaceans, and even other jellyfish. They use stinging tentacles to paralyze and capture their prey.

Social structure: Many species are solitary but can form vast groups called blooms or swarms, sometimes numbering in the millions.

Communication: Jellyfish lack complex behaviors but may use chemical cues, touch, and simple nerve net responses to coordinate movement. Some colors and light emissions may play a role in spacing and reproduction.

• Passive drifting with water currents, sometimes pulsing to swim upwards or evade predators
• Seasonal mass bloom events, where thousands or millions gather
• Bioluminescent flashes as defense against predators
• Some species can revert to the polyp stage, effectively 'rejuvenating' or escaping death

Interactions with other animals

• Sea turtle (predator): Leatherback and other sea turtles eat jellyfish, helping control their populations.
• Small fish (juvenile) (symbiotic): Young fish often take refuge among jellyfish tentacles for protection from predators.
• Ocean sunfish (Mola mola) (predator): Ocean sunfish are major consumers of jellyfish.
• Humans (competitive): Overfishing of jellyfish predators and ecosystem changes lead to increased jellyfish blooms, sometimes clashing with human activities.

Conservation

IUCN Red List status: Not Evaluated (most species); generally not threatened

Threats

• Climate change altering ocean temperatures
• Habitat modification and pollution
• Overfishing of natural predators
• Hypoxia (low oxygen zones) caused by eutrophication

Conservation efforts

• Monitoring jellyfish populations through global networks
• Fisheries management to restore predator populations
• Reducing nutrient pollution to limit blooms

How to help

• Support organizations promoting healthy oceans and reducing pollution
• Participate in citizen science by reporting jellyfish sightings
• Advocate for responsible fisheries and marine conservation
• Reduce personal plastic and nutrient pollution

Jellyfish facts for kids

Jellyfish are soft, squishy sea creatures that float in the oceans all over the world. They have tentacles that can sting and come in many colors, shapes, and sizes. Some jellyfish are tiny, and some are as big as a grown-up!

• Jellyfish have no bones, brain, or heart.
• Some jellyfish light up in the dark!
• The oldest jellyfish lived before the dinosaurs.
• A jellyfish's body is mostly water.
• Some jellyfish stings can be very painful, while others don't hurt at all.

Did you know? There is a jellyfish called the 'Immortal Jellyfish' that can live forever by turning back into a baby—again and again!

Watch & learn

• Jellyfish: Beautiful and Deadly Creatures of the Ocean — National Geographic. National Geographic's coverage provides in-depth visuals of various Scyphozoa, highlighting their unusual beauty, stinging adaptations, and roles in ocean ecosystems. This documentary features captivating underwater footage and expert commentary.
• Life Story: Magnificent Jellyfish and Their Lifecycle — BBC Earth. BBC Earth's meticulously filmed segment explores Scyphozoa jellyfish from their mesmerizing movement to complex life histories, offering a rare glimpse into their transformation and survival in the wild.
• The Strange Science of Jellyfish — PBS Nature. This PBS Nature episode delves into the biology and evolutionary marvels of jellyfish, featuring interviews with leading marine biologists and stunning close-ups of Scyphozoa species in their natural habitats.
• Jellyfish: Ocean Drifters – Their Role in Marine Ecosystems — Smithsonian Channel. Smithsonian Channel explores the ecological significance of Scyphozoa, examining how jellyfish impact food webs and what their blooms reveal about ocean health in this concise, informative feature.
• Jellyfish Invasion: How Scyphozoa Are Changing Oceans — BBC Earth Lab. BBC Earth Lab investigates jellyfish population explosions and their effects on fishing, tourism, and marine biodiversity, using scientific explanations and field footage to explain this growing phenomenon.