Download Giant Clams - Deepak Apte

G
iant Clams belong to a group of
animals commonly called bivalves (i.e.
Giant Clams
animals with two hinged valves). These
boneless animals belong to Phylum Mollusca
(i.e. soft bodied animals) and are commonly
seen in tropical reefs in shallow waters.
Giant Clams are impressive in many ways.
They can grow over a meter and are the
largest living mollsucs. Their fluorescent
mantle never fails to catch attention when
one dives or snorkels in shallow coral reefs.
Giant Clams have developed the ability to
cultivate plant in their body tissue (usually
called Zooxanthellae, a unicellular algae),
demonstrating an unusual phenomenon of
endosymbiosis. Endosymbiosis can be seen
in many animal species, however, Giant Clams
are unique because they have improved this
strategy significantly. Unlike other animals
they cultivate zooxanthellae in a special
Text and Photographs by Deepak Apte
Principal Scientist, Project Giant Clam, BNHS
circulatory system, thus keeping higher
number of symbionts per square unit.
Giant Clam: Myth and Science
In the West, Giant Clams remained a lore
in adventure stories. Several horror movies
and novels depicted these animals as
dangerous beasts ambushing divers. The
myth is mainly because their large size. There
is, however, some truth in these stories. The
powerful adductor muscle and heavy valves
of these large animals can seriously damage
fingers or foot. This is mainly due to the fact
that the clam has a natural tendency to close
its valves at the slightest of touch to its
mantle. The best way to avoid freak
accidents is to watch clams from a distance.
These animals are purely plankton feeders
(microscopic animals and plants) and have
no taste for human flesh!
derasa
4. Crocus or Boring Giant Clam Tridacna crocea
5. Giant Clam Tridacna gigas
6. T. tevoroa
7. T. rosewateri
The most remarkable feature of a Giant
Clam is its fluorescent and colourful mantle.
At present there is no explanation why the
8. Horse’s Hoof or Strawberry Giant Clam
Hippopus hippopus
9. China Clam Hippopus porcellanus
mantle shows so much colour variation. It is
the colourful mantle of the Giant Clams that
makes it the most sought after mollusc in
Only three species are known to occur in
India. Tridacna maxima is the most common
followed by Tridacna squamosa and Hippopus
marine aquarium trade. On close observation
one can see tiny iridophores, which are the
eyes of the Giant Clam.
hippopus. Hippopus hippopus is known to
be restricted to only Nicobar Islands,
however, not much information is available
Giant Clams are anchored to a solid
substrate, and have therefore adopted a
special mode of reproduction called
at present.
broadcast spawning, i.e. they release eggs
and sperms in open water. Giant Clams
like most molluscs are ‘simultaneous
What is unique about Giant Clams?
Species diversity
There are nine species of Giant Clams in
the world:1. Small Giant Clam Tridacna maxima
2. Scaly Giant Clam Tridacna squamosa
3. Smooth or Southern Giant Clam Tridacna
Short larval life (less than 80 hrs)
Slow growth rate
Found exclusively in Tropical Reefs
Grow in shallow waters (maximum
depth 40 m)
hermaphrodite’, i. e. each individual is a male
and female. It produces both eggs and
sperm. However, since self fertilization is
Permanently anchored on a coral
substrate
Symbiotic association with Zooxanthellae
excluded by nature, simultaneous release of
egg and sperm is not possible. Every Clam is
dependent on a mate, which can be a
Synchronized mode of spawning
Long life (over 100 years)
conspecific. Clams staying in colonies
therefore have to find ways to spawn
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together. Synchronized spawning is achieved
by a chemical transmitter called Spawning
Inducing Substance (SIS). Conspecifics
detects the presence of SIS in water as it
passes over the chemoreceptor situated
close to the incoming siphon. Both egg and
sperm carry SIS, which ensures that the
other animals are not terminating their germ
cell release too early.
To compensate the high mortality of larval
life, each clam produces more than 500 million
eggs and sperms.
Substrate requirements
Giant Clams require a specific substrate.
All Giant Clams found in Lakshadweep prefer
to anchor on two species of corals, Porites
lutea and Porites solida. In the absence of
these two species, Giant Clams will not be
able to survive. Interestingly, Giant Clams
occupy a specific niche on a coral reef. High
wave action, especially in monsoon can easily
dislodge Giant Clams from their anchor, thus
they avoid anchoring very close to breaking
Porites solida one of the most preferred
substrate of Giant Clam
waves. Giant Clams avoid proximity to the
sea bed, as they are filter feeders and sand
can choke them to death.
vulnerable to predation, there are
several natural predators of the juvenile
giant clams. Shells from group
Threats to giant clams
1.
east Asia region, Giant Clams are
considered delicacy. Their populations
have therefore crashed beyond
2.
Small Giant Clam Tridacna maxima
3.
Pyramidellidae or Muricidae (rock shells)
bore holes in juvenile Clams and suck
out the flesh killing Giant Clams.
Harvesting for food: In the entire south8.
recovery in many parts of the world.
Harvesting for medicine: Chinese pay a
fortune for the adductor muscle of Giant
enough sunlight for photosynthesis by
Zooxanthellae and water temperature
regimes between 18-25 degree Celsius
Clams, which is considered to be
aphrodisiac.
Harvesting as souvenirs: Empty shells
for optimal growth of Zooxanthellae. In
the event of temperature rise (now
mainly due to global warming, a recent
of Giant Clams are traded in large
numbers all across the world as
souvenirs. Large shells of T. gigas were
even used as bath tubs for children.
Internationally, Giant Clams are used
commercially as aquarium specimens,
4.
Scaly Giant Clam Tridacna squamosa
5.
shells and shellcraft.
Habitat loss: Fast deterioration of coral
reefs is one of the important cause of
Giant Clam decline. Polluted waters are
also responsible for its decline.
Poor densities of adult Clams: Giant Clams
spawn synchronously. If adult Clams are
dispersed far off from each other
chances of fertilization are poor.
6.
Horse’s Hoof Giant Clam Hippopus hippopus
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7.
Absence of habitat: Larvae of Giant
Clams are free floating and will drift with
water currents up to 80 hours. If suitable
habitat or substrate is not available on
the sea floor these larvae will die.
Predation: Though adult Clams are less
Bleaching: Giant Clams cultivate
Zooxanthellae in their mantle. Thus,
they need shallow waters, which allow
9.
global threat), the Zooxanthellae will die,
thus killing Giant Clams.
Accidental killing: Sometimes fishing in
shallow reefs can uproot Giant Clams,
thus causing mortality. There are
instances of Giant Clam deaths due to
10.
boat anchors.
Late maturity: Giant Clams reach
maturity very late (over 20 years and in
some species over 30 years). Juvenile
Clams are more beautiful because of
the intricate scales on the valves and
most sought after as souvenirs. Large
populations of giant clams are harvested
even before they reach maturity.
Together they are responsible for the
thinning of the adult population in many
parts of the world.
11. Pollution: Giant Clams are very sensitive
to heavy metals, hence higher
mapping Giant Clams on 21 islands. We intend
to cover the remaining 15 islands in 20062007. Giant Clams are indicator species of
reef ecosystem and their populations provide
us with insights to the health of the
ecosystem. Giant Clams are an easy and
effective monitoring tool. The present study
on Giant Clam is the first and most
comprehensive research study in the Indian
subcontinent.
Project Giant Clam offers volunteer
opportunities for young post graduates in the
field of zoology or life sciences. However, one
needs to commit atleast 5-6 months at the
field station in Lakshadweep. One of our
Bleaching of Giant Clam T. maxima
concentrations of heavy metals are fatal.
12. Diseases: Clam mortality is known to
occur due to bacterial infections,
flatworms and boring sponges.
Protection status of Giant Clams
WLPA: All three Indian species of Giant Clams
are protected under Schedule I of Wildlife
(Protection) Act, 1972. Thus, any use of a
Clam dead or alive is an offence.
volunteers Ms. Avani Patel has just completed
her stint at Lakshadweep and is working
fulltime on ‘Project Giant Clam.
At the end I must say, after countless dives
Darwin Initiative, United Kingdom. ‘Project
at several islands in Lakshadweep, every time
Giant Clam’ aims to conserve Giant Clams
I decend down the reef, I encounter
through community participation. The first
something I have not seen earlier. It is this
year of field work produced some remarkable
countless bounty of nature that instils a sense
insights to these endangered animals. The
of responsibility we owe to this natural realm.
project findings can be seen at
Whether a Giant Clam or a tiny sea slug,
www.lead.org.
their shrewd survival strategies give me some
In the first year of ‘Project Giant Clam’,
of the best lessons of natural history.
we have finished underwater surveys for
CITES: Giant Clams have been listed in CITES
Appendix II since 1985. The species listed
in Appendix II are deemed not currently
threatened with extinction, but are at risk
of becoming so unless trade is regulated.
International trade of species listed in CITES
Appendix II is permitted provided appropriate
export permits are issued. All living or dead
clams, including all readily recognizable parts
and derivatives are subject to the treaty’s
provisions for Appendix II species.
Project Giant Clam
BNHS has established a field station in
Lakshadweep to study ecology, biology and
conservation of Giant Clams. This project is
being implemented in collaboration with
LEAD international with financial support from
Healthy giant clam habitat has a rich fish population
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