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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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67
Figure 2 Sperm release by broodstock of
Tridacna crocea
40
seconds after induced spawning by means of an intragonadal
injection of serotonin through the byssal orifice.
and therefore must be quickly fertilized by fresh
sperm. The fertilization procedure is performed in the
hatching tanks. The recommended protocol is to store
eggs at a density of 25 mL
-1
and add sperm
considering a volumetric ratio of 1:200 for egg:sperm
(Heslinga et al., 1990), in order to avoid polyspermic
events. At two hours post-fertilization (pf) the
fertilized eggs have all sank to the bottom (Mies et al.,
2012). The top layer of the water column of the
hatching tanks can be siphoned to remove floating
debris. Aeration spots can be turned on to keep the
oxygen levels high. With a homogenous water column,
eggs should be sampled and a minimum 80% should
be fertilized to yield a commercially viable batch
(Ellis, 1998).
After fertilization, a daily addition of 5 ppm of
broad-spectrum antibiotics has greatly improved
survival (Fitt et al., 1992). Cephalosporin and
streptomycin are suitable choices (Fitt et al., 1992;
Mies et al., 2012). Antibiotics can be used on a daily
basis during the first week and gradually removed the
following days. Tanks should be kept undisturbed
during the embryonic development and the
trochophore stage. A simple sampling can be
performed to evaluate hatch rate and trochophore
activity.
4.2 Veliger stage
After all larvae have reached the veliger stage,
approximately 30 h pf at 27
℃
(Mies et al., 2012), the
hatching tanks are drained and larvae are transferred
to the raceways. This procedure is performed slowly,
by submerging a 70-µm mesh cloth in a bucket
containing filtered seawater and siphoning with a hose.
The veligers are then placed in the raceways and
stocked at a density of 10 larvae mL
-1
(Fitt et al.,
1984). Slightly stronger aeration can be turned on.
Veliger larvae have fully formed digestive tract and
feed on available phytoplankton. To increase survival
rates, green algae, haptophytes and diatoms may be
added to the larval tank at a final concentration of
approximately 5 × 10
4
cells mL
-1
(Fitt et al., 1984;
Murakoshi, 1986). The haptophyte species
Isochrysis
galbana
has proven the most suitable and with an
adequate fatty acid profile (Fitt et al., 1984; Fitt et al.,
1986). Added phytoplankton can be either live and
cultured or dead and preserved as found in
concentrated products. Giant clam larvae fed every
two days until metamorphosis have yielded the fastest
growth rates (Mies et al., 2012).
Giant clam larvae are aposymbiotic and during veliger
stage first acquire symbionts (Fitt and Trench, 1981;
Heslinga et al., 1984; Mies et al., 2012). The
symbiotic relationship is established within 24 hours
after
zooxanthellae
acquisition,
translocating
photosynthetically fixed carbon in the form of
glycerol (Mies et al., unpublished data). The
importance of the early seeding of zooxanthellae is
highlighted by Mies et al (2012), where is shown that
veliger larvae containing symbionts in the digestive
tract grow 25% faster than aposymbiotic veliger
larvae.
Zooxanthellae from different taxa have been
successfully seeded into giant clam larvae (Fitt et al.,
1986; Mies et al., 2012). Freshly isolated and highly
motile cells from a conspecific have produced the
higher acquisition and survival of veliger larvae (Fitt
and Trench, 1981; Fitt et al., 1986). Zooxanthellae can
be retrieved from the tissue of other organisms by
blending and filtering in a 40-µm mesh. This
symbiotic relationship has proven so fundamental that
no juvenile tridacnid clam has ever been found
without zooxanthellae in its mantle tissue.
Zooxanthellae additions are recommended on days
four and six pf (Mies et al., 2012) and also
occasionally after larvae have metamorphosed