Sea Urchins
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40 free Sea Urchins trivia questions with answers — trivia quiz, new questions added Oct 2026.
Sea urchins look like living pincushions, but there is far more going on beneath the spines than most beachgoers guess. They have been grazing the seabed since long before the dinosaurs, they turn up in every ocean from the tropics to the poles, and a single species can reshape a whole coastline. This quiz turns the best sea urchin fun facts into questions. It covers how the animals are built and how they move, what they eat and what eats them, their odd senses and long lives, their place on dinner plates around the world, and the part they have played in science and folklore. It is pitched for curious beginners, students and families, with a handful of tougher questions for divers, aquarium keepers and marine biology fans. Every answer comes with a short explanation, so you leave knowing more than when you began.
30 of 40 questions with answers and explanations. Play the quiz
Q 01Which marine mammal's teeth and bones can be stained purple by a lifetime of eating sea urchins?
Sea otter
Sea otter numbers shape whole coastlines: where the otters eat urchins, the grazers cannot strip the seabed's seaweed forests bare.
Q 02The word "urchin" in the name sea urchin is an old English word for which spiny land animal?
Hedgehog
Hedgehog is what urchin first meant, and the spiny sea animals were once called sea hedgehogs outright.
Q 03Which Greek philosopher gives his name to the lantern, a sea urchin's toothed chewing organ?
Aristotle
Aristotle likened the urchin to a horn lantern in his History of Animals, and naturalists later pinned the name Aristotle's lantern on its chewing apparatus.
Q 04On Japanese sushi menus, which three-letter word is used for the rich orange lobes of sea urchin?
Uni
Uni is eaten raw as sashimi or on sushi, and Japanese demand for it is strong enough to have raised fears of overfishing.
Q 05The sea urchin "roe" prized by chefs is really which organs, found in both males and females?
Gonads
Gonads are the only meaty, edible parts of a sea urchin, and they double as the animal's nutrient store.
Q 06Which color names the Pacific sea urchin that can live for more than a century?
Red
Red sea urchins were long thought to live only 7 to 10 years, until studies showed some Canadian ones are around 200 years old.
Q 07Divers smash purple sea urchins with small hammers to help which towering seaweed grow back?
Kelp
Kelp drifts away and dies once urchins chew through its lower stems, leaving the bare seabed that biologists call an urchin barren.
Q 08Which flat, disc-shaped beach find is really a burrowing sea urchin covered in velvety spines?
Sand dollar
Sand dollar skeletons wash ashore bleached white, which reminded beachcombers of large silver coins such as the old Spanish dollar.
Q 09Sea urchins crawl, feed and steer hundreds of spines without ever growing which organ?
Brain
Brain is one thing a sea urchin never grows: a simple ring of nerves around its mouth runs the spines and the crawling instead.
Q 10Which small Indian Ocean crustaceans carry a live sea urchin on their backs for protection?
Crabs
Crabs of the carrier crab species grip the urchin with their two rear pairs of legs, and the urchin gets a free ride to new feeding grounds.
Q 11Which symmetry, shared with starfish, is easy to see on the dried, spineless shell of a sea urchin?
Five-fold
Five-fold symmetry only appears as an urchin matures: its free-swimming larva starts life with a left side and a right side, like ours.
Q 12Sea urchins creep along the seabed on hundreds of tiny, water-powered suckers known by which name?
Tube feet
Tube feet work by hydraulics, with the urchin pumping water in and out of each one, and they can double as breathing organs.
Q 13Which short word do biologists use for the rigid shell that encloses a sea urchin's internal organs?
Test
Test plates grow along with the animal, so a sea urchin never has to shed its shell the way a lobster or an insect does.
Q 21Which Māori name is used for New Zealand's native sea urchin, traditionally eaten raw?
Kina
Kina is a delicacy at home, but its quality is too variable for the export trade to Japan that local fishermen would like.
Q 22On Italian menus, sea urchin listed as ricci is eaten raw or stirred into sauce for which staple?
Pasta
Pasta is only one outlet: Mediterranean cooks also use the urchin to flavour omelettes, fish soup and even mayonnaise.
Q 23Oscar Hertwig's 1870s studies of sea urchin eggs first showed what really happens during which event?
Fertilization
Fertilization, he showed, is the fusion of a sperm and an egg cell, and it was easy to watch because sea urchin eggs are transparent.
Q 14A sea urchin's shell plates are made of which compound, also the main ingredient of eggshells?
Calcium carbonate
Calcium carbonate also forms the urchin's teeth, and oceanographers monitor urchin health as one way to gauge ocean acidification.
Q 15Which squishy seabed animals, despite having no spines, are the closest living relatives of sea urchins?
Sea cucumbers
Sea cucumbers and urchins form their own branch of rounded, armless animals, set apart from the starfish and brittle stars.
Q 16Sea urchins and starfish belong to which animal phylum, the largest found only in the sea?
Echinoderms
Echinoderms number about 7,600 living species, yet not one of them has managed to colonise fresh water or dry land.
Q 17Which long-bodied North Pacific fish with crushing molars is a specialist predator of sea urchins?
Wolf eel
Wolf eel pairs tend to mate for life and share a cave, and despite the name the species is not a true eel at all.
Q 18A 1983 die-off killed over 97% of long-spined sea urchins in which sea, letting algae smother coral?
Caribbean Sea
Caribbean Sea reefs lost their main grazer almost at a stroke, and many were overgrown by seaweed as coral cover and biodiversity fell.
Q 19In 2014, Guinness named which species, armed with venomous pincers, the most dangerous of all sea urchins?
Flower urchin
Flower urchin pincers snap shut on contact and inject venom, and claws that break off can keep pumping it in for hours.
Q 20In Danish and English folklore, fossil sea urchins were kept as charms against what?
Lightning
Lightning was the fear because people took the rounded fossils for thunderbolts fallen from the sky, and kept them to ward off witchcraft too.
Q 24Where on a regular sea urchin's round body is the mouth it uses to scrape algae off rocks?
On its underside
On its underside is the mouth, while the anus sits at the summit of the shell, so a sea urchin eats from below and expels waste above.
Q 25Sea urchin fossils go back more than 450 million years to which geological period?
Ordovician
Ordovician seas held the first urchins, and the group later came close to dying out: only two lineages survived the great Permian extinction.
Q 26Which many-armed starfish collapsed after 2013, letting purple sea urchins overrun California reefs?
Sunflower sea star
Sunflower sea star numbers crashed from a wasting disease between 2013 and 2015, removing one of the urchins' main predators.
Q 27Researchers suggest the whole spiny body of a sea urchin may work as one big version of which organ?
Eye
Eye is the surprising answer because most sea urchins have no eyes at all, yet they still shy from light and hide in crevices.
Q 28The common heart urchin, a burrowing sea urchin, is nicknamed the sea what, after a vegetable?
Potato
Potato is the nickname for this heart-shaped urchin cloaked in yellowish spines, which buries itself ten to fifteen centimetres deep.
Q 29Which US state bred native collector urchins and released them to eat invasive seaweed smothering a coral reef?
Hawaii
Hawaii raised the urchins from about a million larvae, after years of sucking the alien seaweed off Kaneohe Bay with a marine vacuum pump.
Q 30Sea urchins are closer relatives of which animals than they are of insects, snails or worms?
Vertebrates
Vertebrates and urchins share a branch of the animal tree, and the purple sea urchin's genome turned out to share more than 7,000 genes with ours.