Activity
of the pearl oyster Pinctada margaritifera
in the
Polynesian atolls
Introduction
The
oysters' valve movements
were measured on several occasions in situ during the work on
the PGRN programme(1991-1999), using a
valvometer. This device was originally developed for measuring
the activity of mussels and determining whether their shell movements
were affected by sea pollution: mussels were therefore used in that
case as sensitive sea pollution detectors.
Here the valvometer was used to study the rhythmic activity of
the pearl oyster.
Methods
used
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First of all, the valvometer,
which was re-baptised the nacrometer, was adapted to the size
of the pearl oysters (50-150 mm). The oysters were glued onto
a PVC disk. |
A pointer placed in contact with the edge of the oyster valve
is moved upwards
whenever the oyster shell opens up in order to filter the
water it contains. The pointer is set in a horizontal rod
supporting a magnet to which the motion is transmitted. |
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A gauge set in the
PVC disk measures the changes in the magnetic field, and
transmits an impulse which is proportional to the degree of
aperture of the oyster valve to a data acquisition device
placed in a waterproof box. The duration of the data
acquisition period can be adjusted as required.
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Results
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The
accompanying figure shows that the opening of the oyster shell
obeys a rhythmic circadian pattern: the degree of aperture is
maximum at night and smaller during the day. The tendency for
the valves to open more widely and less widely coincide
exactly with the sunset and the sunrise, respectively. In
addition, the
valve is more active during the day, when many large closing
movements occur, than during the night. |
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This figure gives a series of
measurements obtained during a period of several days. The oyster shell opened very little on the first day,
due to the stress induced by the manipulations, and
approximately 48h elapsed before a steady rhythm was set up.
This rhythm was remarkably regular, and a tendency was again
observed for the valves to open less
widely and to close more frequently during the daytime. |
Conclusions
Very few studies have been published in which a circadian rhythm has been
found to occur in bivalves.
Although oyster shells are always open, their behaviour differs
conspicuously between the day and night: the aperture of the valves is
smaller during the day, but the valve movements are ampler and more
numerous during the day than during the night. Although this pattern
of activity seems to be perfectly correlated with the light, oysters
exposed to strong light during the night did not respond to this
change at all and remained wide open when approached by a diver. This
behaviour differs from that observed during the day, when they react
by closing even when a diver is simply passing by. The possibility
that predators may be present during the day probably explains this
behaviour.
The underwater findings suggest a hypothesis as to one of the probable
consequences of this difference in the oysters' opening behaviour
between the day and the night. The maximum night-time aperture
corresponds to the maximum spreading of the mantle up to the tips of
the oysters' growth tufts, whereas
the mantle is practically retracted during the daytime. The
secretion of nacre may therefore occur only at night at the tips of
the tufts. It is possible that the presence of predators during the
day may inhibit the spreading of the mantle: at this time of day,
parrot-fish are often to be seen grazing on the detritus covering the
oyster-shells, for example.
It is difficult to say whether the difference between the diurnal and
nocturnal behaviour of the oyster valves may be due to differences in
these animals' physiological activity. The few attempts made so far to
measure their filtering and respiratory activities did not throw any
useful light on this question. The answer would be of interest, since
it would help to improve the accuracy of the energy balances, which
have been drawn up assuming the pearl oysters' physiological activity
to be constant at all times.
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The
extremely regular rhythm of the oysters' valve movements
suggests that pearl
oysters could be used like mussels as sea pollution detectors
in the atolls. It would be worth testing this possibility by
monitoring the oysters' valve movements at a few of the pearl
oyster research stations, using an
instantaneous system of data transmission connected to
the laboratory. |
Laboratoire Conchylicole de Méditerranée. Station
IFREMER de Sète Boulevard Jean Monnet BP 171 34203 Sète
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Atoll_site_