Pearl
oysters (Pinctada margaritifera) are benthic molluscs which are farmed
under pelagic conditions, on collectors set in deep waters, facing the
bottom. In
view of the high death rates which were occurring, it was decided to study the
problems potentially associated with this
intensive working of the pelagic environment. One point on which special
attention has focused is the planktonic food webs. The questions addressed
here can be summarized as follows: Is the trophic
capacity of a lagoon sufficiently large to be able to meet the requirements of
both the natural stocks and the artificially reared oysters?
In
view of the large bacterial biomass present in the atoll lagoons, it seemed to
be worth assessing the contribution of this nutrient to the pearl oysters'
overall diet on the atoll of Takapoto. This atoll is one of the most intensive
pearl farming regions in the whole of Polynesia.
The
study in question was performed in collaboration with IFREMER. The rate of
predation carried out by pearl oysters on heterotrophic bacterioplankton was
determined using small aquariums containing given concentrations of bacteria (which
were killed by exposure to heat and stained with dye) and algae. The changes
in the abundance of the labelled bacteria versus the algae (I. galbana,
retention rate = 100%) was used to determine the retention rates of the
bacterioplankton by the pearl oysters.
The
results obtained showed that the retention rates of the bacterioplankton by
the pearl oysters were practically negligible (only 2 to 3% of all the
bacteria were retained). The oysters are able to
efficiently retain only particles with a diameter of at least 3 µm.
Based on these experimental data and on the field measurements performed
annually at the Takapoto lagoon, the contribution of free and attached
bacteria to the diet of the pearl oysters can be said to amount on average to
only 5 to 10 % of the total carbon potentially usable by the pearl oysters.
Bacteria attached to particulate matter account for most of this modest
contribution.
| The
large bacterial biomass
therefore belongs to a size class which the pearl oysters are unable
to use directly, and this can also be said of most of the
phytoplankton species inhabiting the lagoons. The lacking trophic link
between the pearl oysters and the picoplankton (bacteria and
phytoplankton) is therefore probably provided by heterotrophic
nanoplankton (Loret et al. 2000). |
| The
rates at which bacterioplankton are ingested by organisms such as some
of the corals and sponges
inhabiting the reefs have been established (see for example
Ferrier-Pagès et al. 1998). On
the Tuamotu atolls, these rates are
not usually high enough with respect to the
entire water column to affect the abundance of the bacteria
significantly, apart from a few special
cases. |
-
In
the ocean waters entering the lagoons via the hoas, the
bacteria are less abundant
(which shows that some bacteria have been consumed by the benthic
populations present in larger numbers on the crests of the reefs) and their
growth rates are higher (which shows that they have made use of the
organic matter excreted by the benthic populations) than in the
surrounding ocean (Torréton and Dufour, 1996b).
-
In
the mainly very shallow, highly oligotrophic Tekokota atoll lagoon,
which is widely open to the sea, the bacteria are
2.7 less abundant and their growth rates are
9 times higher than those inhabiting the surrounding ocean (Torréton
et al., submitted for publication).
This
page was
based on
:
Jonquières
G, Amouroux J-M, Benett A, Blanchot J, Bougrier S, Buestel D, Caisey X,
Delesalle B, Dormoy J-M, Dufour P, Gearion P, Hautefeuille F, Loret P, Pagès
J, Pellan A, Pouvreau S, Robert S, Teissier H, Tiapari J, Torréton J-P (1995)
Etude de la nutrition de l’huître perlière Pinctada
margaritifera. Programme Général de Recherches sur la Nacre (PGRN)
Rapport final de la fiche 19. 155
pp
Torréton
JP, Dufour P (1996b) Temporal and spatial stability of bacterioplankton
biomass and productivity in an atoll lagoon. Aquatic
Microbial Ecology 11:251-261
Torréton
J-P, Pagès J, Talbot V (soumis.) Bacterioplankton
and phytoplankton biomass and production in Tuamotu atoll lagoons.
References
:
Ferrier-Pagès
C, Allemand D, Gattuso JP, Jaubert J (1998) Microheterotrophy in the
zooxanthellate coral Stylopora pistillata: effects of light and ciliate density. Limnol
Oceanogr 43:1639-1648
Loret
P, Pastoureaud A, Bacher C, Delesalle B (2000) Phytoplankton composition and
selective feeding of the pearl oyster Pinctada
margaritifera in the Takapoto lagoon (Tuamotu archipelago, French
Polynesia): in situ study using optical microscopy and HPLC pigment analysis. Mar
Ecol Prog Ser 199:
55-67
Atoll_site_