Several dark purple sea urchins on some rocks. Around them is some kelp which looks to have bite marks in them.

A deep dive into sea urchins

2 minute read

There’s a good chance you’ve come across a sea urchin while at the beach, but what exactly are these delicate, sometimes spiky shell-like globes? We take a closer look at these intriguing creatures.

What is a sea urchin?

Sea urchins are a type of animal, specifically, a spiny marine invertebrate known as Echinodermata, meaning ‘spiky-skinned’.

There are currently 950 known species, with sea urchins found in waters across the globe – from the warm tropics to the frigid polar. They are also one of the most widespread marine animals on Earth, with species recorded from the shallow shoreline to depths of over 6,000 metres.

Like its close relative, the starfish, sea urchins do not have blood and use a water-filled vascular system to move. They also have radial symmetry, with five lines of pores arranged around a central point. These pores accommodate slender tube feet, which often have suckered tips – the sea urchin uses these feet to move, but can also propel itself along with its spines.

Paul Naylor | Image credit: A sea urchin and starfish

What do sea urchins eat?

Sea urchins are mainly herbivores, feeding on algae such as seaweed and kelp but they will also scrape algae from rocks and other hard surfaces.

While plant material makes up most of their diet, sea urchins are also opportunistic feeders and will eat other foods when available. Some species may consume sponges, mussels, barnacles, sea cucumbers and even dead animals that sink to the seabed.

Sea urchins are considered one of the most effective and voracious grazers in the ocean. A single urchin can spend much of its time feeding.

Sea urchin eating kelp | Image credit: Stefan Andrews/Ocean Image Bank

How do they eat?

Sea urchins’ mouths are on the underside of their bodies. As they move around, they scrape algae into their mouths using an ‘Aristotle’s lantern’. This complex five-sided structure includes jaws, teeth-like plates made of calcium carbonate, and specialised muscles to control them.

As they constantly scrape their teeth against hard rocks, their teeth wear away over time. Fortunately, sea urchins have continuously growing teeth, which means new layers replace the worn teeth keeping them sharp and effective throughout the urchin's life.

What predators eat sea urchins?

Although sea urchins protect themselves with a coat of sharp spines, they are an important food source for many marine animals. Crustaceans such as crabs and lobsters as well as octopuses, sunflower sea stars, sea otters and fish such as wolf eels, pufferfish, triggerfish and wrasses make up some of their predator list.

These hunters have adapted a variety of methods to bypass the urchin's tough shell. Sea otters famously crack urchins open with rocks as do seabirds who will drop them from a height, while octopuses use their powerful beaks to break through the shell. Crabs and lobsters crush them with strong claws, and fish such as triggerfish have robust teeth to help bite through the spines and skeleton.

A green sea urching on a rock. The rock looks to have calcium carbonate deposits to the right of the sea urchin

A green sea urchin | Image credit: NatureDiver

By feeding on sea urchins, these animals keep marine ecosystems in balance though. In areas where natural predators decline – often due to overfishing – sea urchin populations can grow extremely rapidly, decimating areas by overgrazing kelp forests. They can create stretches of devastated seafloor with far less habitat for other marine life to thrive. These areas are named 'urchin barrens'. It is therefore incredibly important for the health of the ocean that their population is kept in check by predation.

A bright orange sea urchin with reddish tipped spines on a rock

An orange coloured sea urchin | Image credit: Rohan Holt

Why do sea urchins wear hats?

It might sound odd, but many sea urchins will actively pick up items from the seafloor using their feet and spines to wear them. Things like pebbles, shells or even human litter like bottle caps and coins have been recorded as some of a sea urchin's accessories. This is not a fashion statement, however; they just add these items to act as camouflage – protecting them from predators – shield them from strong light in shallow water and possibly make themselves heavier so that they aren't pulled across by strong currents. Sea urchins can also adjust the hat depending on changing conditions. Despite this complex behaviour, sea urchins do not have a brain, so these actions are simply an instinctive response rather than a conscious decision.

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What sea urchins are found in the UK?

Three species of sea urchin can be found in UK waters: the common sea urchin, the green sea urchin, and the small common heart urchin (known as a ‘sea potato’).

The common sea urchin is the largest of the three species, growing up to 15cm in diameter, and its tube feet are longer than in other urchin species. It’s often a pink or purple colour, but can also be green, red or brown. They’re a subtidal species, so live in deeper water, choosing rocks which are always underwater and aren’t exposed at low tide.

Common sea urchins are a keystone species, playing an important role in supporting the biodiversity of the surrounding area. They graze on algae, helping to control levels so other species have space to grow and flourish. 

A orb-shaped sea urchin on the sea bed. It is light pink in colour and is covered in tiny thread like needles

A common sea urchin seen around Loch Carron | Image credit: Paul Naylor

The green sea urchin is much smaller (growing up to 5cm), but its long, purple-tipped spines make it look bigger than it is. This species has a flattened shape.

It’s often found among seaweed and kelp, which it feeds on, but you may also see them on rocks that are exposed at low tide. It lives in waters around the UK, but particularly in the North Sea.

The small common heart urchin is often called a sea potato, not just because of its spud-like appearance, but because it burrows in the sand. This species has golden, backward-facing spines which helps it to dig down into the seabed – it tends to live around 8-10cm under the sediment.

Small common heart urchins grow to around 9cm in diameter and are very fragile. As it lives in its self-made den in the sand, this sea urchin feeds on dead organic matter around it.

A heart-shaped beige urchin laying on a beach. It is surrounded by sand, some diftwood can be seen in the background

A heart urchin on a Pembrokeshire beach | Image credit: Richard Harrington

Are they poisonous?

While some species of sea urchin are venomous, those native to the UK waters aren’t. Stepping on them can be painful due to their spikey spines, however, and there is a risk of the wound becoming infected, so it’s still worth being careful while at the coast.

Some venomous species can be found in tropical waters, such as the flower urchin (Toxopneustes). Native to the Indo-Pacific, this species appears to be covered in tiny flowers, but these are actually specialised pincers that can grasp attackers and deliver a venomous cocktail of toxins. In humans, the venom can cause extreme pain, breathing difficulties, and even paralysis. This makes the flower urchin considered the most venomous sea urchin in the world.

A flower urchin. It looks like a bouquet of pink floers in the shape of a dome.

A flower urchin | Image credit: April_Rose/inaturalist

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