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Fun and Interesting Facts about Sea Urchins
[Sea Urchin Phylum: Echinodermata] [Subphylum: Echinozoa] [Scientific Name: Echinoidea]
There are around 950 species of the sea urchin animal living on or near the seabeds of almost all marine habitats, from shallow coral reefs to deep polar seas at depths approaching 5,000 metres.
This segment contains interesting facts about the sea urchin phylum, including where they live, what they eat, and how this type of echinoderm with long spines reproduce.
Sea Urchin Geographical Range and Habitat
A cosmopolitan distribution confirms that sea urchins live and thrive in tropical coral reefs and even in many polar regions, especially:
- Around tropical and subtropical ecosystems (e.g. Caribbean Sea, Indo-Pacific, Thailand, and the Great Barrier Reef).
- Near the rocky coastal shores of most temperate zones (e.g. California, Japan, and the Mediterranean Sea).
- The deep seabeds of the ice cold waters around Antarctica and North America.
Common habitats of sea urchin species are seagrass beds, kelp forests, or rocky substrates found at the intertidal zones where herbivores can graze on different types of algae.
Pro Tip: There's no shortage of funny fish names, and the sea urchin classification is often called sea hedgehogs due to the unusual internal complex structure of their spines which are made of single-crystal calcite.
Sea Urchin Physical Characteristics
The key feature of a regular sea urchin is its round body shape and mostly symmetrical, 5-part form (pentaradial).
In contrast, irregular urchins have more of a flatter elongated oval outline, as seen in the sand dollar anatomy and with heart urchins (Spatangoida).
Apart from the radial symmetry, the globular body is made of fused calcareous plates that are covered in 'movable spines' of various sizes.
Here's the thing:
In general, the external skeleton (called a 'test') is rigid and made of calcium carbonate, with either a spherical or flattened shell. But, there is no obvious head on top.
Instead, sea urchins use their sharp spines (which are venomous in some species) for defence, movement, and for burrowing.
Even though there is no brain, they have a nerve ring that encircles the mouth and contains radial nerve cords that extend outward from it. Sea urchins are sensitive to touch and changes in light.
Pro Tip: Our diving injuries section contains expert advice about sea urchin spine treatment with tips for avoiding marine life envenomation.
What Do Sea Urchins Eat?
Even though sea urchins species are mostly herbivorous grazers, many will supplement their daily diet with other food sources, depending on their habitat.
In general, the nutriment for most echinoderms comes from:
- Clam Worm (Alitta succinea)
Detritus (e.g. dead fish)- Encrusting coralline algae
- Filamentous algae
- Kelp
- Plankton (especially for juvenile urchins)
- Seaweeds
- Sea cucumbers (Holothuroidea)
- Sea sponges
- Sea squirt tunicates (Ascidiacea)
Interesting Fact: The feeding method occurs through the use of a 5-toothed jaw mechanism known as Aristotle's lantern. Urchins use it to bite or scrape algae and coral polyps, and then guide the food to their mouth via feet-like tubular projections.
Different Types of Sea Urchins
Banded Sea Urchin (Echinothrix calamaris)
Some of the vernacular names used for Echinothrix calamaris include the banded sea urchin, the living pin cushion, and the double-spined urchin.
Banded sea urchins are commonly found in the tropical waters of the Indo-Pacific, East Africa, the Red Sea, and Polynesia. In fact, they are the most common of all long-spined urchins seen in Hawaii.
This particular species of sea urchin are invasive to coral reefs and frequently encountered when snorkeling or scuba diving in Thailand.
Characteristic and Behaviour
Even though most members of the sea urchin classification are fairly easy to identify underwater, banded sea urchins tend to inhabit shallow coral formations and rubble areas.
Nonetheless, Echinothrix calamaris are also renowned for wedging themselves into tight crevices among the coral framework.
Moving on...
Most of the familiar kinds of sea urchin are nocturnal grazers, feeding on green algae and detritus (dead and decaying carcasses).
Even though banded sea urchins move slowly, they get a lot of protection from the sharp spines that are attached to the oval shaped calcite skeleton or shell (known as a "test").
Colour and Size
Adults urchins grow to about 20 centimetres (8 inches). They have dull black (or gray white) long primary radiolas. These sharp spines are hollow and brittle, so they're easily broken.
They also have shorter secondary spines nestled amongst them. These short spines are yellow or golden brown and they contain the venom.
The important part...
Even though the flower sea urchin (Toxopneustes pileolus) is a lot more venomous than the banded urchin, the spines are sharp enough to puncture human skin.
While not venomous like some other urchins, needle injuries can still be painful and prone to infection, but rarely life threatening.
You may also see a balloon-like anal papilla - a spotted sac marked with black and white spots. This is the part of the body that channels away waste products.
Pro Tip: One of the lesser known facts about sea urchins is that their spines provide some shelter for some juvenile fish, such as the cardinal fish species from the family Apogonidae.
Black Sea Urchin (Arbacia lixula)
Burrowing Urchin (Echinometra mathaei)
Cake Urchin (Meoma ventricosa)
Chaetodiadema Granulatum
Crowned Sea Urchin (Centrostephanus coronatus)
Diadema Setosum
European Edible Sea Urchin (Echinus esculentus)
Fat Sea Biscuit (Clypeaster rosaceus)
Five-holed Keyhole Urchin (Mellita quinquiesperforata)
Flower Urchin (Toxopneustes pileolus)
Only a handful of the 950 species of sea urchin are venomous. Even so, the poisonous urchins have potent defences and they're capable of delivering a painful sting.
Check out these little-known facts about the flower urchin (Toxopneustes pileolus), including where they're found and why they're regarded as the most dangerous sea urchin in the world.
Globular Sea Urchin (Mespilia globulus)
Green Sea Urchin (Strongylocentrotus droebachiensis)
Halloween Urchin (Tripneustes gratilla)
Hatpin Urchin (Centrostephanus longispinus)
Heart Urchins (Spatangoida)
Despite a fairly wide distribution, heart urchins are most commonly found in the sand or muddy seabeds of temperate and tropical seas.
That said, the major hotspots for this particular species of sea urchins are the Caribbean Sea, Indo-Pacific regions, and the Red Sea.
Still, the North Atlantic, North Sea, and the Mediterranean provide ideal habitats for one of the best known of all, the common heart urchin (Echinocardium cordatum) - frequently called sea potatoes.
Characteristic and Behaviour
In contrast to the typical echinoderms characteristics of crawling over the reef, heart urchins tend to bury themselves in soft sand or a similar kind of sediment.
They have a tendency to inhabit shallow coastal waters and use their tube feet to dig burrows and create a network of tunnels. This means many species are buried out of sight in only a few metres of depth.
Shape and Size
The reason why they're called heart urchins is because their rigid skeleton (called a "test") is heart-shaped - more noticeable when viewed from above.
Most sea urchins have a globular body shape. But most members of the order Spatangoida are oval shaped with bilateral symmetry.
It's rare for any of the common types to grow much bigger than ten (10) centimetres long (4 inches). Even so, the red heart urchin (Meoma ventricosa) is one of the biggest, growing up to fifteen centimetres long (6 inches).
To compare, the New Zealand heart urchin (Echinocardium australe) is one of the smallest species, typically reaching forty (40) millimetres (1.6 inches).
Interesting Fact: The International Union for Conservation of Nature (IUCN) does not see any specific risk for the population status of common heart urchins (Spatangoida).
Helmet Urchin (Colobocentrotus atratus)
Lime Urchin (Diadema antillarum)
Long-spined Sea Urchin (Diadema paucispinum)
Pacific Purple Sea Urchin (Strongylocentrotus purpuratus)
Pink Sea Urchin (Pseudocentrotus depressus)
Purple Sea Urchin (Paracentrotus lividus)
Red Urchin (Astropyga radiata)
Red Sea Urchin (Mesocentrotus franciscanus)
Rose Flower Urchin (Toxopneustes roseus)
Reticulated Sea Biscuit (Clypeaster reticulatus)
Sand Dollars (Clypeasteroida)
The early forms of Clypeasteroida (flattened, disc-shaped, burrowing sea urchins) first appeared around 60 million years ago.
Check out these interesting facts about sand dollars (echinoids), such as where they come from, what they eat, and how "sea cookies" reproduce.
Slate Pencil Urchin (Heterocentrotus mamillatus)
Sputnik Urchin (Phyllacanthus imperialis)
Ten-lined Urchin (Eucidaris metularia)
Sea Urchin Reproduction
From the age of two (2) sea urchins start to become sexually mature and reproduce sexually. Most echinodermata species have separate sexes (male and female individuals).
The hard skeleton contains five gonads for males to produce sperm. Females produce eggs that have a fertilisation envelope to block multiple sperms.
Key takeaways:
Following a natural trigger for spawning season, such as a rise in water temperature, sexual reproduction results in external fertilisation when eggs and sperm are released into the open ocean.
After fertilisation, the embryos start to develop into planktonic triangular shaped larvae (called pluteus). They drift with the water currents for several weeks or months before they eventually settle on the ocean floor and transform into juvenile sea urchins.
Sea Urchin Predators and Threats
The ecological role of ocean urchins is an important one. Still, even though aquatic herbivores help to control algal growth, an overpopulation of sea urchins may also cause overgrazing.
In turn, this may reduce biodiversity and lead to rocky coral reefs being stripped of filamentous algae and kelp - a phenomenon known as "urchin barrens".
But wait - there's more:
Sea urchins face a number of natural predators as well as some human-driven threats. For example, fish families (especially triggerfishes and wrasses), sea stars (starfish), sea otters, most crab species, and humans collectively eat sea urchins.
All that being said, some of the worst human-induced threats to the long term survival of sea urchins include:
- Climate change (resulting in varying degrees of coral bleaching).
- Coastal habitat degradation and destruction.
- Underwater garbage pollution.
- Overfishing (particularly in countries where ocean urchins are considered as being a culinary delicacy, such as Europe and Japan).
Interesting Fact: In 1999, the IUCN Red List of Threatened Species assessed the conservation status of many sea urchin families as being of "Least Concern" (LC). However, they list the European Edible Sea Urchin (Echinus esculentus) as "Near Threatened" (NT).
Related Information and Help Guides
- Echinoderms Examples, Characteristics, and Behaviours
- Crown of Thorns Starfish Facts and Information
- Fun and Interesting Facts about Sea Stars (Starfish)
- Sea Cucumber Species Facts and Information
- Sand Dollar Anatomy: How Do Sand Dollars Reproduce?
Note: The short video [3:35 minutes] presented by "Deep Marine Scenes" contains extra sea urchin facts with genuine footage of these spiky echinoderms living in their natural environment.