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Int'l Journal of Marine Science 2012, Vol.2, No.9, 62
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variation in mantle pattern and colouration, are the
most popular choices for decorating aquariums. The
marine ornamental trade has experienced a significant
growth in the past decades and giant clams are among
the best-selling invertebrates (Olivotto et al., 2011).
So much that approximately 200,000 live giant clams
were exported from the Pacific in the year of 2007
(Friedman and Teitelbaum, 2008).
Along with the increase in exports comes an
increasing pressure on the natural stocks. As a result
of overexploitation, giant clams are locally extinct in
several Asian and Micronesian countries (Heslinga et
al., 1984; Gomez et al., 1994). Considering that larger
specimens are especially targeted and that the
recruitment rate in tridacnids is slow (Braley, 1988),
the recovery of populations has been hampered.
Aquaculture and restocking programs were
established in Micronesia and seeds were transplanted
into the adjacent reefs (Heslinga et al., 1988), but not
enough to restore the previous natural conditions. As a
consequence, the major constraint in successfully
aquaculturing giant clams has been the acquisition of
large, healthy and numerous broodstock.
1.3 Ecology
Protandrous simultaneous hermaphrodites (Nash et al.,
1988), giant clams at first can function solely as a
male and eventually mature the female gonads, only
then functioning simultaneously both as a male and a
female. As broadcast spawners, the gametes are
released in the water column and externally fertilized.
In a spawning event, an individual releases sperm and
after around 15 to 30 minutes undergoes gamete
reversion, when eggs are first released.
The fecundity among tridacnids is high, with a single
individual of
Tridacna gigas
reported to spawn 500
million eggs in a single event (Crawford et al., 1986).
Mean egg size for giant clams is 100 µm (Lucas, 1988)
and once fertilization occurs, an embryonic
development of approximately ten hours (Fitt and
Trench, 1981) begins to take place. At 12 hours
post-fertilization (pf), the first of three larval stages,
the trochophore larva, is hatched (Lucas, 1988; Mies
et al., 2012). The trochophore larva is planktonic and
free-swimming, but with incomplete digestive tract
and therefore non-feeding. At 24 hours pf the veliger
stage is attained (Fitt et al., 1984; Mies et al., 2012),
made evident by the presence of calcareous shells and
a more complex locomotion apparatus composed by
velum and cilia. Newly formed veliger larvae use both
endogenous and exogenous food sources, especially
by filter-feeding on live phytoplankton (Heslinga et al.,
1984; Crawford et al., 1986). During veliger stage,
symbiotic zooxanthellae are also acquired through
filter-feeding (Fitt and Trench, 1981). The last larval
stage is the pediveliger stage, usually reached at one
week pf (Fitt et al., 1984). Pediveliger larvae are
marked by the presence of a foot and can crawl more
frequently
than
swim,
nearing
settlement.
Metamorphosis is completed ca. 15 days pf (Fitt et al.,
1984) and is evidenced by the upward positioning of
the shell, a consequence of a regulation of balance by
the statocyst. At metamorphosis, an individual
averages 200 µm in shell length (Heslinga et al., 1984).
Natural larval mortality is high among giant clams,
generally with less than 1% survival from egg to
juvenile (Beckvar, 1981; Mies et al., 2012).
Giant clam larvae of larger species such as
Tridacna
gigas
and
T. derasa
have been successfully reared
without any external food additions (Heslinga et al.,
1990). However, for most of the cases the addition of
phytoplankton has proved essential for larvae to
survive metamorphosis (Fitt et al., 1984; Fitt, 1993).
While there have been no quantifications, it has also
been determined that symbiotic zooxanthellae also
contribute to the nutrition of giant clam larvae with
translocated metabolites (Mies et al., unpublished
data). Adult clams are mixotrophic filter-feeders and
organic carbon is acquired by feeding on
phytoplankton and dissolved organic matter (Klumpp
et al., 1992; Klumpp and Griffiths, 1994). Additional
sources of food are found in photosynthates
translocated by symbiotic zooxanthellae, such as
glucose and glycerol (Muscatine, 1967; Ishikura et al.,
1999). It has been shown that such endosymbiotic
association can supply over 50% the amount of carbon
required by an adult giant clam (Klumpp and Lucas,
1994).
The symbiotic relationship between zooxanthellae and
juvenile or adult giant clams is obligatory for the host.