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The
pearl oyster Pinctada
margaritifera (Linné, 1758) var. cumingi
(Reeve), which produces black pearls, has given rise in
French Polynesia to a flourishing aquacultural industry. This
recently developed branch, which achieved an annual turnover of
approximately one billion French Francs in 1998, is already the
second largest French aquacultural activity.
Since
the profession underwent a few setbacks (diseases fatal to the
oysters) at the beginning, and since there was a general lack of
scientific knowledge about the pearl oyster and its natural
habitat, a vast research programme (the PGRN) was launched, and
the site chosen for the field research involved was the atoll of
Takapoto, in the heart of the French
Polynesian pearl farming area.
A
doctoral thesis which was an integral part of this programme
summed up the
results of nearly 4 years of research on the habitat, ecology,
physiology, growth and reproductive cycle of this bivalve, which
is now being intensively reared.
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Outline
of the doctoral thesis
: the aim of this research project was to develop a
deterministic model accounting for the growth and
reproduction of P.
margaritifera in its natural ecosystem (the atoll
lagoons). To draw up this model, it was necessary: (1)
to analyse the hydrobiology of the Takapoto
lagoon (the physical and chemical composition of the
waters, the trophic network, the food supply potentially
available); (2) to study the physiology of this bivalve
(including its nutrition, its system of filtration, and
its digestive, respiratory and excretory processes) both
under laboratory conditions and in
situ; (3) to record the annual patterns of growth
and reproduction in
situ. All these results were then fed into a
biological model which was used to develop a computer
software program simulating the pearl oysters' growth
patterns and reproductive cycle depending on the
surrounding conditions.
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The
satisfactory level of agreement obtained between the
predictions of the model and the findings obtained in
the field study confirmed the overall validity of the
model. |
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At
the scientific level, this model includes and confirms
all the scientific knowledge available so far about the
nutrition, growth and reproduction of the pearl oyster.
It throws light on how the oyster adapts to its
surroundings. It can also be used routinely at the
rearing stations to calculate the oysters' food
requirements. |
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From
the pearl farming point of view, this model provides
pearl growers with useful information about the time
required for pearls to form, when they should be
collected, how to optimise the numbers raised per clutch,
and the overall trophic capacity of a lagoon. It has
emerged that although the lagoons are not being
overworked, there exist some local problems due to the
very dense oyster populations which are being reared in
some places.
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The
lagoon habitat
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Several
field campaigns were carried out with a view to
determining the physical and chemical
characteristics of the lagoon waters (their temperature,
salinity and oxygen levels, for example) and the
suspended particulate matter they contain (the mineral
and organic matter; the taxonomic composition), which
provide the pearl oysters with their food. Measurements
were carried out at 10 different sites. Extremely
detailed records were made at some of the sites for
several years. |
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Generally
speaking, the data collected on the Takapoto lagoon were
found to be quite homogeneous: the mean temperature of
the water is 29°C, the salinity level
is 39‰, and the oxygen content is always at
saturation level. These parameters show only very slight
seasonal variations. In addition, this isotropic habitat
is characterised by transparent waters containing very
little particulate
organic matter (POM = 0.4 mg.l-1).
The food potentially available to the pearl oysters is
therefore rather sparse, but on constant supply. |
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Mean
values and variations in the MOP in the Takapoto lagoon
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Lastly,
the taxonomic composition of the POM was determined,
based on all the data collected so far on this subject.
It emerged that the POM consists mainly of particles
less than 2 µm in size (picoplankton). The picoplankton,
which these bivalves usually have difficulty in
retaining, consist of detritus, cyanobacteria, bacteria
and pico-eucaryotes. The fraction larger than 2 µm (nano
and microplankton), which these bivalves are usually
able to retain more easily, amounts to only 1/3 of the
total POM and consists of detritus, phytoplankton,
protozoairians and a small proportion of zooplankton.
The food supply potentially provided by the POM is
therefore not only sparse, but it also consists mainly
of picoplankton, which is not of a very suitable
size for bivalves to feed on. And yet pearl oysters
thrive in these oligotrophic waters. This paradoxical
situation has been explained by the results of the
ecophysiological studies described below.
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Ecophysiology
of the pearl oyster
In
order to determine and account for the pearl oyster's growth and
reproductive patterns in its natural surroundings, all the basic
physiological activities were investigated both in experimental
laboratory studies and in
situ. Some of these activites, i.e., the Filtration (CR) and
Retention (RE)
of suspended
organic matter (POM), determine the animals' food intake,
i.e.,
their energy gain.
Other activities, i.e., the
Biodepositing (PF+F),
of Excrement (U) and Respiration (R),
result in a loss
of energy during the intake and metabolism of food. The
ecophysiological studies therefore consisted of investigating
each of these physiological activities more closely. Their
responses to changes in the surroundings were then expressed in
the equations on which the model for the growth and reproduction
of the pearl oyster was based.
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In
the studies on the Filtration processes (CR, l h-1),
pearl oysters were found to adapt remarkably well to the
lack of nutrients in their surroundings: the
filtration capacity of Pinctada
margaritifera was found to be ten
times higher than that of the other bivalves studied so
far in the literature, since a mean filtration rate of
1500 l day-1
was recorded in these oysters at the age of 2
years. The organ responsible for the filtration
process (the branchiae) is in fact particularly well developed in these
bivalves.
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The
other physiological activities of the oyster are much
more commonplace. The results of the
Retention
studies showed that pearl oysters, like most other
bivalves, are not very well equipped for efficiently
retaining picoplankton (<2 µm), although the
suspended particulate matter consists mainly of these
tiny organisms. This inability is partly due the fact
that there are so few latero-frontal lashes adorning the
branchial filaments. In other bivalves, these lashes (or
cilia) are
more numerous and serve to capture particles < 2 µm
in size. |
The
results of the Biodeposition studies showed
that (1) pearl oysters produce pseudo-faeces, even at the very
low turbidity levels present in the lagoon habitat, and that (2)
these oysters' intestinal absorption rates are in line with
those commonly observed in bivalves, i.e., approximately 55 %.
The
substances thus absorbed are therefore used in the catabolic
processes (Respiration). The respiratory
consumption rates were also in line with the normal values
recorded in most of the bivalves inhabiting tropical
regions: at the age of 2 years, a pearl oyster consumes 4
mg O2
h-1.
Growth
and reproductive processes
Whenever
the energy gains are greater than the losses,
Growth
(Ps+Pg), storage and Reproduction (Pr) processes occur, and in the opposite case, the oysters lose weight. These
processes of bio-energy balance and exchange have been simulated
in the model. However, the validity of the model still required
to be confirmed by collecting further field data and comparing
them with the theoretical predictions. The pearl oysters' growth
and reproductive cycles were therefore closely studied in three
cohorts placed under normal rearing conditions in the Takapoto
lagoon. These cohorts, which were aged 1, 2 and 3 years at the
beginning of the study, were reared under the usual pearl
farming conditions. Every 15 days for a period of one year, a
batch of oysters was sacrificed and biometric, biochemical and microscopic analyses were
carried out.
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In
the very stable lagoon environment,
the oysters' pattern of Growth in terms of the
increase in the weight of their flesh (Pg) and that of
their shells (Ps) was found to be extremely regular,
showing no seasonal variations.
The
small variations observed
in the weight of the flesh from one month to another
were mainly due to changes in the weight of the gonad.
These variations, which were mainly correlated with
events in the reproductive cycle, increased with the age
of the oysters.
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The
Reproductive
habits of the pearl oysters fit a well known
pattern: owing to the great stability of their
environment, P.
margaritifera breed practically all year round. This
pattern of reproduction is fairly common among tropical
bivalves.
Owing
to the high temperatures, the gametogenetic cycle is
short (it occurs roughly once a month), and almost 5
complete reproductive cycles a year are therefore
possible in the case of
adult oysters. However, the gametes emitted on
each occasion amount to only 10 % of the animal's body
mass.
The
pearl oysters' reproductive strategy therefore differs
from those of the bivalves inhabiting more temperate
surroundings, where reproduction usually occurs only
once a year but much larger amounts of gametes are
emitted, since these can weigh as much as 50 % of the
body mass. On a year-to-year basis, however, the animals'
total reproductive investment is exactly the same in the
case of both strategies.
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The
ecophysiological growth model
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The
results described above were integrated into a model
simulating and predicting the growth and reproductive
behaviour of a cultured pearl oyster, depending on the
environmental conditions.
In
practical terms, this growth model is a computer
software program into which all the functional data
mentioned above were integrated, using specific
scientific procedures.
The
data fed into the model (the inputs) were those obtained
on the oysters' habitat (the nature and proportions of
the suspended particles and the taxonomic groups to
which they belong). The central part of the model focuses on the
pearl oyster's physiological functions. The outputs are
predictions about the oysters' growth and reproductive
activity. |
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In
each of the age-groups studied (consisting of oysters
aged 1, 2 and 3 years), the predictions
of the model were very similar to the field data
collected, especially as far as the increase in the
weight of the shells and the reproductive cycle were
concerned. This
model, which is the first of its kind to deal with a
tropical bivalve, can therefore be said to be both valid
and reliable. It is now being used as a tool in the
framework of both practical and scientific applications.
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From
the scientific point of view, this model has made it possible
not only to integrate all the findings available so far on the
pearl oyster's habitat and physiological functions, but also (1)
to elucidate the role played by this bivalve in the trophic food
web, (2) to identify the type of reproductive strategy it
uses and (3) and to quantify the fluxes generated by pearl
oysters at various time-intervals.
From
the practical point of view, it has provided professional oyster
growers with a useful tool (for managing the density of the
oyster populations reared and predicting their growth, and the
dynamics of the rate of pearl formation). This model is also of
great value to the administrators responsible for managing the
lagoons (determining the maximum stock, granting concessions,
etc.).
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A
simulation of the process whereby nacre
is deposited on the nucleus of a pearl |
For
further information
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Reports and publications
Niquil
N, Pouvreau S, Sakka A, Legendre L, Addessi L, Le Borgne R,
Charpy L, Delesalle B (2001) Trophic web and carrying capacity
in a pearl oyster farming lagoon (Takapoto, French Polynesia).
Aquatic living resources 14: 165-175
Buestel D.,
Pouvreau S. (2000) Evolution spatio-temporelle de la matière en suspension,
nourriture potentielle pour l’huître perlière Pinctada
margaritifera, dans le lagon de Takapoto (Tuamotu,
Polynésie Française). Oceanologica Acta 23, 193-210.
Pouvreau S., Bodoy A., Buestel (2000) D.
In situ suspension feeding behaviour of the
pearl oyster, Pinctada margaritifera :
Combined effects of body size and weather-related seston
composition. Aquaculture 181, 91-113.
Pouvreau S.,
Tiapari J., Gangnery A., Lagarde F., Garnier M.,
Teissier H., Haumani G., Buestel D., Bodoy A. (2000) Growth of the Black-Lip pearl
oyster,
Pinctada
margaritifera, in suspended culture under
hydrobiological conditions of Takapoto Lagoon (French
Polynesia). Aquaculture 184, 133-154.
Pouvreau S.,
Gangnery A., Tiapari J., Lagarde F., Garnier M., Bodoy
A. (2000)
Gametogenic cycle and reproductive effort of the
tropical blacklip pearl oyster, Pinctada
margaritifera (Bivalvia: Pteriidae), cultivated in
Takapoto atoll (French Polynesia). Aquat. Living Res. 13,
37-48.
Pouvreau S., Bacher C., Héral M. (2000) Ecophysiological model of growth and reproduction of
the black pearl oyster, Pinctada margaritifera,
in the planktonic food web of Takapoto lagoon (French
Polynesia). Aquaculture 186, 117-144.
Pouvreau S. (1999). Étude et modélisation des mécanismes impliqués dans la
croissance de l’huître perlière, Pinctada
margaritifera, au sein de l’écosystème
conchylicole de l’atoll de Takapoto (Polynésie Française).
Th. doct. Halieutique & Aquaculture, ENSA, Rennes
(France), 267 pp (+annexes).
Pouvreau S., Jonquières G., Buestel D. (1999) Filtration by the pearl
oyster,
Pinctada
margaritifera, under conditions of low seston load
and small particle size in a tropical lagoon habitat.
Aquaculture 176, 295-314.
Pouvreau S., Bodoy
A., Buestel D. (1998) Détermination du bilan énergétique chez l'huître
perlière, Pinctada margaritifera, et premier modèle
écophysiologique de croissance dans le lagon d'atoll de
Takapoto (Polynésie Française), Bilan des
connaissances et avancée des travaux. IFREMER, Rapport
Interne de la Direction des Ressources Vivantes, Taravao,
Tahiti, RIDRV 98-01 : 69 pp.
Pouvreau S., Prasil V. Growth of the black-lip pearl
oyster,
Pinctada margaritifera, in
9 culture sites of French Polynesia : Synthesis of
several sampling designs conducted during 1994-1999.
Aquat. Living Res.,
à paraître
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Research
laboratories
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In
Polynesia :
IFREMER-COP
BP
7004
Taravao,
Tahiti
Polynésie
Française
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In
Mother Country :
CREMA
(CNRS-IFREMER)
BP
5
17137
L'houmeau |
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Atoll_site_