40 Fun Facts About Sea Urchins

Which marine mammal's teeth and bones can be stained purple by a lifetime of eating sea urchins?

Sea otter numbers shape whole coastlines: where the otters eat urchins, the grazers cannot strip the seabed's seaweed forests bare.

The word "urchin" in the name sea urchin is an old English word for which spiny land animal?

Hedgehog is what urchin first meant, and the spiny sea animals were once called sea hedgehogs outright.

Which Greek philosopher gives his name to the lantern, a sea urchin's toothed chewing organ?

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.

On Japanese sushi menus, which three-letter word is used for the rich orange lobes of sea urchin?

Uni is eaten raw as sashimi or on sushi, and Japanese demand for it is strong enough to have raised fears of overfishing.

The sea urchin "roe" prized by chefs is really which organs, found in both males and females?

Gonads are the only meaty, edible parts of a sea urchin, and they double as the animal's nutrient store.

Which color names the Pacific sea urchin that can live for more than a century?

Red sea urchins were long thought to live only 7 to 10 years, until studies showed some Canadian ones are around 200 years old.

Divers smash purple sea urchins with small hammers to help which towering seaweed grow back?

Kelp drifts away and dies once urchins chew through its lower stems, leaving the bare seabed that biologists call an urchin barren.

Which flat, disc-shaped beach find is really a burrowing sea urchin covered in velvety spines?

Sand dollar skeletons wash ashore bleached white, which reminded beachcombers of large silver coins such as the old Spanish dollar.

Sea urchins crawl, feed and steer hundreds of spines without ever growing which organ?

Brain is one thing a sea urchin never grows: a simple ring of nerves around its mouth runs the spines and the crawling instead.

Which small Indian Ocean crustaceans carry a live sea urchin on their backs for protection?

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.

Which symmetry, shared with starfish, is easy to see on the dried, spineless shell of a sea urchin?

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.

Sea urchins creep along the seabed on hundreds of tiny, water-powered suckers known by which name?

Tube feet work by hydraulics, with the urchin pumping water in and out of each one, and they can double as breathing organs.

Which short word do biologists use for the rigid shell that encloses a sea urchin's internal organs?

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.

A sea urchin's shell plates are made of which compound, also the main ingredient of eggshells?

Calcium carbonate also forms the urchin's teeth, and oceanographers monitor urchin health as one way to gauge ocean acidification.

Which squishy seabed animals, despite having no spines, are the closest living relatives of sea urchins?

Sea cucumbers and urchins form their own branch of rounded, armless animals, set apart from the starfish and brittle stars.

Sea urchins and starfish belong to which animal phylum, the largest found only in the sea?

Echinoderms number about 7,600 living species, yet not one of them has managed to colonise fresh water or dry land.

Which long-bodied North Pacific fish with crushing molars is a specialist predator of sea urchins?

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.

A 1983 die-off killed over 97% of long-spined sea urchins in which sea, letting algae smother coral?

Caribbean Sea reefs lost their main grazer almost at a stroke, and many were overgrown by seaweed as coral cover and biodiversity fell.

In 2014, Guinness named which species, armed with venomous pincers, the most dangerous of all sea urchins?

Flower urchin pincers snap shut on contact and inject venom, and claws that break off can keep pumping it in for hours.

In Danish and English folklore, fossil sea urchins were kept as charms against what?

Lightning was the fear because people took the rounded fossils for thunderbolts fallen from the sky, and kept them to ward off witchcraft too.

Which Māori name is used for New Zealand's native sea urchin, traditionally eaten raw?

Kina is a delicacy at home, but its quality is too variable for the export trade to Japan that local fishermen would like.

On Italian menus, sea urchin listed as ricci is eaten raw or stirred into sauce for which staple?

Pasta is only one outlet: Mediterranean cooks also use the urchin to flavour omelettes, fish soup and even mayonnaise.

Oscar Hertwig's 1870s studies of sea urchin eggs first showed what really happens during which event?

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.

Where on a regular sea urchin's round body is the mouth it uses to scrape algae off rocks?

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.

Sea urchin fossils go back more than 450 million years to which geological period?

Ordovician seas held the first urchins, and the group later came close to dying out: only two lineages survived the great Permian extinction.

Which many-armed starfish collapsed after 2013, letting purple sea urchins overrun California reefs?

Sunflower sea star numbers crashed from a wasting disease between 2013 and 2015, removing one of the urchins' main predators.

Researchers suggest the whole spiny body of a sea urchin may work as one big version of which organ?

Eye is the surprising answer because most sea urchins have no eyes at all, yet they still shy from light and hide in crevices.

The common heart urchin, a burrowing sea urchin, is nicknamed the sea what, after a vegetable?

Potato is the nickname for this heart-shaped urchin cloaked in yellowish spines, which buries itself ten to fifteen centimetres deep.

Which US state bred native collector urchins and released them to eat invasive seaweed smothering a coral reef?

Hawaii raised the urchins from about a million larvae, after years of sucking the alien seaweed off Kaneohe Bay with a marine vacuum pump.

Sea urchins are closer relatives of which animals than they are of insects, snails or worms?

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.

Sea urchins in clear shallows hold shells and seagrass over their bodies, probably to block what?

Sunlight is the likely threat: urchins kept in shallow tanks shied away from ultraviolet light and died after long exposure to it.

The purple sea urchin of the Mediterranean uses its mouth and spines to dig snug hollows in what?

Soft rock gives way slowly, and the urchin ends up with a cavity that fits it exactly and to which it keeps returning.

Which French port city's region holds a dedicated sea urchin food festival known as an oursinade?

Marseille and its neighbours eat the local purple urchin the Mediterranean way: raw, sometimes with a squeeze of lemon.

The green sea urchin's scientific name, droebachiensis, comes from the town of Drøbak in which country?

Norway treats the species as a pest, because green urchins have been eating their way through its coastal seaweed forests.

What name is given to the free-swimming, long-armed larva that develops from a sea urchin's egg?

Pluteus larvae take months to develop, then sink to the seabed and transform into a tiny urchin in as little as one hour.

Which British biochemist shared a 2001 Nobel Prize after discovering cyclin proteins in sea urchin eggs?

Tim Hunt made the find at Woods Hole in 1982 and named the protein cyclin because its levels rose and fell with each cell division.

Which California fish, born female before turning male, eats sea urchins with its protruding teeth?

Sheephead finish the job with a plate of modified throat bones that grinds the urchin's shell into small pieces.

Which sieve-like plate on top of a sea urchin's shell lets seawater into its hydraulic plumbing?

Madreporite plates work like a pressure-equalizing valve, feeding the water system that powers each of the urchin's hundreds of suckers.

Which piece of armor names the smooth, flat-spined sea urchin that clings to wave-battered shores?

Helmet urchins trade sharp spines for a smooth mosaic of flattened plates, and in trials resisted moving water three times better than a rival species.

One old folk belief held that fossil sea urchins were the petrified eggs of which animals?

Snakes were said to make the stones by magic from foam at midsummer, and people carried them as amulets against poison and battle wounds.