The shells of the European Flat Oyster, Ostrea edulis Linnaeus, are described as generally rounded but often distorted in outline. It is a fact that many of the shells of this species found on the seashore are mis-shapen and irregular around the edges. So what kinds of irregularities are there in oyster shells and what might the causes be? Here are a few ideas put very briefly. There are possibly five main categories of reason for abnormality in overall shape in the shell.
Figures 1 to 16 are photographs of modern oysters that have been collected from beaches but they illustrate features frequently noticed in shells from archaeological excavations.
1. Genetics or Environment
Figure 1 and Figure 2 (above) show the exterior and interior surface respectively of a right valve. The shell is elongated and the axis of growth appears to have changed during development. The reason is unknown but could be genetic or environmental. The oyster may have been moved either by natural or man-made causes; or forced to grow in a new direction by an obstruction such as another oyster; or subjected to a new influence like a change in direction of strong current.
2. Settlement Substrate
Figure 3 and Figure 4 (above) show different views of the external surface of a left valve. The heel of the shell nearest to the hinge or umbone is strongly concave and inverted barnacles are embedded within its matrix. In this instance the irregularity has been caused by the original settlement of the young oyster on a hard epibiont-encrusted surface. This could have been a rock or another oyster shell. The oyster has subsequently developed over and around this substrate – assuming its shape and engulfing the barnacles.
3. Weight compensation
Figure 5 and Figure 6 (above) present the exterior and interior views respectively of left valve. The shell is very heavy, thick and of great age – as shown by the extensive ligament scar with many growth lines. A characteristic lateral extension or ear has developed adjacent to the hinge area. Its function is uncertain but a tentative suggestion is that it may have developed to increase the surface area to compensate for the greater weight when on softer sediments, i.e. to stop the oyster sinking.
Figure 7 and Figure 8 (above) again show the external and internal surface respectively of a left valve. This is also a very thick old shell and bears a lateral extension near the hinge which has itself grown at an angle. In this case, the ear is more pronounced due to damage by an infesting organism immediately below the extension. The damage probably resulted from an infestation by a marine Polychaete tube worm.
4. Infestation Damage
Figure 9 and Figure 10 (above) show the exterior and interior of a thick and heavy left valve. The shell is riddled with infestation burrows. Some of these, such as the very large one in the centre of the outer surface would have been made by a boring bivalved mollusc. It is clear that during life the oyster shell was heavily colonised – probably most frequently by Polydora ciliata and P. hoplura, since the remedial sculpturing of the margin of the shell reflects the extensive damage caused by at least one U-shaped burrow.
Figure 11 and Figure 12 (above) show the inner and outer views of a right valve with a marked lateral margin indentation or cleft that has similarly and clearly been caused by infestation damage.
Figure 13 and Figure 14 (above) show the outer and inner surface view of a left valve. The posterior margin is indented. This has an unidentifiable external cause that could be infestation, predation or overcrowding when young.
Figure 15 and Figure 16 (below) give the exterior and interior view of a left valve. The inside of this shell has provided a settlement surface for oyster spat. Three of these have grown into young oysters of very irregular shape. Two of them are touching and impinging on each others’ growth. This is a typical situation of the overcrowding frequently found in naturally-occurring un-managed native oyster beds.
P.S. Trying to understand the reasons for variations in Ostrea edulis L. oyster shells is a pet subject of mine so there are lots of postings about this subject in Jessica’s Nature Blog. For more information and photographs of oyster shells click here on OYSTER VARIATIONS.
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