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Home › Types of Sea Life › Marine Species › Invertebrates › Echinoderms › Urchins

Fun and Interesting Facts about Sea Urchins

[Sea Urchin Phylum: Echinodermata] [Subphylum: Echinozoa] [Class: Echinoidea]

There are about 950 species of sea urchin 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 species, 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:

• Detritus (e.g. dead fish)
• Encrusting coralline algae
• Filamentous algae
• Kelp
• Plankton (especially 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

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.

Threats and Predators

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).

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