THE STEINHART AQUARIUM - Gulf of Guinea Science ...

Commercial propagation shows promise of
success and may offset some taking of natural
populations.
Remarks: Despite seafarers’ tales of fellow
crew members being caught in this clam’s traplike
valves, it is unlikely that this could occur.
Giant Clam
Tridacna gigas (Tridacnidae)
Distribution: Found throughout the tropical
Indo-Pacfic.
Habitat: Benthic on rocky or sandy substrate
of coral reefs and lagoons to depths of 20 m.
Appearance: T. gigas is the largest and
heaviest of all living mollusks. The pale
or clear spots on the mantle, known as
“windows,” allow sunlight to enter and
stimulate the photosynthesis of symbiotic
algae (zooxanthellae). The two holes in the
mantle are the siphons, one is fringed with
tentacles for the intake of water for food and
oxygen, the other tube-like for the discharge
of large volumes of water if the clam’s shell
closes rapidly
Diet: Zooxanthellae provide most of the
clam’s nutrition as well as a ready supply of
oxygen. In turn, the clam provides protection
and access to light for the zooxanthellae. Like
other clams, also uses gills to filter small food
particles from the water and to carry out gas
exchange.
Reproduction and Development: Tridacnas are
broadcast spawners, releasing sperm and eggs
into the open water in great numbers. Fertilized
eggs develop into planktonic larvae that
settle and attach to the substrate with byssal
threads. Byssal threads become less necessary
as increasing weight holds the animal in place.
Mortality/Longevity: Very long-lived. One
resource suggests 100-year lifespan, another
200 years!
Conservation Status: Listed as vulnerable
by the IUCN. The giant clam is declining in
numbers, and is extinct in many parts of its
former range. In Southeast Asia and the Pacific
islands, the flesh is considered a delicacy. In
China, the adductor muscle is believed to be an
aphrodisiac and sells for large sums. Recently,
commercial propagation shows some promise
of success.
Remarks: Giant clams have sometimes been
California Academy of Sciences
59
called “killer clams,” with stories of humans
caught and drowned by sinister attacks on
arms and legs. Fortunately for divers, these
tall tales are untrue. The most common injury
associated with giant clams are hernias, back
strains, or broken toes, self-inflicted on people
trying to lift these huge animals from the water!
Maxima Clam aka Small Giant Clam
Tridacna maxima (Tridacnidae)
Distribution: Has widest range of giant clam
species. Indo-Pacific coastlines from east Africa
to China, Southeast Asia, Australia, and Pacific
islands.
Habitat: On reefs or sand, often embedded in
coral in shallow water.
Appearance: Aptly named, the small giant clam
is less than a third the size of T. gigas. Grows to
40 cm. Mantle is usually bright blue, green, or
brown, but may be orange or lavender. Shell
has distinctively furrowed edges.
Diet: Nutrients from photosynthesis of zooxanthellae
and from filter-feeding.
Reproduction and Development: Broadcast
spawner. Like other Tridacnas, juveniles are
males when first sexually mature, but later
become hermaphrodites. Sperm and eggs
are released at different times to avoid selffertilization.
Mortality/Longevity: Tridacna clams, especially
the mantles, are preyed upon by mantis and
cleaner shrimps, various snails, crabs, wrasses,
triggerfishes, and angelfishes.
Conservation Status: Classified as Lower Risk
– conservation dependent on IUCN Red List;
also Appendix II of CITES.
Squamosa Clam
Tridacna squamosa (Tridacnidae)
Distribution: South Africa; Red Sea to Samoa,
Tonga and Marshall Islands.
Habitat: Shallow reef flats to depths of about
20 m. Live vertically oriented with the hinge
side down.
Appearance: Width to 40 cm. The rows of large
leaflike flutes on the surface of the valves are
diagnostic. Like all Tridacna giant clams the
mantle color of individuals is highly variable
due to the presence of zooxanthellae as well as
pigments of brilliant green, blue, red, violet, or
brown in the mantle tissue.