The giant clam is the world's largest bivalve. This smaller species of giant clam have exquisite iridescent blue and green mottling on the mantle. Bivalves are mollusks with two convex halves, or shells, that hinge along one edge with the animal in between.
The shells are shut together and held closed by two large adductor muscles, this seals the animal shell, protecting it from predatory attack.
Photo: Brian Hall
November 1, 2000
The Giant Clam
This is Genevieve Johnson talking to you from the Odyssey. While
provisioning in Christmas Island in preparation for our next leg in search
of sperm whales, we have had several opportunities to snorkel and dive on
the coral reefs.
There is one creature that we continue to see over and over again, it is a
huge invertebrate of mythical stature, considered by many to be the
embodiment of the tropical pacific, the giant clam.
The giant clam is the world's largest bivalve. This smaller species of giant
clam is one of several different giant clam species found in tropical
waters, and has exquisite iridescent blue and green mottling on the mantle.
Bivalves are mollusks with two convex halves or shells that hinge along one
edge with the animal in between. The shells are shut together and held
closed by two large adductor muscles, this seals the animal shell, protecting its
vital organs from predatory attack.
The clam is a filter feeder, as seawater passes over the gills for gas
exchange, food particles are also captured and passed into the mouth.
The species of Giant clam we have consistently encountered here in Kiribati
are slightly smaller than the enormous 2-meter long specimens, the creatures
that have captured our imagination from early Polynesian times to present
day Hollywood, where stories are told of a divers hand or foot being trapped
in a closing shell. Of course this rarely happens since the clam has no
interest in trapping a large foreign body inside its shell and will not
fully shut when a strange object is inside.
As we dove down to take a closer look, the clams would immediately withdraw
into their ridge shaped shells, the corrugated structure increasing the
strength of the two halves upon closure.
The large shell is built up over time as the animal secretes proteins and
inorganic substances such as calcium, which it then adds like building
blocks to the thick, rough outer layer of the shell, its size and form being
a function of the rate of shell deposition.
Sometimes foreign bodies such as grains of sand can become lodged inside the
clam between the shell and the mantle, if this happens secretions can build
up around the grain, perhaps to reduce irritation, a round smooth structure
is formed that may eventually result in a pearl.
Giant clams were once a regular component of most tropical pacific reefs,
but have been exploited in many areas. The thick muscle inside the shell is
highly prized on the Asian market. Illegal poaching of this species
throughout its range in the pacific has led to its extinction in many areas,
and where it is still present, it is extremely rare. The largest species of
giant clam is now listed under the Convention on International Trade in
Endangered Species, (CITES) as a threatened species. As we continue to
explore the world's oceans we hope to encounter more of these animals, maybe
even the rare giant clam of enormous proportions.
Log by Genevieve Johnson
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