Symbiosis
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40 free Symbiosis trivia questions with answers — science & nature quiz, new questions added Oct 2026.
No species gets by entirely alone. Across reefs, rainforests, farms and even the human body, different organisms strike up close, long-running relationships: some trade favours, some simply hitch a ride, and some take without giving anything back. This quiz covers the main kinds of symbiotic relationship and the examples that turn up in every science class, from fish that shelter among stinging tentacles to bacteria that feed plants, along with stranger cases involving body-snatching fungi, glowing microbes and freeloading birds. It also touches on the idea that complex cells began as a merger of simpler ones. It is pitched for middle school and high school students revising ecology, for teachers who need a ready-made review, and for anyone who enjoys odd animal facts. Every answer comes with a short explanation, so you can play it straight through or print it as a worksheet.
30 of 40 questions with answers and explanations. Play the quiz
Q 01The isopod Cymothoa exigua enters a snapper through its gills and ends up replacing which body part?
Tongue
Tongue tissue dies once the isopod's claws cut off its blood supply, and the animal then grips the stub and sits in its place. The fish can use the intruder like a prosthetic.
Q 02The slender pearlfish hides from predators by slipping inside the rear end of which seafloor animal?
Sea cucumber
Sea cucumber breathing does the work: the animal draws water in through its anus, so the pearlfish only has to wait for the door to open. Most of these lodgers do their host no harm.
Q 03Which tiny animals of the genus Demodex live unnoticed in the hair follicles around human eyelashes and noses?
Mites
Mites of two Demodex species live on human faces, and one DNA study found them on every adult it tested. They crawl over the skin at night and usually cause no trouble at all.
Q 04Which zombie video game series based its outbreak on Ophiocordyceps, a real fungus that hijacks ants?
The Last of Us
The Last of Us imagines the fungus jumping to humans. In real rainforests it steers an infected ant to climb a plant and lock its jaws onto a leaf, then grows a spore-filled stalk out of the ant's head.
Q 05Which fish, coated in a protective layer of mucus, lives unharmed among the stinging tentacles of sea anemones?
Clownfish
Clownfish gain a home that few predators dare enter, and they pay rent by driving off anemone-eating fish and fanning their host with their fins.
Q 06Burdock burrs, which spread by hitching harmless rides on animal fur, inspired which everyday fastener?
Velcro
Velcro began when Swiss engineer George de Mestral took a close look at the burrs stuck to his dog's fur and copied their tiny hooks. The plant gets its seeds carried away, and the animal loses nothing.
Q 07Which bird lays its eggs in the nests of reed warblers and dunnocks, leaving them to raise its chicks?
Cuckoo
Cuckoo chicks heave the host's own eggs over the rim of the nest with their backs, so the foster parents pour all their effort into feeding one giant impostor.
Q 08Which tree-dwelling rainforest mammal has grooved fur that grows green algae, helping to hide it from jaguars?
Sloth
Sloth fur is a small ecosystem of its own: besides the algae that tint it green, it shelters moths found nowhere else but on these animals.
Q 09Which Christmas plant grows on tree branches and steals their water and nutrients through a root-like organ?
Mistletoe
Mistletoe still makes some food with its own green leaves, so it is only a part-time thief. Birds spread it by wiping the sticky seeds off their beaks onto fresh branches.
Q 10Which deep-sea fish dangles a lure that glows thanks to light-making bacteria living inside its tip?
Anglerfish
Anglerfish owe their glow to the bacteria packed into the lure's tip. The lure itself is a modified fin ray that draws prey within reach of the fish's jaws.
Q 11Bees get nectar and flowers get pollinated: which term describes a partnership where both species gain?
Mutualism
Mutualism is nature's win-win deal, and it has steered evolution: the shapes of many flowers exist largely to suit the animals that carry their pollen.
Q 12Which term, from Latin for 'sharing a table', describes one species benefiting while the other is unaffected?
Commensalism
Commensalism covers orchids perched on tree branches and birds nesting in tree holes: the guest gains a home, and the host barely notices.
Q 13A flea drinks a dog's blood and gives nothing back: which term describes one species living at another's expense?
Parasitism
Q 21Which cell parts are thought to descend from bacteria swallowed by a larger cell long ago?
Mitochondria
Mitochondria still carry their own small loop of DNA and split in two on their own schedule, much as free-living bacteria do.
Q 22Which plant cell structures are thought to descend from free-living cyanobacteria engulfed long ago?
Chloroplasts
Chloroplasts divide in a way that closely resembles their free-living relatives, a likeness one botanist noticed as early as 1883, long before DNA could back him up.
Q 23Which biologist argued that cell organelles began as bacteria in a paper rejected by about fifteen journals?
Lynn Margulis
Parasitism differs from hunting because the freeloader is far smaller than its victim and usually keeps it alive, often staying in or on the same host for a long time.
Q 14First used in biology by a German fungus expert in the 1870s, the word 'symbiosis' comes from Greek for what?
Living together
Living together is all the Greek roots promise, which is why biologists count any close, lasting bond between species, whether it helps, harms or makes no difference to the partners.
Q 15Which African bird perches on rhinos, zebras and giraffes, eating the ticks buried in their skin?
Oxpecker
Oxpecker visits are not pure kindness: the birds also peck at wounds to keep them open and drink the blood, which is why some biologists class them as freeloaders.
Q 16Which fish uses a sucker on top of its head to hitch rides on sharks, turtles and whales?
Remora
Remora suckers are a dorsal fin reshaped into a ridged oval disc. Sliding backward tightens the grip, and swimming forward breaks it.
Q 17Which crusty growth on rocks and tree bark is really a fungus with algae housed inside its body?
Lichen
Lichen pairs a fungus that provides the body with algae that make food from sunlight, a team tough enough to live on bare rock, in hot deserts and across the Arctic tundra.
Q 18Which sea animals house zooxanthellae, tiny algae that feed them sugars made from sunlight?
Corals
Corals turn ghostly white when overheated water makes them throw out the algae, an event known as bleaching. The algae provide much of the colour as well as the food.
Q 19Rhizobia bacteria living in the root nodules of peas and beans capture which element from the air for the plant?
Nitrogen
Nitrogen gas is useless to plants until the bacteria turn it into ammonia. In return the plant feeds its lodgers with carbon-rich acids.
Q 20Which shelled filter-feeders fix themselves to the skin of humpback and gray whales for a free ride?
Barnacles
Barnacles on a whale are carried through food-rich water without any effort, and a single humpback may haul up to 450 kg of them at no real cost or benefit to itself.
Lynn Margulis finally saw the paper printed in 1967. As a young graduate she had married the astronomer Carl Sagan, and she later wrote popular science books with their son Dorion.
Q 24Which term, from Greek for 'fungus root', names the underground partnership between fungi and most land plants?
Mycorrhiza
Mycorrhiza is a trade: the plant hands over sugars made in its leaves, and the fungus pays with water and minerals such as phosphorus drawn from the soil.
Q 25Which tropical American insects grow underground fungus gardens, feeding the crop with vegetation they carry home?
Leafcutter ants
Leafcutter ants use the greenery as compost for a fungus that feeds their larvae. Next to humans, they form some of the largest and most complex societies on Earth.
Q 26Which thorny trees house ant colonies inside hollow thorns and are defended by them against leaf-eating animals?
Acacias
Acacias of the bullhorn kind pay their guards with nectar and protein-rich leaf tips. The ants repay them by attacking browsers and clearing away seedlings that might grow up to shade the tree.
Q 27Which sap-sucking garden insects are stroked by ants to release honeydew, earning protection in exchange?
Aphids
Aphids are treated like a tiny dairy herd: the ants stroke them with their antennae to release drops of the sugary waste and drive away the predators that come to eat them.
Q 28Which small reef fish greets larger fish with a dance, then picks dead tissue and pests off their skin?
Wrasse
Cleaner wrasse work from fixed stations that their 'clients' visit, and a waiting fish will hold a special pose so its groomer can reach the gills and even swim into its mouth.
Q 29Which wood-eating insects depend on microbes in their guts to break down the tough cellulose they swallow?
Termites
Termites owe their diet to gut microbes that turn wood fibre into fuel the insects can absorb. DNA shows these colony builders are really a branch of the cockroach family tree.
Q 30Bacteria living in the human large intestine make a form of which nutrient, needed for blood to clot?
Vitamin K
Vitamin K takes its letter from 'koagulation', the Danish word for clotting. Gut bacteria such as E. coli make its K2 form, and newborns are given an injection of the vitamin to prevent bleeding.