Q: A colleague and I are disputing the existence of pearls in edible
mussels (Mytilus edulis). On occasions when I have collected and eaten mussels
in Scotland, I have sometimes found small, white pearl-like objects. My friend
believes them to be something else, and says that mussels never contain
pearls. Who is right and, if there are pearls, are they similar to the pearls
found in oysters?
A: The proof appears to be in the pudding, or more precisely, in the
picture.
The two mussels in the photograph (right) were found on a beach on the
east coast of the Coromandel Peninsula on New Zealand鈥檚 North Island. The
objects inside were similar to pearls and were partially attached to the
shell.
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Apparently mussels do form pearls. When one is walking along a pristine
Coromandel beach, beautiful ocean on one side and lush green bush on the other
and you find a pearl, you just have to say: 鈥淭he world is my mussel.鈥�
A: Most people associate pearls with oysters, and indeed the vast bulk of
commercial pearls come from the 鈥減earl oyster鈥�, which are bivalves of the
genus Pinctada.
However, a wide range of other molluscs can and do produce pearls. British
pearls were known to the Romans, and have been worked sporadically since then.
These occur in the freshwater mussel Unio, which is found in rivers such as
the Tay, and are still worked on a small scale today.
Even the humble edible mussel can produce pearls, although the quality is
generally low compared with Unio or Pinctada pearls. Examining smashed mussel
shells on a seabird 鈥渁nvil stone鈥� on the island of Rum, I found about 1 in 20
contained blister pearls.
A couple of years ago there were some newspaper reports of a quite large
spherical free pearl (one that is not attached to the inside of the shell)
being found by a mussel eater in a restaurant in Perth.
A: On the Atlantic coasts of North America and northern Europe, including
the west coast of Scotland, pearls are commonly found in blue mussels (the M.
edulis mentioned by the questioner).
Pearl formation is caused by the mussel鈥檚 response to the encystment of the
metacercaria larvae of the trematode parasitic worm Gymnophallus between its
mantle and shell.
Heavy infestation of the parasite leading to large numbers of pearls being
formed in the mantle tissues is rarely fatal to the mussel, but it severely
reduces the commercial value of a mussel population as it makes them
practically inedible without damaging the eater鈥檚 teeth.
The problem is virtually eliminated in commercially grown mussels, by
harvesting them before they are five years old. This prevents the pearls
growing to a size where they become a problem.
A: The mussel M. edulis can indeed contain pearls. These pearls are formed
as a result of infection by a parasitic flatworm, probably Gymnophallus
bursicola.
The flatworm enters the mussel as a small larva and lodges in the mantle
epithelium. As a protectant, the mussel encapsuIates the flatworm with layers
of nacreous shell material, and so a pearl is formed. Although the complete
life cycle of the parasite is poorly researched, the common eider duck
Somateria mollissima and the American scoter, Oidemia nigra, which feed on
mussels, are believed to act as definitive hosts while M. edulis serves as the
intermediate host. Presumably, there are enough unencapsulated flatworms
present in the mussel to ensure completion of the life cycle.
Unfortunately, mussel pearls, unlike those that are formed by the tropical
pearl oysters Pinctada, are of no commercial importance. Indeed, the high
incidence of pearls in bottom-grown mussels on the Atlantic coasts of North
America, Denmark, Britain and France can cause a major problem in marketing
the mussels.
However, this problem can be eliminated if mussels are grown to harvest
over a shorter period of time, often on ropes. This ensures that the mussels
are marketed before the pearls reach a detectable size.
A: Pearls do indeed occur in marine mussels of the genus Mytilus and also
in the genus Modiolus (the edible mussel and the horse mussel respectively).
They are usually small and of poor colour. The best pearls are found, however,
in the fresh water mussel Margaritifera margaritifera, also known as the
mussel pearl.
These are widely distributed in soft water, especially in clean, fast-
flowing rivers with a gravel bed (these are also usually good trout and salmon
rivers). In the past, pearl fishing was often carried out by travelling people
who used a glass-bottomed bucket to locate them. The shells don鈥檛 always
contain a pearl but those that do can be recognised by blemishes on the shell
surface and only these deformed shells should he taken by pearl hunters.
This week鈥檚 questions
Hot tin mining: In 1889 it was annotinced that the 鈥渙nly known continuous
lode [of uranium ore] in the world鈥� had been found at the Union Mine in
Grampound Road, Cornwall. Does anybody know what happened to this deposit?
There were plans to use the uranium to replace gold in jewellery. Was this
ever done? If so, it would add a certain frisson to the Antiques Road
Show.
David Fisher
No address supplied
It ain鈥檛 half hot: Why does our temperature go up when we are ill?
Keren Bagon (aged 10)
Radlett, Hertfordshire
Burning issue: What is the cause of the strange smell like burning plastic
that fills the carriages of British Rail InterCity trains whenever they slow
down?
Pete Newman
No address supplied
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