Cnidaria
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40 free Cnidaria trivia questions with answers — trivia quiz, new questions added Oct 2026.
Cnidarians are some of the strangest animals in the sea. Jellyfish, corals, sea anemones and their relatives have no bones and no heart, yet they have drifted, stung and built their way through every ocean since long before the first fish appeared. This quiz collects the best fun facts about the phylum Cnidaria. It covers how these animals sting and feed, how tiny coral animals raise reefs that shelter whole communities of fish, which species are dangerous to swimmers, and which ones hold records for size, age and sheer oddness. A few questions touch on the science, from body plans to life cycles. It is written for curious readers, students meeting the phylum in a biology class, and anyone who has watched a jellyfish pulse past an aquarium window. The early questions are friendly and the later ones are tougher, and every answer comes with a short explanation.
30 of 40 questions with answers and explanations. Play the quiz
Q 01Which tiny sea creature can turn its adult body back into a juvenile polyp and start life again?
Immortal jellyfish
The immortal jellyfish reverts to a polyp when starved, injured or old, then grows into fresh young jellies. In theory the cycle can repeat forever, though in the wild most are eaten or fall sick first.
Q 02Which stinging drifter with a gas-filled float is a colony of specialised units rather than a single animal?
Portuguese man o' war
The Portuguese man o' war is built from many small units called zooids, each specialised for hunting, digestion or reproduction. Its name comes from a sailing warship it was thought to resemble.
Q 03Because a jellyfish's gut has just one opening, which body part must also serve as the animal's anus?
Mouth
The mouth of a jellyfish sits at the tip of a stalk hanging beneath the bell, and waste leaves the same way food came in. No cnidarian has a separate exit.
Q 04The Great Barrier Reef, the biggest structure made by living things, lies off which Australian state?
Queensland
Queensland's coast is fringed by more than 2,900 individual reefs stretching over 2,300 kilometres. The whole system is big enough to be seen from space.
Q 05Which group of sea jellies steers around obstacles with 24 eyes and includes one of the world's most venomous animals?
Box jelly
The box jelly carries six eyes on each of four sensory clubs, some complete with a lens, cornea and retina. A severe sting from the largest species can stop a heart within two minutes.
Q 06Which kitchen liquid is kept in jugs on Australian beaches to stop a tropical jellyfish's stinging cells from firing?
Vinegar
Vinegar permanently switches off stinging cells that have not yet fired, so no more venom goes in. Urinating on a sting, despite the popular belief, does not ease the pain and can make it worse.
Q 07Which tiny freshwater cnidarian, named after a monster of Greek myth, regrows completely when cut in half?
Hydra
Hydra can rebuild a whole animal from small pieces of tissue, each half growing the end it lost. Biologists also prize it because it does not appear to age at all.
Q 08Which Pacific island nation is home to Jellyfish Lake, where snorkellers drift among swarms of golden jellies?
Palau
Palau opens Jellyfish Lake, on the island of Eil Malk, to snorkellers but bans scuba diving there. Its golden jellies cross the lake and back every single day.
Q 09Which giant jelly of cold northern seas, the culprit in a Sherlock Holmes story, can trail tentacles 30 metres long?
Lion's mane jellyfish
The lion's mane jellyfish is one of the largest known jellies, with a bell that can pass two metres across. Its longest tentacles may outstretch a blue whale.
Q 10Which collective noun for a group of jellyfish is also the name of a traditional British fishing boat?
Smack
A smack of jellyfish can swell into a vast swarm when conditions suit, filling fishing nets until the gear is damaged. Scientists usually call such a gathering a bloom.
Q 11Which small banded reef fish shelters among a sea anemone's stinging tentacles, protected by a coat of mucus?
Clownfish
The clownfish seems to wear mucus that fails to trigger the anemone's stinging cells. In return for shelter it drives off anemone-eating fish and keeps its host clean.
Q 12What name is given to the event in which heat-stressed corals expel the algae that feed and colour them?
Bleaching
Bleaching turns a coral pale or completely white. The lost algae supplied up to 90 percent of its energy, so a coral that stays that way may starve.
Q 13Which giant sea reptile lives almost entirely on jellyfish and often swallows drifting plastic bags by mistake?
Leatherback sea turtle
Q 21A jellyfish can sense food or danger from every direction because its body has which type of symmetry?
Radial
Radial symmetry arranges the body around a central point like the spokes of a wheel, with no left or right side. It suits animals that drift or stay put rather than chase.
Q 22Which small crustacean carries a stinging sea anemone in each claw, like a cheerleader's pair of pom-poms?
Boxer crab
The boxer crab grips its anemones with tiny hooks on claws that seem to serve no other purpose. Feeding and digging are left to its walking legs.
Q 23Many reef corals release eggs and sperm together in a mass spawning on nights shortly after which lunar phase?
Full moon
The leatherback sea turtle follows jellyfish across whole oceans, with Pacific animals swimming from Indonesia to feed off California. An estimated third of Pacific adults have swallowed plastic.
Q 14Biologists call the bell-shaped swimming stage of a jellyfish by the name of which figure from Greek mythology?
Medusa
Medusa had snakes for hair, and the trailing tentacles of a swimming jelly suggested her snaky locks. Corals and sea anemones skip this stage and live only as polyps.
Q 15The phylum name Cnidaria, coined for animals armed with stinging cells, comes from the Greek word for which plant?
Nettle
Nettle is knide in Greek, and the comparison fits, since each stinging cell shoots out a coiled thread like a tiny harpoon. The C at the start of Cnidaria is silent.
Q 16Home to at least a quarter of all marine species, coral reefs are nicknamed the sea's version of which habitat?
Rainforests
Rainforests of the sea is the usual nickname, and the comparison is earned on very little ground. Coral reefs cover less than 0.1 percent of the world's ocean area.
Q 17Which colourful reef fish grinds up coral rock as it feeds and excretes the leftovers as white beach sand?
Parrotfish
The parrotfish digests the edible parts of the rock and passes the rest as sand, helping to build beaches and small islands.
Q 18Which ring of coral encircling a lagoon did Charles Darwin explain as a reef growing around a sinking volcano?
Atoll
An atoll is what remains once the volcanic island has sunk from view and only the coral rim survives. Darwin conceived the idea during the voyage of the Beagle.
Q 19Jellyfish raised aboard a NASA space shuttle in 1991 struggled with which skill once back on Earth?
Telling up from down
Telling up from down depends on tiny crystals that roll with gravity inside pockets around a jelly's bell. Animals reared in orbit grew up without that pull and swam clumsily back home.
Q 20In 2013 a swarm of jellyfish clogged the cooling pipes and shut down which kind of facility on Sweden's coast?
Nuclear power plant
A nuclear power plant at Oskarshamn, on the Baltic coast, had to close its third reactor when jellyfish blocked the cooling-water intake. That unit has ranked as the most powerful of its type in the world.
A full moon cues many corals to spawn on the same night, which matters because eggs and sperm released alone would never meet. Several species often join in at once.
Q 24A glowing green protein from the crystal jelly Aequorea victoria won its discoverers a 2008 Nobel Prize in which field?
Chemistry
Chemistry honoured the protein because its gene works as a glowing marker that shows researchers when other genes switch on. Osamu Shimomura collected about a million of the jellies while studying it.
Q 25Which tiny Australian jellyfish, about a cubic centimetre in size, causes a delayed and sometimes fatal sting syndrome?
Irukandji
Irukandji is the name of an Aboriginal people from the coast north of Cairns. Jack Barnes, who identified the jelly in 1964, proved its guilt by letting it sting him, his son and a lifeguard.
Q 26Which yellow-orange reef dweller with a painful sting resembles a stony reef-builder but is closer kin to jellyfish?
Fire coral
Fire coral is named partly for the burning pain of its sting. It only resembles the true stony corals and sits on a separate branch of the cnidarian family tree.
Q 27Many corals clone themselves when an outgrowth on the parent polyp swells into a new animal, a process called what?
Budding
Budding lets one coral polyp multiply into a colony of genetically identical animals. Some sponges, flatworms and tunicates copy themselves in the same way.
Q 28Which large, venomous sea star preys on living coral polyps and damages whole reefs during population outbreaks?
Crown-of-thorns starfish
The crown-of-thorns starfish pushes its stomach outside its body and dissolves the coral tissue beneath it. It is named for the venomous, thorn-like spines that cover its upper surface.
Q 29A microscopic jellyfish relative that survives without using oxygen lives as a parasite in the muscle of which fish?
Salmon
Salmon carry Henneguya zschokkei, which has lost the mitochondrial genes that let cells burn oxygen for energy. It is one of very few animals known to live entirely without the gas.
Q 30Fossils show that jellyfish have drifted the oceans for at least how long?
500 million years
500 million years is the minimum, and some estimates run past 700 million. That makes jellyfish the oldest group of animals with more than one organ.