Upstairs in Stromness Museum you’ll find the Victorian Natural History gallery. Given the grandeur of the large taxidermy collection, you would be forgiven for passing by a small display of Echinoderms, such as starfish and sea urchins. Since the museum is closed at the moment, I will bring them to you. This blog presents five fascinating facts about these familiar underwater creatures. I include some photos taken whilst enjoying my lockdown pastime, snorkelling around Orkney.
1. Echinoderm means ‘spiny skin’.
The spines are not actually in the skin however, they are part of the skeleton. Echinoderms include starfish; brittle stars; urchins; sea cucumbers and feather stars. They share ‘radial symmetry’ which means their arms extend out from a central point. Most of them have five arms/sides.
2. Some sea urchins bury themselves under the sand.
These unusually shaped urchins are part of our collection of Echinoderms. They’re known as ‘Sea Potatoes’ or ‘Heart Urchins’ and it is easy to see why. They create a mucous-lined burrow about 15-30cm under the seabed, which includes a breathing tube, an eating tube and an excreting tube! They can also live up to 20 years. The star-shaped pattern on their shell is a clue that these sea potatoes are related to starfish.
3. The mouthparts of the sea urchin are called ‘Aristotle’s Lantern’,
This can be seen in the labelled diagram below from our collection. Around 2370 years ago, Greek philosopher Aristotle wrote in his book Historia Animālium: “In reality the mouth-apparatus of the urchin is continuous from one end to the other, but to outward appearance it is not so, but looks like a horn lantern with the panes of horn left out.” Isn’t it incredible that when Aristotle wrote this, Orcadians were living at places like the Iron Age village at Broch of Gurness.
4. Echinoderms play a vital role in the food web.
Sea urchins eat seaweed (algae) amongst other things and thankfully they are messy eaters, which means the debris they leave behind is eaten by lots of smaller animals. Urchins ‘thin out’ the algae, allowing a more diverse community of organisms to live on the seabed. This is a bit like the affect grazing cattle have on natural grassland. The food web is a delicate balance though, as in the absence of predators, such as crabs and lobsters, urchins can decimate whole populations of algae such as kelp.
Starfish eat by expelling their stomach out through their mouths and over their prey. Safe to say we won’t be inviting them to any dinner parties!
5. Starfish and urchins move along by using their ‘tube feet’.
The feet are powered by a water vascular system, which pumps water around the animal’s body. As muscles in the tube feet contract, water is forced towards the end of the foot, extending it out and attaching itself further along its route.
Bonus echinoderm fact: The Orkney word for sea urchin is ‘Scaddy Man’s Heid’.