Inside the shell of the oyster, the animal is wrapped around by a fleshy cloak called the mantle. The mantle creates the shell – with the interior surface of which it is always in contact. Shell growth rate depends on water temperature, season, available nutrients, and dissolved minerals.
In oysters the shell is built up layer by thin layer. Shell growth is not continous but punctuated. In warm weather, when food is abundant, growth is faster than in cold weather with a poor nutrient supply. Reproduction and extreme climatic conditions slow down growth too – although it never really stops, just gets very slow.
Each additional growth layer is a little longer than the previous one. Day by day, month by month, year by year, the oyster shell slowly increases in diameter from the origin near the ligament to the outer margin of each valve of the shell. Because the growth in diameter of the shell is achieved by adding new thin layers one on top of the other, the shell gets thicker as well.
The incremental shell growth is visible as concentric growth lines on the exterior of the shell. There are many narrow growth shoots from day to day growth. They tend to be narrower in winter and wider in summer thus indicating season of growth. These small increments are grouped into wider bands that represent each year of growth.
The appearance of the growth lines is different for left and right valves. The right valve is generally flat and smooth with the lines clearly visible. The left valve, on the other hand, tends to be cupped to accommodate the meat of the animal. The outer surface of the left valve has ill-defined radiating ribs and the concentric growth shoots stand proud and look like frills; the overall look in an unworn shell is a bit like a tiered or layered petticoat.
The growth lines make interesting natural patterns, as shown in these photographs. However, they are also evidence for growth rate, past climate, and age (as in the number of years old at death).
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