Farming the Sea
63
large eshy gills are covered with microscopic whip-like appendages
called cilia which, heating rhythmically, push food particles trapped
in a mucous secretion towards the mouth. Although this lter
mechanism selects the size of particles eaten, it cannot prevent
potentially harmful bacteria from being ingested along with the
normal food of the shellsh. Therefore, where the water is polluted,
particularly with human sewage, these microbes may collect within
the shell sh and on its gills and mande—the eshy lining of the
shell. F ortunately it has been known for some time that such
contaminated shell sh can be purged of these unwanted microbes
by ushing with sterile, running sea—water—indeed mussels have
been cleansed on a commercial scale in Wales since 1928.
Different shell sh require different conditions and times to
cleanse themselves but in mussels the procedure is as follows: the
shell sh are kept in sterile sea—water for a day during which the gut
and gill cavity are cleared of bacteria. They are discharged along
with other detritus bound up in strings of mucus. The contaminated
water is then replaced with freshly sterilized sea—water. After a
further day, the mussels afè collected and their outsides washed with
chlorinated water. Finally, the shell fish are checked to ensure that
they are clean and then packed ready for market.
Modern cleansing techniques are denitely adequate to handle
everyday bacterial contamination of shell sh but do they also
remove disease—causing viruses? Some cases of infective hepatitis (a
liver disease producing jaundice and fever) have been attributed to
eating oysters infected with the virus causing this disease, but
evidence has been inconclusive partly because the virus is difcult
to study even in the laboratory. Far more evidence relates to polic—
viruses which make more convenient experimental subjects. Several
scientists have demonstrated _that the oyster will take up these
viruses—in one study the viral concentration became 10 to 60 times
that in the surrounding water. Equally, other evidence indicates
that, during cleansing, viruses are expelled at a rate which closely
parallels that for bacteria. One must also remember that the
concentrations of viruses used in many experiments exceeds that
normally found over commercial shell sh grounds. Strict rules
govern where shell sh may be taken for human consumption. On
the whole, then, there seems no cause for alarm, bUt oysters and
other shell sh growing in the neighbourhood of sewage outlets
should be regarded with suspicion by the amateur collector.
63
large eshy gills are covered with microscopic whip-like appendages
called cilia which, heating rhythmically, push food particles trapped
in a mucous secretion towards the mouth. Although this lter
mechanism selects the size of particles eaten, it cannot prevent
potentially harmful bacteria from being ingested along with the
normal food of the shellsh. Therefore, where the water is polluted,
particularly with human sewage, these microbes may collect within
the shell sh and on its gills and mande—the eshy lining of the
shell. F ortunately it has been known for some time that such
contaminated shell sh can be purged of these unwanted microbes
by ushing with sterile, running sea—water—indeed mussels have
been cleansed on a commercial scale in Wales since 1928.
Different shell sh require different conditions and times to
cleanse themselves but in mussels the procedure is as follows: the
shell sh are kept in sterile sea—water for a day during which the gut
and gill cavity are cleared of bacteria. They are discharged along
with other detritus bound up in strings of mucus. The contaminated
water is then replaced with freshly sterilized sea—water. After a
further day, the mussels afè collected and their outsides washed with
chlorinated water. Finally, the shell fish are checked to ensure that
they are clean and then packed ready for market.
Modern cleansing techniques are denitely adequate to handle
everyday bacterial contamination of shell sh but do they also
remove disease—causing viruses? Some cases of infective hepatitis (a
liver disease producing jaundice and fever) have been attributed to
eating oysters infected with the virus causing this disease, but
evidence has been inconclusive partly because the virus is difcult
to study even in the laboratory. Far more evidence relates to polic—
viruses which make more convenient experimental subjects. Several
scientists have demonstrated _that the oyster will take up these
viruses—in one study the viral concentration became 10 to 60 times
that in the surrounding water. Equally, other evidence indicates
that, during cleansing, viruses are expelled at a rate which closely
parallels that for bacteria. One must also remember that the
concentrations of viruses used in many experiments exceeds that
normally found over commercial shell sh grounds. Strict rules
govern where shell sh may be taken for human consumption. On
the whole, then, there seems no cause for alarm, bUt oysters and
other shell sh growing in the neighbourhood of sewage outlets
should be regarded with suspicion by the amateur collector.