Sea urchins in the UK – More than just a pretty shell
Sea urchin (Echinus esculentus) skeletons or shells are often seen as little flashes of colour in the mass of broken shells and pebbles on the shoreline. However, these marine creatures have gone on a long journey to get to this stage.
Unlike other creatures, sea urchins do not shed their shells and the only time you will find the test (skeleton or shell of a sea urchin) is when they have died and the spines and internal organs have been washed away or eaten.
In the UK there are three species of sea urchin – Common or edible sea urchin(Echinus esculentus), shore sea urchin (Psammechinus miliaris)and the small common heart urchin or sea potato (Echinocardium cordatum)
Sea urchins are echinoderms, related to starfish, sea cucumbers and brittle stars. Their scientific name “Echinus” comes from the Greek word for hedgehog and it is easy to see why.
Common Sea Urchin
The common (or edible) sea urchin is found in UK waters and is a powerful omnivorous grazer. They are a subtidal species meaning that they live in deeper water that is not exposed at low tide.
They will eat barnacles as well as algae and can ‘graze’ whole areas of the sea bed and rocks clear. A complex structure of plates and muscles supports five chisel-like teeth that they use to scrape food from underwater surfaces and are known as “Aristotle’s lantern”. History has it that Aristotle, a Greek philosopher and naturalist compared the mouth’s shape with a “horn lantern” as ancient lanterns were made out of horns and had 5 sides.
Their skeleton, also known as a test is composed of calcium carbonate and is usually bright red with white points where their spines attach. Some may appear pink and others have a purplish-blue hue. The white spines are quite short compared to other urchins around the world. Some of the biggest can grow to over 20cm across.
The urchins have long tube feet which are extended when they need to anchor themselves or move around and are usually longer than the thicker spines. Between the feet are small pincers that they use to remove debris. Without these cleaning pincers, the urchin would find life difficult to collect debris as it moved.
Shore Sea Urchin
This sea urchin is much smaller than the common sea urchin. However, its spines are much longer which gives it an overall appearance of being larger than it actually is.
These sea urchins are pale green with spines that have purple tips and can be up to 6cm across. Beneath the spines, their test is usually no bigger than a golf ball. Rather than round, these sea urchins are much flatter
These sea urchins are intertidal meaning they can be found in shallow water that will leave them exposed when the tide goes out as well as in deeper water.
As they live in shallow water they are the most common sea urchins to find when you are rock pooling. They are often camouflaged by seaweeds and shells that get caught in their spines as the shallow water moves with the tide.
Common Heart Urchin
The common heart urchin is sometimes called a sea potato. This is because of their rounded potato shape and size and also their burrowing habits.
They are fairly common but because they spend a lot of time 10-15 cm beneath the sandy sea bed they are not seen that often. They use their dense yellow and gold spines to burrow into the sand. The spines all point backwards making this task easier and on their undersurface, they have spatula-shaped spines that make burrowing more efficient.
The test on this sea urchin is oval and this means there is a front and a back end. They will construct a channel that runs to the surface of the sand and their elongated tube feet will stretch up this channel for respiration and collecting the particles that they feed on.
How to find a sea urchin shell (test)
As with any creature, their lives will end. Many will be victims of deep-sea trawling and habitat destruction and the species is near threatened. Some will be washed ashore in storms or predated by otters and seals. The fragile shell and spines will remain even after the squishy insides have been eaten or washed away.
Slowly over time, the action of the waves will remove the spines from the shell, eventually leaving just the familiar inner shell. To find a whole shell is a special treat on any beach adventure and a treasure from any adventure. The intact shells will have a hole in the top and small hooks on the inside around the base.
The common sea urchin will have a red test with the coloured bands still visible and the much smaller shore sea urchin will have a green test. The test from a common heart urchin will be a cream heart-shaped oval with a cross pattern on the top.
Where to find sea urchins in the UK
Sea urchins can be found all around the coast. The common sea urchin is found in deeper water and will only be found while diving.
The best places and dives I have found for finding these sea urchins and their tests include
- St Abbs
- Scapa Flow
- Sound of Mull
- Isle of Skye
- Shetland Islands
- Skomer Island
- Eddystone Reef
The shore sea urchin is much easier to find and will be found in rock pools around the coast. Take time to move seaweed and pebbles out of the way to see if they are hiding beneath.
The common heart urchin is hard to find as they are usually burrowed into the sand, but look out for depressions in the sand or small mouse-like ‘things’ crawling across the beach at low tide.
You will quite often see pieces of sea urchin on the beach and they stand out against the shells and seaweed with their bright red and green colours.
Sea urchin fossils
Fossilised sea urchins are called echinoids and are common fossils to find. It is rare to find their spines intact but quite often complete tests can be found. Most are from the Cretaceous period, about 100 million years ago.
In the UK they can be found on the Jurrasic Coast at Charmouth, Hooken Cliff in Devon, Ardnish Point on the Isle of Skye, Danes Dyke near Bridlington and Swanage in Dorset as well as a number of other locations.
Threats to Sea Urchins in the UK
The common sea urchin is considered “Near Threatened” in the IUCN Red List of Threatened species and this is partially due to over-exploitation as well as environmental changes. Their gonads are often eaten as a delicacy. Sea urchins are hunted by otters and seals who keep their colony numbers in check naturally. If the population of sea urchins spirals upwards as predator numbers decrease then their grazing habits can destroy entire habitats in a short period of time.
Sea urchins are vulnerable to rapid changes in their habitat. One of the biggest threats is the increase in rainfall as the climate changes. In Scotland, rainfall has increased by 20% since the 1960s. This combined with glacial melting has resulted in a decrease in the salinity of coastal waters. Research at the Scottish Association for Marine Science found that the sea urchins could tolerate some reduction in salinity but a significant drop below 21‰ (normal salinity is around 31 parts salt per thousand (31‰) resulted in physical deterioration to the sea urchin.
Research has also found that the chemicals found both in new plastics and those washed up on U.K. beaches caused deformities in sea urchin larvae. Sea urchin larvae raised in water tainted by chemicals found in plastics showed serious deformities in contrast plastics that had never been chemically treated did not cause deformities.