Pearls are considered the gemstones of the seas, rare and beautiful. Pearls are often used as symbols of all things pure and precious. And they have often been the inspiration for stories, such as John Steinbeck’s The Pearl. The process of pearl formation is relatively simple. An “intrusive foreign body” enters the oyster’s shell and acts as an irritant between the flesh of the oyster and its shell. This irritation induces the oyster to secrete a hard, pearly substance to coat the “foreign body”. This pearly substance, known as nacre or more commonly as “mother of pearl”, is the same substance used to create the inside of the shell itself. Over time, as the oyster continues to lay down layers, a pearl is formed. Today we have harnessed the ability to culture pearls, by taking advantage of this process. Pearl farmers introduce an irritant, such as a grain of sand, to the oyster, thereby inducing it to begin this process of pearl formation.
This process is not unique to the oysters we recognize as “pearl forming oysters”, but common to all bivalves (group of animals including oysters, mussels, and clams). The ubiquity of this process raises the question of why this process evolved. Most bivalves feed by filtering particles out of the water column and are likely to frequently encounter sand grains. If all it takes is a sand grain, why don’t we find pearls in wild oysters more often? Bivalves have mechanisms for expelling foreign particles, such as sand grains, from their shells. Furthermore, the process of coating a foreign sand grain in a hard calcium carbonate matrix is energetically costly. Resources that could be invested in growth, are instead devoted to creating a pearl. So if bivalves have a process to expel these foreign particles and it is costly to coat them, why do we see pearl formation at all?
The nucleus of a pearl
Despite being first reported in 1906, there is a fact about pearls that many people don’t know. In the “nucleus” or heart of an oyster pearl, is not a benign “intrusive foreign body like a sand grain, but in fact a parasite. Unlike a sand grain, a parasite represents a threat and a challenge. Whereas a sand grain can be expelled, parasites have mechanisms to minimize the likelihood of being expelled from the oyster (and increase the likelihood of successfully infecting the oyster). These parasites have the ability to attach themselves to the flesh of the animal, making it more difficult for the oyster to expel the parasite. In response, if the oyster can’t expel the parasite, it does the next best thing; it builds a cage around the parasite. It encases the parasite in a beautiful pearly cell of calcium carbonate (anyone else reminded of Han Solo trapped in carbonite?). It’s an everlasting evolutionary arms race between the parasite and the oyster (see Seth’s post on the Red Queen Hypothesis and Chris' post on Evolutionary Arms Races). And in fact, recent evidence has revealed that this battle has been waged between parasite and host for over 400 million years.
Travel back in time to the Devonian Period
Approximately 400 million years ago, there existed a group of organisms known as ammonites. This now extinct group was quite proliferous during the Devonian Period, with over 10,000 species having been identified in the fossil record. When looking at these fossils, paleontologists noticed the existence of pits in the molds left behind by the long deteriorated shell. In the 1960s, paleontologist M.R. House first commented on the similarity between these pits and those that might result from pearl formations. Recently, a group of researchers from Switzerland and Germany re-evaluated these pits.
These researchers, by analyzing fossils that had been collected from all over the world, found marked similarity between the pits in the ammonite fossils and the pearls formed around trematode parasites in modern day bivalves, suggesting that the process of pearl formation dates back to at least the Devonian period. For the first time, this suggests that trematodes may be far older than previously believed. What’s more, they found evidence of these two groups of organisms, ammonites and their parasites (possibly related to modern day trematodes), evolved in synchrony.
The authors identified four distinct types of pit formation patterns, which were associated with different groups of ammonites. What does this mean? Co-speciation. These patterns provide evidence that the parasites were evolving and diverging into distinct species, in close association with the development of distinct species of ammonites. They documented parasites evolving with their host, and occasionally, the host ammonite finally outrunning the parasite (see Red Queen Hypothesis).
Back to today
Parasites and hosts are constantly battling. The manifestations of these evolutionary races can sometimes take on incredible results, like the beautiful pearl. So next time you see a pearl, especially a “natural” or “wild caught” pearl, thank not only the oyster the built it, but the parasite that started it all.
And there you have it, a little pearl of wisdom for you.
Baets, K. D., Klug, C., & Korn, D. (2011). Devonian pearls and ammonoid-endoparasite co-evolution. Acta Palaeontologica Polonica, 56(1), 159-180.