Photo by Larry Basch.
It's always a pleasure
to see one of these beauties on the reef. Giant clams (faisua) are large,
colorful and, surprisingly, they are part animal and part “plant”. That's
because giant clams, like corals, have plant-like cells (zooxanthellae) in their
tissues that produce free food for the clams. When a clam opens its shell and
spreads out its pretty mantle, it exposes these solar panels of zooxanthellae
to the sun to make food, like a plant unfolding its leaves.
We
have two native species of giant clams in our local waters, Tridacna maxima
and T. squamosa, which look fairly similar. They grow to about 12-15 inches
in shell length, although most found today are smaller because the larger ones
have been over-harvested. The largest and most famous species of giant clam (T.
gigas), which grows as big as a large suitcase, is not native here but has
been recently imported.
Over
the past 15 years, mariculture efforts by DMWR have introduced T. gigas
and T. derasa, and re-introduced Hippopus hippopus, a shallow-water
species that was formerly present.
Because giant clams need sunlight,
they inhabit shallow, clear waters down to about 60 feet deep. They grow very
slowly; one local clam was 18 years old. They spawn repeatedly over their life
span and release millions of eggs each time they spawn, but most young clams do
not survive, so the adults have to live a long time and spawn many times to insure
that the population survives. Larval clams swim in the water for about a week,
then settle permanently onto the reef to grow.
The clams are a favorite
food item throughout the South Pacific and their accessibility in shallow waters
and slow growth make them susceptible to overfishing. That is very much the case
in American Samoa, where few remain on many of our reefs near populated areas.
There is a growing concern that our population of giant clams may be getting too
few and far between to spawn successfully.
Partly for that reason, there
has been an interest in growing these clams in hatcheries to supply markets for
food and the aquarium trade. DMWR has operated a hatchery here for many years
and tried to encourage local production by supplying small clams for local 'farmers'
to grow them on their reefs. That effort has met with limited success for several
reasons. Considerable dedication is needed because it may take several years to
grow the clams to a commercial size, and the clams have to be protected from poachers
and predators. And there are always fa'alavelave events that call for contributions
of giant clams if anyone has some. In general, giant clam mariculture here has
usually supplemented family needs rather than create a commercial business. Periodically,
the idea of raising the clams to a small size for the aquarium trade is talked
about as a possible market.
Harvest regulations in American Samoa (in
2005) are: giant clams taken for personal consumption must be at least 6 inches
in shell length, or if sold, a license is required and giant clams must be at
least 7 inches in shell length and sold with the clam still in its shell.
10. Giant clams (faisua)
At the same time,
the clam also gets some food by drawing water through its siphon and filtering
out any tiny food particles (zooplankton). Perhaps that's why giant clams grow
so large – they have two very different ways to get food.
