As you read this list of sea urchin facts, you'll understand they are much more than their spines and venom. They are not only ecologically important, but they also support many economies as they are a culinary delicacy.
Sea urchin roe or gonads are precious and expensive. It is particularly notable for its richness in omega-3 fatty acids essential for brain function and heart health.
Sea urchins belong to the family Echinoidea3. They are known for their spiny, spherical bodies, making them easily recognizable underwater.
As part of the phylum Echinodermata, which encompasses a diverse array of marine life, sea urchins share their family with other intriguing creatures like starfish, sand dollars, and sea cucumbers.
These spiky invertebrates are incredibly diverse, boasting over 950 known species. Smaller species are usually found in shallow waters, while larger sea urchins prefer deeper ocean parts.
Furthermore, sea urchin species come in different colors. The most common colors are yellow, red, black, brown, and purple.
An average sea urchin can measure around 3-10 cm in diameter. The Red Sea Urchin is the largest species (diameter of more than 18 cm) and is native to the eastern Pacific Ocean along the west coast of North America. On the other hand, the Ericillo Sea Urchin is the smallest, measuring less than 1 cm in diameter.
Next up on our list of sea urchin facts: Sea urchins have no bone structure but an outer shell-like structure called a "test" made from calcium carbonate. Their hard shell, along with their spines, is their primary form of protection.
Furthermore, the test protects and supports their body and organs. The test comprises small plate segments that enclose the entire sea urchin. Additionally, most sea urchins have radial symmetry, which you can see upon examining a dried urchin shell.
Sea urchins are one of the few animals that thrive in all oceans. They live in the warm waters of the tropics to the icy waters of the Arctic. They also thrive in shallow areas with crashing waves to the deep sea, reaching depths of over 5000 meters.
Additionally, they can inhabit various habitats, including rocky shores, coral reefs, seagrass beds, sandy or muddy bottoms, kelp forests, or a rocky sea bed.
One interesting fact about sea urchins is that they aren't picky eaters. While their primary food source is kelp, they can also eat other types of seaweed, plankton, and decomposing matter from fish or sea creatures. They can also consume bivalves and periwinkles.
Their mouths are on the underside of their body. They have a unique feeding structure called Aristotle's lantern, which consists of five tooth-like systems. Each tooth is supported by its jaw, allowing independent movement.
Though these creatures are beautiful, you should never touch them with your bare hands! The sea urchin spines are very sharp and can sting when they pierce human skin. Moreover, they quickly break off. Once stung, these spines can get stuck on your skin, producing more discomfort for the victim.
Though the sting of most sea urchins isn't life-threatening, it can still be dangerous. Tiny claw-like pincers called pedicellariae release venom. Depending on the sea urchin species, the victim can experience localized pain, muscle aches, loss of strength, inability to move, breathing difficulties, and allergic reactions.
Sea urchins have separate sexes, with distinct male and female individuals. During the mating season, which can occur at different times of the year depending on the species and location, sea urchins release their reproductive cells into the water column simultaneously or in staggered intervals1.
Once released into the surrounding water, the eggs and sperm undergo external fertilization. The sperm, released in large numbers, must locate and fertilize the sea urchin eggs. This process occurs in the water column, increasing the chances of successful fertilization due to the wide dispersal of gametes.
Baby sea urchins spend a period as plankton or water drifters, relying on their cilia to move and navigate for food. They can eat phytoplankton and zooplankton. However, the urchin plankton is vulnerable to predation and environmental factors such as currents and temperature.
They go through multiple molting stages, shedding their exoskeleton as they grow, allowing them to increase in size and develop the structures necessary to transform into juvenile sea urchins (typically around 6-8 weeks).
According to the Guinness World of Records, the Flower Sea Urchin is the world's most dangerous sea urchin. The Flower Urchin's venom can cause paralysis, extreme pain, muscle contractions, and breathing difficulties. Divers must be careful, for its sting can cause drowning.
Furthermore, unlike other urchins that inject venom through the spines, these urchins inject venom through their flower-like appendages called pedicellariae. So the next time you see a flower urchin underwater, approach it cautiously.
Primary spines are the longer and more prominent ones on a sea urchin's surface. They are typically thicker and provide structural support, stability, and protection to the sea urchin. Additionally, primary spines aid in locomotion, allowing sea urchins to move across surfaces or navigate through their habitat.
Secondary spines, called tube feet and pedicellariae, are smaller and shorter spines found between the primary spines.
People around the world eat sea urchins. Sea urchin gonads, or Uni in Japan, is a premium seafood with a salty taste and creamy texture. Their global market reached 100 million dollars in 2014 and has continued to rise.
Traditional Japanese restaurants serve them raw. However, they can also flavor sauce, soup, and pasta. Due to their high demand and the low supply of quality gonads, Uni is very expensive. A single 200-gram box of premium gonads costs $100 in the U.S. However, in Japan, it can sell for up to 500 dollars!
The Red Sea Urchin can live for over a century. These species don't show any signs of senescence or aging. Scientists even discovered that a 100-year-old individual can reproduce as a 10-year-old urchin. Though the exact reason for their longevity is unknown, researchers suggest their slow growth and metabolism are contributing factors.
Sea urchins, unlike animals with more complex nervous systems, lack a centralized brain or ganglia. Their nerve cells spread throughout their body, allowing for localized processing of sensory information and coordination of basic behaviors.
Read about another marine invertebrate with a simple nervous system. Take a look at our jellyfish facts to know more about them.
Certain sea urchin species are threatened or endangered due to habitat loss2, pollution, overfishing, and climate change. However, other species are abundant and widely distributed.
Though these spiky species serve the ecosystem well by controlling the algae population and promoting coral recruitment, their overpopulation can equally harm the habitat. The Purple Sea Urchins, in particular, have become pests due to overpopulation. Their over-proliferation has caused the overgrazing of kelp forests and led to empty spaces called urchin barrens.
Urchin barrens negatively impact ecosystem health and the fishery industry. As kelp forests continue to vanish, economically important fish like abalone and Red Sea Urchin vanish.
We hope you enjoyed this list of fascinating facts about sea urchins!
Related: To further explore the animal kingdom, check out some of the other animals that start with S.
Vives, A., Rubilar, T., Herrero-Pérezrul, M. D., & Ceballos-Vázquez, B. P. (2021). Reproduction of the sea urchin Tripneustes depressus (Camarodonta: Toxopneustidae) in Bahía de La Paz, Baja California Sur, Mexico. En Bahía De La Paz, 69(Suppl.1), 202–218.
Agnello, M. (2017). Introductory Chapter: Sea Urchin - Knowledge and Perspectives. In InTech eBooks.
Pontheaux, F., Roch, F., Morales, J., & Cormier, P. (2021). Echinoderms. In CRC Press eBooks (pp. 307–333).
Chinny Verana is a degree-qualified marine biologist and researcher passionate about nature and conservation. Her expertise allows her to deeply understand the intricate relationships between marine life and their habitats.
Her unwavering love for the environment fuels her mission to create valuable content for TRVST, ensuring that readers are enlightened about the importance of biodiversity, sustainability, and conservation efforts.
Fact Checked By:
Mike Gomez, BA.