The evidence for infestation by Polydora hoplura Claparede in European Flat Oyster shells (Ostrea edulis Linnaeus) is distinct from that caused by the related but smaller marine polychaete worm Polydora ciliata Johnston. [This kind of damage was described in the earlier post Ancient & modern Polydora ciliata type burrows in Flat Oyster shells].
The Polydora hoplura bristleworm is about 50mm long. It makes large U-shaped burrows in oyster shells. The much smaller P. ciliata creates very small burrows on the outer surface of the shell; these are relatively harmless – although severe infestations make the shell unsightly and therefore unsuitable for serving at the table in restauarants. P. hoplura, on the other hand, damages only the margin of the shell – on the inner surface.
The P. hoplura worm (in its mud tube) settles between the inner edge of the shell and the fleshy mantle. The mantle not only covers the living animal but also secretes the shell. Weak acids from the worm’s excretory products dissolve the U-shaped burrow in the shell. The oyster is irritated by this intrusion and usually tries to seal off the pest by forming a thin layer of shell around it. The result is a shelly blister. If too many of these blisters are formed, or if any one of them is particularly large, the oyster may be unable to close its valves properly and this has a major effect on the health of the oyster. The oyster may die and the shell may be severely distorted in shape.
In old empty oyster shells, whether these are found on the beach or in middens on archaeological excavations, the thin encapsulating shell of the ‘blisters’ can still survive intact – but more usually is broken to reveal the empty worm burrow beneath. The importance of identifying this type of infestation in historical deposits lies in the fact that the P. hoplura species has a distribution in the UK restricted to south western coasts. P. ciliata is found on all UK coasts. The presence of hoplura damage in ancient oyster shells found on inland sites is, therefore, an important clue to the place that the oysters came from. This helps to pin-point trade routes.
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