What is a sea urchin?

Characterized by their piercing spikes, the sea urchin is a unique-looking sea creature that boasts so many impressive features. They are echinoderms in the Echinoidea class. There are currently 950 known species of the sea urchin and they are widely distributed across all our oceans. They are also known as the “sea hedgehog” because their spiny round bodies resemble the animal.

Here are some of the most interesting facts about the sea urchin:

1. They are not aggressive but the sea urchin sting is very painful.

Sea urchins don’t attack humans but if you step on them, their spines will sting you. Because they can be found in shallow waters, it is not uncommon for humans to accidentally come in contact with them. The sea urchin sting is extremely painful. Its spike can easily penetrate human skin and create deep puncture wounds.

The most dangerous sea urchin has toxins that can cause paralysis.

The flower urchin is the most dangerous sea urchin. It is named after its flower-like appearance. This is a relatively large species that measures about 6-8 inches. It is mostly found in the Indo-West Pacific region. The flower urchin contains at least two toxins that can cause paralysis, anaphylactic shock and even death.

Toxopneustes pileolus
Image by François Michonneau

2. Aside from spikes, sea urchins also have jaw-like pincers that can release poison.

Sea urchins have two kinds of defense mechanisms. Their first defense is the pointy spikes on their exterior. They also have pedicellariae - tiny, jaw-like pincers that can release venom. Pedicellariae are common among echinoderms like sea stars. In sea urchins, they are located in between the spines.

Vinegar can be used as first aid for sea urchin sting.

The sting of a sea urchin is extremely painful because of its spines. Vinegar can be used to dissolve the spines stuck in the wound. Immerse the affected area in a hot water and vinegar solution for about 30 - 90 minutes. Then, use tweezers to remove the spines. Wash the area thoroughly and take Ibuprofen to help with the pain.

sea urchin sting
Image by Yugyug

3. Although they may look stationary, the sea urchin walks using tiny tube feet.

The sea urchin may look stationary but they actually walk along the sandy seabed. They do this using their tube feet located all around their body. They are like small tentacles with a suction at the end. The sea urchin is able to extend and retract its tube feet to move along the seafloor, climb obstacles and even flip themself over.

They also use their tube feet to breathe and eat.

Their tube feet are multi-functional. Sea urchins also use them to breathe. It is an important part of  sea urchin’s water vascular system and they rely on these tiny tentacles to absorb oxygen and calcium carbonate to make their shells stronger. Additionally, they use the suction ends to grab food while they are rolling around on the reef.

sea urchin tube feet
Image by Giphy

4. Sea urchins have razorlike teeth that can chew through rocks.

Located on the underside of its body are five toothlike plates, which is called “Aristotle’s Lantern”. These teeth are so sharp and strong that they can chew through rocks. Scientists have also learned that these teeth are self-sharpening and continuously grow. Sea urchins use their teeth to bore through rocks that become their hiding spots.

Its mouth was named after the philosopher Aristotle.

It is called “Aristotle’s Lantern” because it was named after the Greek philosopher Aristotle. Aristotle was the first to write about the sea urchin anatomy. He wrote a book called “The History of Animals” and described the sea urchin mouth as a “horn lantern”. Later on, scientists renamed it Aristotle’s Lantern as a tribute to his discovery.

sea urchin mouth
Image by Giphy

5. Sea urchins don’t have eyes but their body acts like one big eye.

Sea urchins don’t have a set of eyes but their body functions like one big eye. This is because the spines and tube feet surrounding their body are light-sensitive. Their whole body is full of light-receptor cells, so they can detect light and darkness.

They have low-resolution vision but can still see with their tube feet.

In one study, scientists tried to detect how clear the sea urchin vision is. They did this by displaying a growing figure, similar to a predator, above the sea urchin. The figure needed to be between 30 and 70 degrees in the sea urchin’s 360 degree area before it was detected. That’s not very good vision but it still works in conjunction with the sea urchin’s lifestyle.

Sea Urchin
Image by Kirt Edblom

6. Underneath their spines, sea urchins live in a suit of armor.

Sea urchins not only have their spines to protect them, they also have full body armor called the test. The test is a shell-like skeletal structure that’s made of calcium carbonate. Aside from its hard material, the test is also made even more stronger because of its structure. It is made of interlocking small plates, which make the structure almost crack-resistant.

The carrier crab uses the sea urchin as a shield.

The sea urchin’s body is so dangerous and well-built that the carrier crab uses it as a shield. These two marine animals have a symbiotic relationship. The carrier crab carries the sea urchin on his back to ward off potential predators. The sea urchin also benefits because it gets to have a share of the crab’s food.

7. The Pacific purple sea urchin has caused a lot of sea kelp deforestation.

The purple sea urchin is one of the most common varieties of sea urchin. It is also one of the most destructive because of its aggressive feeding behavior. They are known to consume entire kelp forests, causing large areas of “urchin barren”.

Sea urchins eating kelp forests has a vast consequence on marine ecosystems.

While it is normal for sea urchins to feed on kelp, the imbalance of their numbers is causing a huge decline in kelp forests. Kelp forests are important in keeping the marine ecosystem thriving. If the sea urchin population remains uncontrolled, kelp forests will continue to disappear.

8. The sea otter is one of the top sea urchin predators.

While they may have an impressive set of defensive structures, the sea urchin is still not immune to being preyed on. The sea otter is considered as one of the top sea urchin predators. These foragers get through the sea urchin’s tough exterior by using a rock to smash the shell.

Sea otters feeding on sea urchins actually help save kelp forests.

Sea otters and other natural predators feasting on sea urchins is essential in saving our underwater kelp forests. They help keep the sea urchin population at bay, so kelp forests can replenish. In coasts where there aren’t a lot of sea otters, urchin populations boom. In turn, this causes whole sea kelp populations to be devoured by urchins.

How endangered is this animal?

Is it safe to eat sea urchins? Not all parts of the sea urchin are edible but there’s one particular section that’s considered a delicacy -- its gonads. Found on the inside of its hard shell, the gonads - also called “roe” - are mostly used in sushi. This is a very popular seafood dish, especially in Japan.

Sea Urchins
Image by sung ming whang
  • Some sea urchin populations in Japan have been depleted because of the high demand.
    The largest consumer of uni is Japan. Because of the high demand for this seafood, overfishing happened. This resulted in the sea urchin populations in Japan being depleted. Now, most of the uni they eat is imported from California waters, where urchins are still thriving.
  • Human activity can also threaten sea urchin populations.
    Because sea urchins commonly live in shallow water, they are more susceptible to human influences. Researchers have found out that fluctuating water temperatures and algal bloom can affect sea urchin population. The study concluded that environmental stressors - climate change, global warming, sea pollution need to be monitored to continue protecting not just sea urchins but other marine life too.









Also Known As

Sea hedgehogs


3 to 10 cm (1 to 4 in), although the largest species can reach up to 36 cm (14 in)


All the oceans and all climates from tropical to polar


From rocky shores to hadal zone depths


Algae, Fish, Barnacles


15-200 years

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