Here we are once more with the intention of sharing information about these beautiful marine gems. On this occasion will continue with the subject of natural pearls and specifically about how these gems are created in nature.
During the era scientific enlightenment in the late 1800′s, scientists from all over the world were searching to understand how pearl oysters were able of producing pearls, and their discoveries were fascinating. But some of the first things they discovered at to do with the internal structure of the pearl, since in order to obtain the truth it was necessary to cut open pearls and inspect their core. And what they found is that pearls are very much like onions, at least structurally.
When an onion is cut in half what we see inside are numerous concentric layers, each stacked on the previous one, and in a similar manner pearls are produced: the original seed that caused the pearl to originate will be found at its core, surrounded by millions of micron thin layers of Aragonite. Perhaps this is a reason why the ancient Greeks gave the name “Margarita” to the pearl, since this is also the word for “onion” (hence the name “Margaritifera” that was given in early times to many pearl producing mollusks, meaning “pearl bearer”) in that language.
In the above images of an onion and a natural pearl (both cut-in-half) we can see their internal resemblance, and going further into the deeper core we can also appreciate how their core is not round, but with each additional coating of nacre (in the case of the pearl) the shape becomes rounder, softer, although most natural pearls I have personally seen are rarely 100% round, most being baroque and a good proportion of them being semi-baroque in shape (mainly in the shape of buttons, bullets and drops).
But of course we do have several types of natural pearls. Many don’t look like the image that we have in our brains as being a pearl, but they are nonetheless pearls. We basically have two major groups of natural pearls: blisters and loose pearls. Of these two groups we would have subgroups as well. Let us begin with the pearls that could be considered the most common.
Perhaps some of the most common natural pearls are those usually referred as blister pearls in English, “ampollas” in Spanish or as “ampulles” in French, and we could even say that these laid the foundation for the eventual production of mabé pearls (also known as blister or half pearls). These pearls are commonly found formed on the pearl oyster’s shell, as a response from a very active “Bio-terrorist” (usually an animal that actively drills through the oyster’s shell). The reasons for this active attack on the oyster’s shell are varied and depend on the species that attacks the oyster, blister pearls being the result of the oyster’s defense mechanism against these intruders.
The varieties of organisms that “attack” the oyster’s shell are huge and include animals such as sponges, polychaete worms and drill mussels. Many of these creatures are not really after the oysters flesh, meaning there not there to actually eat the oyster but that they are actually just looking for a “home” and have been known as “domiciliares” because they usually make their homes inside the oyster’s shell and -unfortunately for the pearl oyster-these actually weaken the shell, making it really brittle and easy to break. Of course, these “Bioterrorists” will also come in direct contact with the oyster’s flesh and this interaction will almost certainly produce blister pearls.
There’s a variety of sponge known as, usually colored with a bright orange red or yellow with a sticky consistency, which grows on a large variety of shellfish here in the Sea of Cortez, and it seems to have a preference for the black-lipped pearl oyster (Pinctada mazatlanica). It can cause small lump like blisters, but I have never seen any pretty specimen of pearl caused by this sponge.
Another creature capable of causing blister pearls is the infamous drill mussel or pholids. These creatures –and here I also have to include the Cliona sponge- are actually filter feeders just like pearl oysters are, so we can be sure that they don’t attack the shell to eat the oyster’s flesh, but they have brittle shells so they need the protection of a hard substance around them. These little creatures can actually bore stone, wood as well as sea shells. We have seen numerous blister pearls formed by the attack of these agents, as well as in one loose pearl. These creatures also have a preference for black-lipped pearl oysters, but may occasionally attack older individuals of the rainbow lipped pearl oyster variety (Pteria sterna).
The group of organisms which we find more interesting in the case of pearl formation of the polychaete worms, mainly those of genus Polydora: slender worms usually with a bright red coloration. These worms have the capacity to infest pearl oysters to the point of weakening them and causing their death and in the process making the oyster produce numerous “mud blisters”, which may eventually become coated with nacre.
We have examined several varieties of the so-called mud blisters and in most instances where we have found are the remains of dead drill worms, as well as good quantities of very organic mud. It would be difficult to fully identify what causes this variety of blister pearl, but I believe that it is safe to say that it is a combination of the worm´s drilling activity and the entrance of mud due to the disappearance of the drill worm. What caused the drill worm to disappear? Well, we have also seen large numbers of predatory polychaete worms on the oyster’s shell and these may very well go after the drill worms and kill them, leaving their home vacated.
When removing a mud blister and cutting it in half we usually find a protective coating of protein secreted by the pearl oyster that that helps to coat the organic mud and that is in turn coated with nacre.
Unusual blister pearls
Some very unusual specimens have been found that include other varieties of animals as the cause, these include fish and crustaceans. Perhaps the most interesting specimen is that of a small fish that was found in the shell of a Mexican black lip pearl oyster that was fished in Baja California during the last days of the 19th century (this specimen is still kept in the American museum of natural history in New York). The fish was identified as a “pearl fish” (family Carapidae), which are usually associated with many species of clams and oysters and sea cucumbers (please use this link if you want to see an animated diagram of a pearl fish, if you’re a proctologist you will enjoy this). And although we have seen these fish inside oysters we have never had the fantastic opportunity of finding a “fish pearl”.
Pearl fish are not parasitic but instead they find shelter within the oyster’s shells. I believe most oysters would not be offended by the presence of this little fish, but in this particular case may be the little fish died and the oyster preceded to rapidly coat it with pearl or nacre, I don’t believe this could’ve ever happen with a live fish.
Other possible sources for blister pearls
Other organisms that could be turned into pearls -but that I have never seen turned into pearls- are the little shrimp and crabs that are found inside pearl oysters. The little translucent “pearl shrimp” are also found in many other species of clams, such as pen shells, and are typically found within the large Pinctada oysters. The species we find in the Sea of Cortez is Pontonia margarita, and we can usually find two individuals within an oyster (one male and one female, the male usually being the smaller of the two), this species does not seem to affect the pearl oyster.
Another type of crustacean we have seen inhabiting the oyster’s body is the “pea crab”; these little crabs are somewhat soft and quite clumsy, no wonder they need the protection they find inside an oyster’s shell. These little crabs have only been reported as found living inside the Australian silver lip pearl oyster (Pinctada maxima), but the Sea of Cortez has a variety that is only found within the rainbow lipped pearl oyster (Pteria sterna) and this will be the first time this will be reported in writing. The name of this species is still unknown (Pinnotheres sp.) and we usually only find one inside an oyster. We have seen some crabs causing a disturbance within the oyster that could eventually lead to the production of blister pearls, but we have yet to find a “crab pearl”.
So, what do you think about all the life-forms that depend or use a pearl oyster -in a way or another- for their survival? Life is indeed a web, and if you can save one species you will be offering an “umbrella” of protection for many others…
In our next chapter will continue talking about natural pearls and their possible origins, in the meantime I will continue hunting for additional facts and -of course- searching for more mythical pearls: I can clearly see myself wearing a pea crab pearl pendant.