Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

In Someone Else's Backyard

added Friday, June 4, 2010

Taking a trip to someone else’s back yard is always a joy. You never know what you will find. At first, life in this new back yard seems much as it does in your back yard. You see plants and animals that are familiar, behaviors that you have come to know, natural processes that are much the same as back home. Then you notice that something is different – something you haven’t seen before – something you can’t explain. So you take careful note of what you see, recording as many details as possible in the short time you have. You look for clues to explain what you see. But your time here is short and your investigations naturally brief.
Once back home, you do a little research to try to explain what you saw. Your research produces some answers but opens up many more questions. Finally it is time to assemble all your facts and come up with a plausible explanation, knowing that you may never find the answers you seek.

I recently took a trip to the Outer Banks in North Carolina. Miles of pristine seashells imagebeach stretched out on both sides of me with hardly another person in sight. Naturally, I did what most other beachcombers do. I went looking for seashells at the high tide line. Many of the shells I found were familiar to me – being common also on the beaches of Massachusetts, which is a mere 500 miles north as the seagull flies. I found lots of scallops, mussels, oysters and jingle shells. Others were new to me – the ponderous ark and Venus shells. However, whether they were familiar to me or not, one thing about them stood out as different from the shells I have seen on my local beaches. About half of them were black. At first glance, it seemed that the black color would happen randomly – that all shell types had their black representatives. Then on closer inspection, I found mainly 3 types of shells that had black representatives – the oyster, scallop and jingle shells. I found practically no other shell type that had turned black. Considering that some Jingle Shells are naturally colored black, that left two shells with black coloration – the scallop and oyster. Both of these seashells had both black and naturally colored specimens.shells image

Now that I had made my observations in the field, it was time to do a little research. Time to turn on the Internet and surf the web for the info I needed. When one is on vacation and far from any reference library, the Internet can be a godsend.
A search for what causes the black color in seashells turned up very little. In fact, the only reference I found was from a site called “Jessica’s Nature Blog” at It seems that the black color is due to bacteria producing a chemical that stains the shells black.

Dig down into the muck of a quiet bay or marsh and you may get a whiff of rotten eggs. This is a familiar smell to anyone who has spent any time in one of these areas. The reason for this is: In a quiet bay or marsh, there is little turbulence in the water. This allows very fine particles of sand and mud to settle to the bottom. Because this sediment is so fine, it traps the water in place at a certain depth under the sediment. Since this water is not replaced regularly with fresh seawater, the oxygen in the water is soon depleted. Certain bacteria that live where there is no oxygen – called anaerobic bacteria – flourish. These bacteria produce a chemical called hydrogen sulfide. This hydrogen sulfide is what you smell. Just dig down into the sediment, release the gas and you smell rotten eggs. To those of us who grew up spending time in a marsh, this is a familiar and welcoming smell.

If the sediment where this hydrogen sulfide is being produced contains iron, then the iron reacts with the hydrogen sulfide to produce iron sulfide. It is this iron sulfide that stains the shells black.

Now that I have done my research, let me put it all together and come up with a plausible theory for the black shells. Both the oyster and scallop can be found in areas in and around quiet water – places where there is fine sediment and anaerobic bacteria producing hydrogen sulfide. This sand must contain a high concentration of iron. When the shells die, they get buried in the sediment. The hydrogen sulfide from the bacteria reacts with the iron in the sand to produce iron sulfide. This iron sulfide stains the shells. At some future time, the buried shells – which are now stained black - become unburied and get washed up on the beach where I can find them.

Of course, theories are fine to begin with. But they don’t prove anything. To make sure that my theory is correct, I would have to find the area where these shells live, check for iron in the sediment and for anaerobic bacteria producing the hydrogen sulfide. Of course, when one is on vacation and time is limited, this is not possible. So now it is time to head to the lab and see if I can at least recreate these conditions and see if they do in fact turn shells black. To see my experiment, check out this months’ Nature Activity