The most variable of all bivalved marine molluscs, the shells of the Common or European Flat Oyster, Ostrea edulis Linnaeus, reflect the differences in -and changes to – the local environment in which the animal has been living and growing; as well as the conditions that have affected them after death.
General details about Flat Oysters have already been discussed in the earlier post Oyster shells from Studland Bay. In this post I am going to talk about colouring in flat oyster shells. I have also briefly touched on why oyster and other shells are sometimes black. You can link to the post Black oysters at Rhossili Bay here.
Just to recap what was said in the earlier post, the orange and black colours of empty oyster shells that you find on the beach are stains on the shell that result from burial in the sand and mud of the seashore. The deeper and the longer the shell has been lying in the sediments, the blacker it will be stained. Shells buried nearer to the surface are likely to be stained orange. Because wave action can turn the sediments over, together with the objects held in them, you sometimes find shells that are both black and orange. Shells that have only been partially buried – with a part remaining above the surface -may be partly stained orange with some of the shell remaining the original colour.
The precise depth in the sediments at which the staining will change from orange to black will depend on how far down in the loose sediments oxygen survives. As depth increases so does the likelihood of oxygen depletion. Surface layers are oxygenated but deeper layers have no oxygen and are termed anoxic. The depth of the oxygenated sediment will vary from one part of the beach to another, and from beach to beach, and from time to time. However, the boundary is usually between 5 and 15 cm down and is marked by a thin grey layer.
Bacteria live attached to the sediment particles. The kinds of bacteria vary according to whether the sediments are aerobic or anaerobic. Some bacteria can only live in the presence of oxygen; others thrive in the absence of oxygen. In the upper layers there is enough oxygen to reduce all the waste products of the micro-organisms; but lower down it is different. The activities of the bacteria react with iron compounds occurring naturally in the sediments. Anerobic bacteria use fermentation or other processes to break down organic compounds and create hydrogen sulphide, ammonia or methane as by-products. The hydrogen sulphide then reacts with the iron in the sand resulting in black iron sulphides that cause the black staining.
When the black iron sulphides are moved upwards through the sand by burrowing animals or agitation by wave action, the oxygen between the sand grains in the higher levels converts them to ferric oxide which is yellow and stains buried shells orange.
The large old orange-stained oyster shell above was photographed on the strandline at Whiteford Point in Gower, South Wales.
The thick old blackened oyster shell shown below was washed up at Rhossili Bay and is pictured on a piece of driftwood from the strandline.
Sometimes parts of oyster shells may be coloured purple. This colouring is usually confined to patches or concentric bands. Unlike the black and orange stains which occur after the death of the animal and are due to burial conditions, pink or purple patches are part of the oyster’s natural and original colouring and develop during the life of the animal – although not all oyster shells have these colours.
In the picture below you can see an oyster shell that is mostly stained black but has purple patches as well. This specimen was seen in the surf on the water’s edge at Rhossili Bay. The pink or purple colouring survives after death and burial. The reason that the pink and purple colours remain is that they result from pigments that are bound up in the shell structure.
The old heavy oyster shell illustrated below was seen on Whiteford Sands, Gower, and it is coloured both black and orange with purple concentric bands and patches.
The reddish, pink or purple pigments are thought to have been derived from the food of the living oyster. It is a filter feeder, eating small particles including various types of plankton such as the microscopic organisms that constitute red tides.
As the animal grows and lays down new layers of shell (a cross-section through the shell would look a bit like flakey pastry because of all the layers), the pigments are enmeshed or incorporated into the actual crystal structures. Pigments have even been preserved, wrapped up in crystals like this, in fossil brachiopods and fossil bivalves – and have been successfully extracted for the study of the organism’s diet by palaeontologists.
The final photograph of the post shows a fine example of the survival of this purple pigmentation in bands along the outer edges of some of the annual summer growth shoots. Growth of shell in Flat Oysters is greatest in warmer weather when food is also more abundant. Believe it or not, this fresh-looking, unworn, sharp-edged oyster dates from the 9th century. It is nearly 1,200 years old and was recovered during archaeological excavations of an ancient oyster midden underneath the Old Town Cellars at Poole in Dorset, UK.
COPYRIGHT JESSICA WINDER 2011
All rights reserved
Jessica's Nature Blog 
Jessica,
I am the Creative Director with Northeast Boating Magazine. We are doing a feature article for our December/January issue about invasive species in the Northeast. We need a photo of a European Oyster. I noticed you have a few on your blog. Would it be possible to use one with a credit to your blog.
Do you have hi-resolution images of any that are now on your blog?
If so what would you like the credit to say?
Thank you in advance to your help.
David
LikeLike
Fascinating stuff. Tony
LikeLike
Thanks, Tony…. but not half as fascinating as your own art work based on x-rays. Ingenious and beautiful.
LikeLike
Oh god, thank-you so much for that article! I’ve spent two hours trying to identify a bunch of seashells I found on the beach (I knew nothing about shells this morning), wondering why I was always finding “white to brownish” descriptions while most of my shells were bright orange and black.
Thanks to your article, I found another text about the blackening of buried shells, but this text was saying that once blackened shells are open in the air again they will rust and turn orange, is it true? I found lovely black pectens, I’d be sad if the colour changed.
I was also wondering if the whole shell is turning black? I picked up lots of cockles in fact, most of them having dark blue/grey stripes on a white shell. Some were amost entirely black, but most had these stripes: can the burial cause these patterns because of existing stripes on the living shell? I’ve been trying for a long time to find the species of the shells I picked and I can’t find no cockle with dark stripes
In any case, thank-you very much for this very informative piece. I don’t know anything about shells yet but I love natural history and will now read the rest of your blog ♥
LikeLike
Hello, Caroline. I am so glad you found some of the answers to your questions on Jessica’s Nature Blog. I haven’t noticed any changes in black shells that I have collected, back to orange. The shells I have stay black, but they do fade a bit I suppose – at any rate dry shells always look more dull than wet ones. They have never changed colour though.
As to whether the black staining goes all the way through the shell, I don’t know. It may vary depending on the length of time the shell has been buried in anoxic sediments, or it may depend on the porosity of the shell. When I have a moment, I’ll cut through some of my blackened oyster shells to see what they look like inside – I’ll let you know what I find. I think that it is only dead empty shells that are entirely black across the whole surface.
Like you, I have been fascinated by the stripes of black or blue-black colour in cockle shells – stripes that are nothing to do with the natural shell colour as it is formed by the animal within. I have concluded that the stripes are to do with variable burial depths of living cockles at different times of the year. If for some reason, perhaps to do with winter storms or very turbulent water, a cockle is temporarily and accidentally buried in deeper, blacker sediments, then the shell that is formed during that period of time is stained. It is possible to find live cockle shells that are half orange, or half black, as well as striped, indicating that they were living on the boundaries of the different zones of oxygen availability in the sediments. There are several posts about striped shells on my blog. It is always difficult to compare the shells you find on the beach with the illustrations and drawings of perfect specimens in the text books because beach-combed shells have acquired a lot of visible history and damage. I have read that some beach shells, particularly the more robust ones, may have been rolling around on the seashore for a hundred years or more. Time enough to be buried and stained over and over, and to be burrowed into by animals, and worn away by the elements.
LikeLike
Oh your explanation about striped cockles is so interesting, I would never have thought of that – but again, I don’t know much about shells and live far from the sea, I have very limited experience with living shells! I’ve given up identifying my findings precisely, I’m not even sure it can actually be done. But I’m happy enough with the genus and have learned a lot already.
I knew already some shells could be really old, I didn’t think they could be *that* old. All of this is fascinating. Thank-you again!
LikeLike
Hi there, thanks for the insight, I’ve been picking up oyster shells with my kids and was wondering why some are quite a dark black, puzzle solved! We do have alot of iron sand here in Nelson New Zealand, so it’s not surprising. Cheers- Mark D.
LikeLike
Thank you, Mark. I am pleased that you were able to find the information you were looking for on Jessica’s Nature Blog.
LikeLike
Oysters are everywhere in my area especially when we are out boating. Usually they are white and gray, and I don’t pay much attention to them. I am a shell-collector but I rarely collect oyster shells. Yours are interesting and pretty! Also this post has some good information. I will link to you when I get around to writing about oysters.
LikeLike
Thank you, Dustytoes.
LikeLike
Hello,
Thanks for the interesting study.
How long would it take for an oyster shell to be black?
Many thanks,
Phil
LikeLike
Hello, Phil. That is a good question. I am not really certain how long it would take for an oyster shell to turn black when buried in anaerobic sediments. Certainly years, maybe decades, or even hundreds of years. Sorry I cannot be specific.
LikeLike
Such interesting information – walking along the beach will become just a more and more enriching journey. I came across the Blog whilst looking for a dark shiny blue oyster picture.
Keep it up!
LikeLike
Thank you, Irene.
LikeLike
I’m wondering since I just found this and can’t find any info on it, but why would a oyster shell be completely purple?
LikeLike
Hello, Peyton. That is a difficult question to answer without seeing a colour photograph of your shell. Where did you find it? Is the purple colour on the inside of the shell as well as the outside of it? If you would like to send some pictures of both surfaces of the shell to winderjssc@aol.com, I will try to answer the question.
LikeLike
Hi Jessica,
I read your posts about colour variations in oyster shells…my 10 year old son collected this black oyster shell during our recent visit to beach…fossils excites him…he wanted to know more about this shell..when we were searching to learn little more, saw your post about colour variations in oyster shells…it is very informative…he is curious to know about how old it is…some clams has got embedded into this oyster shell…he is planning to give small presentation to his class…if you can provide some more information, it would be really helpful…
He also collected hammerhead oyster shell, sharks eye, male spider conch, pink scallop and a purple oyster shell…he learned about other shells but the black oyster shell is old..
Thanks,
Vijayakumar
LikeLiked by 1 person
Hello. Thank you for writing to me about your son’s black oyster shell. It is difficult to say how old the shell is. Black shells are not necessarily fossils. Most blackened oyster shells found on British beaches may be relatively recent in age and perhaps between a few decades to a few hundred years old. They can become stained black while they are buried in mud or other sediments by the waste products of anaerobic bacteria. The shells are not fossilised by this process. The native oysters in the UK, especially the ones that have lived a long time and developed thick shells, frequently have small burrowing bivalves molluscs embedded in their shells; and the thick shells of dead oysters can also be occupied by these small species of clams. The shells also provide a good surface on which a large variety of creatures can attach themselves. On Jessica’s Nature Blog there are several posts about these epibiont creatures that either infest or encrust the shells – but always with reference to the European Oyster Ostrea edulis. I do not know where your son found his blackened oyster or what species it is but unless it is exceptionally hard and heavy like a rock It is unlikely to be a fossil. Perhaps you could take or send the shell to a museum close to where you live, or send them a photograph, to ask for more information. And it is also possible to look for all the other posts with information about oyster shells on Jessica’s Nature Blog by entering the word ‘oysters’ in the Search box, or look at my other site Oysters etc. Hope this helps.
LikeLike
Thanks a lot for taking your time and responding…I am from India and we collected it from Rameswaram, from your reply it is clear that it is not a fossil…Thanks for the pointers, I will check it.
Thanks,
Vijayakumar
LikeLiked by 1 person