Sea Urchin Facts and Their Hidden Mouths

Prepare to be amazed by the enchanting world of sea urchins! These captivating creatures, adorned with spiky exteriors, hold fascinating secrets beneath their prickly appearance. From their mesmerizing array of species to their vital role in marine ecosystems and unique feeding habits, sea urchins are far from ordinary inhabitants of the seas. Get ready to dive deep and uncover the mysterious facts of these extraordinary sea dwellers!

Sea urchin
Phylum: Echinodermata
Class: Echinoidea

1. They are covered in spines

Sea urchins, a class of marine invertebrates known as Echinoidea, are closely related to starfish and sea cucumbers.

These creatures possess a central shell-like structure known as a test, composed of calcium carbonate, which acts as their exoskeleton. Similar to sand dollars, sea urchins exhibit symmetrical bodies. However, the animals showcase fascinating changes in symmetry as they grow. The larvae sea urchins only exhibit bilateral symmetry. However, as they reach adulthood, they undergo a transformation and acquire 5-fold symmetry. This remarkable change results in their bodies being divided into five sections, each of which reflects the others.

Tuxedo urchin looks like small balls

Their round bodies are covered in spines with a variety of lengths and colors—ranging from brown, green, red, orange, purple, to black—each unique to their respective species. Some species even show their colors through their names, such as the Pacific purple, green sea urchin, white sea urchin, or West Indian sea egg (Tripneustes ventricosus), Atlantic purple sea urchin (Arbacia punctulata), and sea grass or pink sea urchin.

However, not all species have spines, certain urchin shells display a coating of hair-like structures.

These creatures have a size between 3 – 10 cm, with the largest species being the red sea urchin. It boasts a maximum “test” (outer skeleton) diameter exceeding 18 cm and spines that can grow up to 8 cm in length.

They have 5 teeth

Among these spines are their 5 sets of paired tubular feet known as pedicellariae. These tiny suction-cup-like appendages help the species move across the ocean floor, get over obstacles, firmly grasp surfaces, capture food, and readjust their orientation when disturbed by waves or currents. Moreover, sea urchins exhibit an interesting anatomical arrangement, with their mouths located at the lower part of their bodies and their anuses positioned on the upper side.

The Pincurchin in Pokemon is a marine creature inspired by the sea urchin.

2. Habitat

For hundreds of millions of years, sea urchins have thrived in oceans worldwide, staying in the seabed. These remarkable echinoderms live in diverse waters, spanning from temperate and tropical to polar regions. They can be found in various locations, from the intertidal areas to the profound depths of the ocean, astonishingly venturing down to as deep as 16,000 feet.

These shellfish exhibit a preference for desolate regions on the seabed. Nevertheless, they tend to gather in shallower waters unless they encounter exceptionally turbulent conditions. In these cases, they move to deeper waters. To withstand the movements caused by waves, they anchor themselves to rocks.

These species do not swim, they walk along the seabed

The helmet or shingle urchin, scientifically known as Colobocentrotus atratus, thrives in exposed shorelines and possesses exceptional resilience against wave action. Unlike most sea urchins, this species can live out of water for extended periods, making it well-adapted to its environment.

There are about 950 types of sea urchins, with the black or long-spined sea urchin or wana in Hawaiian being the most popular species. They are commonly found in the Indo-Pacific area, with a significant presence on Bawah Island. To know about the other urchin species, you can read more here.

3. Sea urchin Diet

Sea urchins predominantly rely on algae as their main food source, adeptly using their spines to gather it from the seafloor and consume it. Additionally, they opportunistically capture floating food particles from the surrounding water.

They also diversify their diet by eating mussels, sea cucumbers, barnacles, limpets, and chitons. Their diet is not limited to living prey, as they also play a vital role in the ecosystem by scavenging and feeding on deceased and decaying animals, thus facilitating nutrient recycling. Once getting food, these omnivores use their five teeth (which are actually plates) positioned at the center of their bodies to grind them.

An otter eating urchins

Sea urchins are preyed upon by many predators, such as sea otters, starfish, eels, lobsters, triggerfish, crabs, and even humans. To protect themselves from these threats, these echinoderms employ their spines as a natural defense mechanism to deter potential attackers. Some species even have venomous spines.

Despite their spiky armor, some predators have developed strategies to overcome these defenses. Predatory fish can bite off the spines, while lobsters and crabs skillfully use their pincers to break them off. Sea otters, armed with their sharp claws and formidable strength, swipe away the spines before skillfully crushing the urchin’s shell to access and consume its soft insides.

4. Their eyes are all over their bodies

Sea urchins have a fascinating characteristic – their entire body serves as an eye. Rather than having distinct eyes, their entire body is covered in light-sensitive cells that resemble the retina of an eye. Interestingly, these cells are most concentrated around their mouth and tube-like feet, which are nestled among their spines. Consequently, sea urchins not only rely on their feet for movement but also employ them to see.

Despite possessing light sensitivity, sea urchins have a tendency to evade bright light and actively navigate toward darker regions to look for shelter.

Sea urchin adaptions

These creatures don’t have brains, but a modest nervous system. Instead, their nerves emanate from a neural ring encircling their mouths. There are five primary nerves that follow their water vascular system, facilitating the circulation of water throughout their bodies.

All other nerves emanate from these five primary nerves, extending their connections to their mouth, tube feet, and spines. This complex network let the animals control their bodies, while also granting them a heightened sense of touch.

Furthermore, sea urchins have speridia near their main nerves. These organs help the species to perceive gravitational orientation, a crucial aspect for them to maintain balance, especially when they have round bodies.

5. They’re a mixed bag

Sea urchins play a vital role in coral conservation by serving as natural guardians against algae proliferation. Algae overgrowth poses a substantial menace to corals, as it vies for space and impedes their growth. It can even grow directly on their surfaces, depriving them of the vital sunlight necessary for photosynthesis. Hence, sea urchins play a crucial part in regulating the population of algae.

However, an overpopulation of sea urchins is not a good thing. A reduction in their natural predators due to overfishing can trigger a rapid increase in sea urchin populations, leading to a disastrous phenomenon known as “urchin barrens.”

As the urchins eat the majestic kelp forests, the once-thriving ecosystem that depended on these marine forests vanishes, leaving behind a desolate absence of both fish and other creatures. Now, these regions are nothing more than submerged fields dominated by sea urchins.

6. They are venomous

Sea urchins possess spines that can cause considerable discomfort if you accidentally stepped on them. Their spines are shark enough to penetrate your skin, making you sick and causing infection. What’s more alarming is that these spines often break upon contact, becoming lodged inside your skin. Due to their brittleness, removing these spines is a challenging task, even for medical experts. Attempts to extract them may exacerbate the situation as the spines could fragment further, intensifying the pain.

Certain species, such as the deadliest flower urchin (Toxopneustes pileolus) found in the Indo-West Pacific region, even possess venomous spines. This particular species has been listed by the Guinness Book of World Records as the most dangerous of its kind.

Sea urchin in foot
The injury when stepping on urchins

Their venom contains two toxic components: Contractin A and Peditoxin. Contractin A targets nerves and blood, while Peditoxin enhances the potency of the first venom. The combination of these deadly venoms can lead to various adverse effects, including muscle spasms, blood clotting, hypotension, convulsions, respiratory distress, anaphylactic shock, and, in severe cases, even death. So, you shouldn’t touch these sea creatures.

If you got stung, it’s crucial to take immediate action by extracting the spine from the skin and thoroughly cleaning the affected area to avoid infection. You should seek immediate medical attention. With proper sting treatment, survival is possible even after a sting.

The most venomous urchin

Although this species is venomous, there is still a species that comes in contact with this sea urchin – the clownfish. This fish also has a close relationship with toxic sea anemones.

Clownfish deliberately subject themselves to small doses of the venom from flower urchins as a means to develop immunity. Through repeated contact with the spines, they gradually build resistance. This acquired immunity enables them to consume the parasites residing on the flower urchins, creating a mutually advantageous relationship between the two species.

This fish is not the only sea creature that has a symbiotic relationship with the sea urchin. The carrier crab and this spiny species are so tight-knit that the crab is called urchin crab. With the urchin on the back, the crab gets protection and the urchin can go to different places to eat.

7. They are in high demand

Sea urchins are highly regarded as a beneficial delicacy across the globe, especially in Japan. Each year, about 50,000 tons of this species are consumed in this country, constituting a staggering 80% of the world’s total sea urchin harvest. This exceptional demand has transformed the sea urchin export industry in the US and South Korea into a highly profitable venture.

Even the eggs of these species are highly regarded as a valuable delicacy, commanding a hefty price tag. This expensive dish can cost you around $360 to have a kilogram of these eggs.

Sea urchin meal
Urchin ceviche: a unique citrus dish, Only the yellow part of the species is edible, which are their reproductive organ – gonads

In Mediterranean and European cuisines, the sea creatures are often consumed both raw and cooked with a hint of lemon. In Italian cuisine, they are incorporated into pasta sauces, called “bucatini ricci.” Similarly, New Zealand’s indigenous Maori people, known as “Kina,” relish them in their natural raw state.

Chileans also have a fondness for raw sea urchins. In European gastronomy, these marine delicacies impart a delightful flavor to dishes such as tartlets, omelets, soups, and soufflés.

The growing popularity of sea urchins, particularly in Japan, has raised alarm about the potential for overfishing and the consequent threat to their population. Achieving a solution to balance between culinary indulgence and environmental conservation continues is a formidable task.

Uni is the Japanese word for sea urchin. In Spanish, they are known as “erizo de mar,” which translates literally to “sea hedgehog.”

8. Life cycle

In the spring mating season, sea urchins reproduce sexually through broadcast spawning. Their reproduction is external fertilization. Males have five gonads located within their tests, and each gonad is linked to a duct that leads to an opening. Under favorable conditions, they expel their sperm through these ducts into the surrounding water, where it combines with eggs released by females, leading to the formation of fertilized embryos.

After fertilization, the embryo undergoes rapid development, transitioning into a free-swimming larva within a mere 12 hours. These larvae drift along in the water, sustaining themselves by consuming phytoplankton. As they mature, they gradually develop the distinctive test plates observed in juveniles.

They float for several months until settling onto the seafloor. During this period, their shells harden, marking the transition into the juvenile phase. It may take the young 5 years to become adults, while sexual maturity is typically attained within approximately 2 years.

Sea urchins have a long lifespan, with some species surviving for 70-100 years or even longer. Among these, the red sea urchin (Mesocentrotus franciscanus) stands out as an exceptional creature. This giant species potentially boasts a lifespan of up to 200 years, making it one of the oldest animals on our planet.



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We are passionate animal enthusiasts with over a decade of experience studying animals. With a degree in zoology and conservation biology, we've contributed to various research and conservation projects. We're excited to bring you engaging content that highlights the wonders of the animal kingdom. We aim to inspire others to appreciate and protect wildlife through informative content grounded in expertise and passion. Join us as we delve into the captivating world of animals and discover the incredible stories they have to tell.

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