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Int'l Journal of Marine Science 2012, Vol.2, No.9, 62
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69
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The loss of zooxanthellae in the mantle tissue is a
phenomenon called “bleaching”, in which the mantle
colouration turns white and the host perishes as a
consequence of massive tissue fragmentation and
necrotic events (Sudek et al., 2012
)
. However, the
nature of such relationship is slightly different when
the host is still in its larval development. The gametes
released by an adult giant clam are aposymbiotic, with
total absence of zooxanthellae cells (Fitt and Trench,
1981; Fitt et al., 1986; Mies et al., 2012).
Zooxanthellae are first acquired during veliger stage
by filter-feeding in the water column. The symbionts
are resistant to digestion (Fitt and Trench, 1981; Fitt et
al., 1986) and eventually establish symbiosis prior to
metamorphosis (Mies et al., unpublished data). Both
larvae and adult individuals are able to establish
symbiosis with different strains and isolates of
zooxanthellae, originally extracted from corals,
anemones and especially other giant clams (Fitt et al.,
1986).
2 Hatchery System
2.1 Scale
Many different aquaculture systems have been
described for the production of giant clam seeds.
Large-scale systems (Heslinga et al., 1990) tend to be
outdoors and connected to the ocean with no artificial
lighting or manipulating of water physical-chemical
parameters. Rearing protocols can be either of
intensive or extensive character, with both yielding
proper results (Heslinga et al., 1990). Small-scale
systems (Mies et al., 2012) are generally located
indoors and completely recirculated without any water
exchange. Artificial lighting is provided and water
parameters are thoroughly regulated through the use
of protein skimmers and water heaters/chillers.
Rearing techniques are of extremely intensive
character. Independently of the scale, a few
prophylactic filtering practices are commonly
employed in all production systems, such as the
installation of ultraviolet sterilizers and mesh filters.
Both artificial and natural seawater have been used in
the culture tanks.
2.2 Components
The most popular and successful systems used in giant
clam aquaculture were comprised of five different
types of tanks (Figure 1), each one for a particular
purpose (Heslinga et al., 1990; Braley, 1992; Mies et
al., 2012).
Figure 1 Diagram of the five sets of tanks generally used in
giant clam aquaculture and the main tasks and processes
involving each one of them.
The first is a broodstock tank (1), in which adult and
mature clams are kept. The broodstock tank is
generally shallow to facilitate light penetration and
made of rigid material,
e.g.
concrete, in order to hold
large and heavy specimens. Spawning broodstock are
transferred to spawning tanks (2), which are small
volume vessels of easy handling used for gamete
collection and transportation. Gametes are fertilized in
the hatching tanks (3), where eggs and trochophore
larvae are stocked. Hatching tanks are generally
round and made of plastic or fiberglass. When the
majority of the larvae have reached the veliger stage
they are transferred to shallow and flow-through
raceways (4) also made of rigid material to withstand
large water volumes. Raceways are provided with