40 Fun Facts About Cnidaria

Which tiny sea creature can turn its adult body back into a juvenile polyp and start life again?

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.

Which stinging drifter with a gas-filled float is a colony of specialised units rather than a single animal?

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.

Because a jellyfish's gut has just one opening, which body part must also serve as the animal's anus?

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.

The Great Barrier Reef, the biggest structure made by living things, lies off which Australian state?

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.

Which group of sea jellies steers around obstacles with 24 eyes and includes one of the world's most venomous animals?

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.

Which kitchen liquid is kept in jugs on Australian beaches to stop a tropical jellyfish's stinging cells from firing?

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.

Which tiny freshwater cnidarian, named after a monster of Greek myth, regrows completely when cut in half?

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.

Which Pacific island nation is home to Jellyfish Lake, where snorkellers drift among swarms of golden jellies?

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.

Which giant jelly of cold northern seas, the culprit in a Sherlock Holmes story, can trail tentacles 30 metres long?

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.

Which collective noun for a group of jellyfish is also the name of a traditional British fishing boat?

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.

Which small banded reef fish shelters among a sea anemone's stinging tentacles, protected by a coat of mucus?

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.

What name is given to the event in which heat-stressed corals expel the algae that feed and colour them?

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.

Which giant sea reptile lives almost entirely on jellyfish and often swallows drifting plastic bags by mistake?

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.

Biologists call the bell-shaped swimming stage of a jellyfish by the name of which figure from Greek mythology?

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.

The phylum name Cnidaria, coined for animals armed with stinging cells, comes from the Greek word for which plant?

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.

Home to at least a quarter of all marine species, coral reefs are nicknamed the sea's version of which habitat?

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.

Which colourful reef fish grinds up coral rock as it feeds and excretes the leftovers as white beach sand?

The parrotfish digests the edible parts of the rock and passes the rest as sand, helping to build beaches and small islands.

Which ring of coral encircling a lagoon did Charles Darwin explain as a reef growing around a sinking volcano?

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.

Jellyfish raised aboard a NASA space shuttle in 1991 struggled with which skill once back on Earth?

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.

In 2013 a swarm of jellyfish clogged the cooling pipes and shut down which kind of facility on Sweden's coast?

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 jellyfish can sense food or danger from every direction because its body has which type of symmetry?

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.

Which small crustacean carries a stinging sea anemone in each claw, like a cheerleader's pair of pom-poms?

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.

Many reef corals release eggs and sperm together in a mass spawning on nights shortly after which lunar phase?

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.

A glowing green protein from the crystal jelly Aequorea victoria won its discoverers a 2008 Nobel Prize in which field?

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.

Which tiny Australian jellyfish, about a cubic centimetre in size, causes a delayed and sometimes fatal sting syndrome?

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.

Which yellow-orange reef dweller with a painful sting resembles a stony reef-builder but is closer kin to jellyfish?

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.

Many corals clone themselves when an outgrowth on the parent polyp swells into a new animal, a process called what?

Budding lets one coral polyp multiply into a colony of genetically identical animals. Some sponges, flatworms and tunicates copy themselves in the same way.

Which large, venomous sea star preys on living coral polyps and damages whole reefs during population outbreaks?

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.

A microscopic jellyfish relative that survives without using oxygen lives as a parasite in the muscle of which fish?

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.

Fossils show that jellyfish have drifted the oceans for at least how long?

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.

Which name do biologists give the microscopic algae that live inside reef corals and supply most of their food?

Zooxanthellae hand their host sugars made from sunlight and get carbon dioxide and a sunlit perch in return. Their pigments give many corals a yellowish-brown tint.

Which tiny capsule inside a jellyfish's stinging cell shoots out a venomous thread in a few millionths of a second?

A nematocyst launches its thread with an acceleration of about 40,000 g, and one study put the peak in the millions. The thread then injects venom into whatever it has pierced.

Which mineral compound, laid down as crystals of aragonite, do stony corals secrete to build their skeletons?

Calcium carbonate strengthens and protects the soft polyp that makes it, and layer on layer of it builds a reef. Soft corals stay flexible and form only tiny splinters of it inside their tissue.

Which type of drifting colonial cnidarian includes Praya dubia, a deep-sea giant that can rival a blue whale in length?

A siphonophore is a chain of specialised units called zooids, all grown from a single fertilised egg. Praya dubia can reach 50 metres long yet is only about as thick as a broomstick.

Which jelly of warm mangrove lagoons was the first animal without a central nervous system observed sleeping?

The upside-down jellyfish rests with its bell on the seabed so the algae in its tissues can catch the light. When provoked it releases mucus laced with stinging cells.

Which class of cnidarians, whose name means 'flower animals', contains the corals and sea anemones?

Anthozoa never produce a swimming jelly stage, so every member spends its adult life as a polyp. The floral look of those polyps inspired the Greek name.

Which jelly-like layer, mostly water, sits between a jellyfish's two cell layers and makes up the bulk of the bell?

Mesoglea works as a water-filled skeleton that holds a jelly's shape while it swims. Out of the sea it is too soft to bear the animal's weight, which is why a stranded jelly flattens and collapses.

Which tiny free-swimming larva, covered in beating hairs, begins the life of most jellyfish and corals?

A planula drifts or swims until it finds a hard surface, then settles and grows into a polyp. For a coral, that one settler can found an entire new colony.

Which deep-sea cnidarian, Hawaii's official state gem, includes living colonies dated at over 4,000 years old?

Black coral is named for its dark skeleton, the part polished into jewellery, while the living polyps around it are coloured. Its oldest colonies rank among the longest-lived organisms on Earth.

Which rejected phylum name, Greek for 'hollow intestine', was long used for jellyfish, corals and their relatives?

Coelenterata also took in a second phylum of jelly-like animals, because both have a hollow body cavity. Zoologists dropped the name once the two groups proved to have less in common than assumed.