Mostly people just find the empty dry tests or shells of ‘Sea Potato’ sea urchins on the sea shore. Seeing a living specimen seems a lucky find and a real privilege to a beachcomber like myself. The specimen photographed in this blog was found at low tide on the sand at Port Eynon, Gower. It looks quite different from the empty tests you find on the strandline – especially where all the spines have dropped off (see the earlier post Sea Potatoes from Gower).
The Sea Potato, Echinocardium cordatum (Pennant), is also known as the Heart Urchin – partly because of the overall shape of the test but also, surely, because of the heart-shaped patch of specialised spines on its oral or lower surface. You can see this clearly in the above picture where the animal is lying upside down on the sand with its back end towards the left of the picture.
In several of these photographs you can see some small pink bivalved molluscs amongst the sea urchin spines. These are not there by accident. They are very special animals which only live in association with this sea urchin species. They are called Tellimya ferruginosa (Montagu) and are commensal with the Sea Potato or Heart Urchin. Commensal means that they have this close association because they gain benefit from it, probably mostly from an enriched food supply and water flow – but they are not parasites that gain advantage at a cost to the host.
All the spines are mobile. Spines are attached by skin and muscles to small lumps or tubercles on the test. There are several different shapes of spines and groupings of spines. Flat tipped spines occupy the heart-shaped patch on the under surface. These are used in excavating the burrow and for locomotion.
Most of the test is covered by longer, finer, sharply pointed spines. These form special groups in some parts of the body such as the backward directed circlet surrounding the anus at the posterior end of the animal – visible in many of the pictures.
Another special grouping of long spines is found on the upper or aboral surface of the sea urchin. This tuft often projects upwards through the entrance to the burrow occupied by the animal.
Between the groups of spines are shallow grooves with very modified, small and articulated spines called pedicellaria; and extensible tube feet like those found in starfishes. The grooves are the ambulacra and form a shape like a five petalled flower. The tube feet are pushed out by hydrostatic pressure from the inside of the test to the outside via small holes in the test. You can often see these places as reddened dots amongst the spines even if you cannot see the tube feet themselves.
The spiny mat that covers the animal is mostly found in the areas between the ambulacral grooves; these areas are the interambulacra.
For a short video clip of a Sea Potato sea urchin found at the water’s edge, actively moving its spines and still with two of the commensal bivalves in position amongst the peri-anal spines, click on the link below:
Live Sea Urchin at Port Eynon – video clip
The final photograph shows a living Sea Potato in its correct orientation with the aboral or dorsal surface uppermost, and the anterior or head end facing away from the viewer.
Revision of a post first published October 2009
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Jessica's Nature Blog 
It looks like a damp tribble!
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Go on – do tell me – what exactly is a ‘tribble’?
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Reminds me of a shy little hedgehog, all curled up for safety.
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Well spotted. Sea urchins belong to the Class Echinoidea which means “like a hedgehog” because of all the spines covering the body.
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Ah, you’re not a trekkie, then. Sorry. Grew up with it and so it’s part of my internal landscape.
A Tribble was a creature from some unknown planet that was about the size of a guinea pig, and round and furry and without eyes or much else visible, and basically ate, defaecated and reproduced. But they made a lovely pleasing purring sound that had an effect on the central nervous system of humans that meant they were popular as pets. The trouble with Tribbles(the name of the first episode with them in) was that they were born pregnant and they reproduced at an alarming rate; turns out the clue to controlling the reproductive rate was restricting food. Anyway, my brother bought me a (copy) Tribble a few years back, which makes it’s cute noise too, and the sea potatoe looks very like it!
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Love the site! A real taste of Gower when I can’t be there.
I’ve linked to you in a comment on Pharyngula (hope the HTML works!) and linked to your photos of the ‘plastic’ rock pool.
By the way a tribble was an imaginary animal in a Star Trek episode (Trouble with Tribbles) which reproduced at a phenomenal rate and almost caused the demise of the Enterprise. It was more the size of a hedgehog than our lovely sea potato. It’s the only Star Trek episode I know the name of!
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Thank you for doing that, Kitty. I don’t know whether you also noticed that I had photographed ‘Gulls Gobbets’ for the blog as well; these were regurgitated pellets containing brightly coloured pieces of plastic and mussel shells which had probably been picked up from nearby where the multi-coloured rock pool was located. I was also just in the process of following up with photos of the pool taken in August – not so attractive now, and the water obviously polluted and stained red. I am going down to Rhossili again in November and I’ll look out for more evidence of seabirds eating plastic rubbish.
Thanks for the information about the tribbles!
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Tribbles sound delightful! If it wasn’t for their breeding habits, they sound like they would make wonderful pets. I used to have cats and I loved it when they curled up on me and purred. It is very soothing to be connected to the purring. I’d love to have a cat now but it wouldn’t be fair on the poor thing. Although I live in a lovely rural setting that cats would find enticing, there is noone who could care for a cat when I’m not there. Do you know where I could find a tribble? If I could keep it malnourished…….
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I wish I knew where to find one. Guinea pigs are quite similar and travel fairly well. they also make a lovely array of noises, including a purrr! which in Guinea pig means I love you(as it’s usually a precursor to nookie)
My cats are indoor cats. partly to protect them from the urban issues and partly to protect the wildlife from them. They’re also resigned to sometimes being incarcerated in their cat boxes for longish car journeys; usually to my parents’ house at Christmas where the journey is rapidly driven from their minds by lavish applications of cooked turkey, prawns and other delights.
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Oh and as an addendum, recent studies have shown that the frequency at which cats purr has beneficial effects of bone density and can even help heal bone breaks. So the little old lady whose cat sits and purrs on her knee is less likely to have osteoporosis!
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Somehow a guinea pig does not have the right kind of attitude – they might purr for guinea pigs of the opposite sex but I don’t think they would leap up onto my lap and brush their cheek against mine while purring.
You are fortunate to have cats, and cats that travel well. I wish I was still owned by a cat.
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That’s a health benefit of which I was unaware. Looks like I will just have to content myself with the right kind of calcium rich diet and high impact exercise regime to ward off the osteoporosis in lieu of the purring cat.
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GPs can be very affectionate to their owners, but it takes a time to accustom them to handling and then to get them to like it. The deafening chorus of squeaks when you come home(and hopefully feed them) is one of the many things I miss about them now we have none. We had our GP home in the dining room of our last (much bigger) house and the piggies used to emerge when we ate meals and eat at the same time and chirrup at us. We had a couple who were very bright and could do things that astounded us.
I’d miss cats if they weren’t here. I’d miss dogs too; my dog has cancer of the tongue and we don’t know how long she’s got but I will miss her enormously when it does happen.
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In the past, before we could afford a car, our legendary cat Watson used to travel on the train with us. He disdained our carresses but accepted those of total strangers, punishing us for taking him away from home and putting him in a box, but he loved it when he got there. The one time we put him in a cattery for Christmas, the cattery owner refused to ever have him back as he’d created absoulte havoc. Ah, he was a giant among cats!
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Guinea pigs sound like they could be a lot of fun but somehow they still lack the appeal of a cat. Cats and dogs are usually around for a long time and develop a bond with us and us with them. I am very sorry that you are facing the prospect of losing your dog in that way. So sad.
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Watson sounds like a real character. I have looked after some real character cats myself in the past. My first cat used to bring me ‘gifts’ that actually included two live guinea pigs! So I have handled some and know that they are generally more like the tribble you described than the sea urchin in the picture – though I have seen those punk-like guinea pigs with hair that sticks up all over the place and these do look a bit more like giant live sea potatoe sea urchins.
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Your comments have travelled a long way from the original subject matter 🙂 but I must say those are the best photos of Tellimya in situ that I have ever seen. I have previously dug the urchins from their burrows to search for the bivalves when biological recording on the shore. You can find them by sieving the sand that was surrounding the urchins. Seeing them attached to the host animal like that is not, in my experience, a commonplace phenomenon.
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Thank you for the comment, Jan. You are the only person to have noticed the presence of these small bivalves – let alone that there was anything unusual about the shot.
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