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Int'l Journal of Marine Science 2012, Vol.2, No.9, 62
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appropriate irregular and rugose substrata to facilitate
larval settlement and metamorphosis. The substrate
provided is often cut in smaller pieces for removal and
viewing under stereomicroscope. The last tank suits
the purpose of grow-out (5) and is shallow and well lit
to stimulate zooxanthellae growth.
3 Induced Spawning
3.1 Broodstock
Individuals chosen for broodstock are generally very
large in order to spawn a good amount of quality and
ripe eggs. Broodstock must also be very healthy and
allocating energy for reproduction and not for healing.
With the purpose of evaluating the quality and
ripeness of the eggs stored in the gonads of an adult
individual, a gonadal biopsy is usually performed
(Braley, 1984). A thin hypodermic needle is inserted
through the excurrent siphon aperture and a small
tissue fragment is removed. Mature and ripe eggs are
spherical and developing eggs are irregular (Crawford
et al., 1986).
Bivalve broodstock are generally submitted to a
conditioning process before induced spawning (Utting
and Millican, 1997; Sühnel et al., 2012) with the
principle of optimizing egg production. In the case of
giant clams, suggested techniques include the regular
addition of cultured microalgae, increase in
photoperiod
and
keeping
physical-chemical
fluctuations at a minimum (Mies et al, 2011). While
there are no reports on the frequency of spawn of giant
clams
in situ
, several studies recommend that an
individual is given at least six months to recover from
one induced spawning event to another and
successfully undergo gametogenesis (Gwyther and
Munro, 1981; Heslinga et al., 1990). A rotation of
broodstock may be necessary in order to keep
production continuous.
3.2 Evaluation of the method
Several techniques may be employed to induce
spawning in tridacnid clams and mixed results are
obtained. The technique may have a chemical,
biological or physical character. The parameters that
should be considered for the evaluation of the success
of a method should contemplate whether spawning
took place, the spawning response time and the
occurrence of broodstock mortality events (Table 1).
Independently of the method, the individuals are
rinsed clean of epibionts before induced spawning and
placing in the spawning tank to avoid pollution of the
aquaculture system.
3.3 Chemical methods
The most successful method of induced spawning
giant clams is of chemical character, the intragonadal
injection of 1 mL of a 1.0 g L
-1
solution of serotonin
(5-hydroxytryptamine), a neurotransmitter naturally
found in mollusks (Welsh and Moorhead, 1960;
Braley, 1992). There are two different techniques for
injecting a clam with serotonin, depending on the
syringe positioning and place of insertion. The needle
can be inserted through the mantle cavity (Braley,
1985) or through the byssal orifice (Mies et al., 2011).
The mantle cavity injection is based on the insertion
of the needle through the excurrent siphon, in a
downward angle, until the gonadal tissue is reached.
At this point the needle is allowed to pierce a few
millimeters into the gonadal layer and the syringe
contents are ejected. The second technique is based on
the injection through the byssal orifice, an aperture
found underneath the shells from which the byssal
filaments and foot are exposed. The clam is held and
the needle is inserted through the byssal orifice,
piercing a muscle layer and then reaching the gonads.
The main difference between both techniques is the
fact that the mantle cavity injection sometimes reports
broodstock mortality (Ellis, 1998). That is a
consequence of misplaced injections that inadvertently
pierced vital organs, while there are no major organs
in between the byssal orifice and the gonads, with
broodstock mortality yet unreported in the byssal
orifice method. Both serotonin techniques yield high
response rate and quick spawning, with the byssal
orifice technique often inducing the release of gametes
in less than a minute (Mies et al., 2011).
Hydrogen peroxide has also been reported as an agent
of induced spawning in tridacnid clams. By means of
a syringe, 10~20 mL of a 3% solution is introduced to
the incurrent siphon (Beckvar, 1981). Spawning does
occasionally take place, but the majority of the
individuals does not respond and still show many
signs of stress (Fitt and Trench, 1981; Gwyther and
Munro, 1981).