Seashells, Spies of the Ocean

By examining the ridges on seashells, researchers aim to obtain information regarding temperature or pollution of their environment. Researchers who don’t think twice about diving into both warm and freezing waters looking for these precious molluscs.

Mmm...a delicious platter of oysters! Oh, a big scallop! However, these molluscs are not only of culinary interest. Their limestone shells actually enclose precious clues about marine environment. Due to this, some researchers don’t think twice about diving under the ice of the Antarctic, into water at minus 1°C, to collect scallops living there…at the risk of coming back up frozen stiff!

Before fishing for scallops in the South Pole, Laurent Chauvaud, now a researcher in the Marine Environmental Science laboratory in Brest, examined the scallop-shell, their cousins from Brittany. At the beginning of the 90s, he discovered that these bivalve molluscs produced one ridge on their shells every day, a little like trees adding a ring to their trunk every year.

Extremely useful to know how old the shells are! Also useful to obtain information about the conditions that prevailed at the time that these ridges were produced by the animal. With regards scallops, the distance between the ridges are directly connected to the temperature of the water and the concentration of phytoplankton. The warmer the temperature and the more abundant the food, the larger the gaps between these ridges are. In other words, the more the shellfish thrive and eat well, the more “shell” they secrete.

On the other hand, when conditions are unfavourable, the shellfish produces little “shell”. This is what occurred with the Peruvian bivalve, Protothaca thaca, during the El Nino event from 1997-1998. Due to this sea current, the temperature of the water greatly increased, which did not suit theses animal at all: they practically stopped growing for a period of two years. This left a distinctive mark on their shells! This mark is now highly sought after in other shell fossils to know whether phenomena such as El Nino have also occurred during the last thousand years.

But the shells also hide conceal chemical information. Leonardo da Vinci had predicted this, in as early as the sixteenth century, while studying snail and eggshell fossils. Ahead of his time, as usual, he drew up the sclerochonology contours, an old discipline that analyzes shells or skeletons.

By progressively building up their shell, bivalve molluscs engrave characteristics of the marine environment into the limestone. It is possible, for instance, to find what temperature brought about different ridges by measuring their concentration of oxygen. The same method is used to find the temperature of ice cores.

The concentration of metal in their shells (lead, nickel, copper…) enables us to measure and date periods of pollution, such as those linked to mines or oil slicks, for instance. Thus, after the oil tanker Erika was shipwrecked in 1999, Laurent Chauvaud came across a strong concentration of vanadium, a metal found in fuel leaked from the vessel, in the ridges of the scallop-shells. "And these shells could also be used to determine the nitrate contamination periods off the coast of Brittany”, he adds.

However, not all bivalve species produce ridges in such a regular and visual manner as the Pectinidae (the mollusc family to which scallops and scallop shells belong). This is especially the case for oysters: it is not easy to distinguish between the growth bands on the back of their shell. Even more so due to the fact that unexpected ridges can be added after attacks from predators, storms or also during periods of reproduction.

But researchers have more than one trick up their sleeves to make these molluscs “speak”. This is why Franck Lartaud, a young researcher at the Pierre & Marie Curie University in Paris, swapped his microscope for a devise that measured light emitted from elements that are submitted to a bombardment of electrons. "The shells of bivalves are made up of limestone and more precisely of bicarbonate of calcium. When oysters are producing their shell, the calcium is sometimes replaced by other items found in the surroundings, for example, manganese. Manganese has the particularity of lighting up and becoming colored when it is provoked by an electric current. But, above all, it has been demonstrated that this substance is more concentrated in the shell in summer than in winter, and that it can even be used to distinguish between daily ridges!” he explains. Jackpot, the oyster “spilled the beans!" “And this method can also be applied to oyster fossils", Franck adds.

Yet, the advantage of the bivalve mollusks is that they have been in existence for a very, very long time: for over 200 million years in the case of oysters! So you can understand that these molluscs can prove to be real “informers” on past climatic conditions. Indeed, they can do much better than the rings on trees that enable us to determine the temperature over a period of 8,000 years, or the ice cores from the South Pole over a period of “only” 800,000 years. Another advantage of these marine archives is that we come across a lot of them in the fossil deposits all over the earth’s surface. "Thereby, we were able to estimate that the temperature of the sea covering the Paris area at the beginning of Eocene, that is 52 million years ago, was 20°C in winter. And that increased to 30°C in summer”. Nice temperatures to have a dip, don’t you think?

So, are you convinced that these molluscs are good informers? It is worth noting that coral is also an excellent informer. Everyone leaves a trace somewhere…

Key facts

  • The shell of a mollusc is composed of a complex mix of calcium carbonate and organic matter.
  • The calcium can be partially replaced by other elements, for example, lead, nickel, magnesium or manganese.
  • The production of the shell is linked to the animal’s metabolism (reproduction, nutritious input), as well as environmental parameters (temperature, tides, day/night changes, pollution).
  • A succession of lines can be found on a shell called growth ridges. These ridges are also visible on the inside of the shell.
  • In theory, it is possible to determine the age of a seashell by counting these ridges. But sometimes it is necessary to resort to more complex methods…
  • Sclerochronology is a technique used to determine the age, history and environment of a fish, a shell or coral.
  • Certain bivalves, such as oysters, first appeared 200 million years ago. They can therefore provide information on climate conditions in very ancient periods.

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