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Int'l Journal of Marine Science 2012, Vol.2, No.9, 62
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Table 1 Parameters considered for the evaluation of the success of an induced spawning method performed in tridacnid clams.
Method
Spawning
occurrence
Average time for
spawn
Broodstock
mortality events
Character
Reference
Serotonin injection
(mantle cavity)
high
minutes
yes
chemical
Braley (1985), Ellis (1998),
Mies et al. (2011)
Serotonin injection
(byssal orifice)
high
seconds/minutes
no
chemical
Mies et al. (2011)
Hydrogen peroxide
low to medium
minutes/hours
no
chemical
Beckvar (1981), Fitt and
Trench (1981), Gwyther
and Munro (1981)
Macerated gonads
medium to high minutes/hours
no*
biological
Fitt and Trench (1981),
Gwyther and Munro (1981),
Heslinga et al. (1990)
Thermal stress
low to medium
hours
yes
physical
Fitt and Trench (1981),
Heslinga et al. (1990), Ellis
(1998)
Note:
*while no induced clams perish due to the employment of this method, it does require the sacrifice of adult broodstock.
3.4 Biological methods
When an adult individual begins spawning activity,
non-described pheromones are also released along
with the gametes. These substances trigger spawning
activity in other nearby individuals, resulting in an
epidemic spawning event. The introduction of
macerated gonads of a conspecific is a technique
based on the simulation of an epidemic spawning
event. This technique requires the sacrifice and
removal of ripe gonads of conspecifics. The gonadal
tissue is then macerated and suspended in a solution of
ca. 50 g of gonadal tissue L
-1
(Heslinga et al., 1990)
and finally approximately 10 mL are introduced to the
incurrent siphon of a broodstock clam. This procedure
is relatively successful (Fitt and Trench, 1981;
Gwyther & Munro, 1981), but spawning may take
place hours after stimuli and it requires the sacrifice of
broodstock.
There are alternative biological methods employed in
the induced spawning of other bivalve species, such as
the simulation of a phytoplankton bloom (Velasco et
al., 2007), but most of them remain untested in
tridacnid clams.
3.5 Physical methods
Stressful conditions often cause giant clams to spawn,
as it is made evident with traveling and transported
broodstock. Most physical techniques are based on the
generation of a stressful event. The thermal stress, or
heat shock, is widely used in the oyster and scallop
industries and it is the sole physical technique
successfully used for induced spawning in giant clams.
This can be accomplished in two different manners,
either in raising the water temperature or exposing the
individuals to sunlight (Heslinga et al., 1990). After a
few hours under heat stress, the individuals are
returned to the original tank and natural conditions,
where spawning activity is expected to commence.
While having the advantages of being costless and
producing low mortality, this procedure is frequently
unsuccessful (Fitt and Trench, 1981) and it generally
takes several hours for individuals to respond and start
spawning (Ellis, 1998).
4 Larval Rearing
4.1 Early stages
Induced broodstock are placed in the spawning tanks
once stimuli have been made. Spawning activity is
marked by the release of gametes through the
excurrent siphon (Figure 2). Individuals spawning
sperm are kept in separate tanks from those spawning
eggs. Sperm is the first gamete released and is active
only for approximately one hour (Ellis, 1998). After
sperm release is ceased, egg release commences and
can last several hours. Egg cells, however, are viable
for a shorter period, around 30 minutes (Ellis, 1998)