They are among the largest clams to have existed on Earth!
They 'farm' algae in their bodies. The algae provides
the clams food.
are globally endangered due to overcollection as a delicacy.
seen? These enormous clams are sometimes seen on our undisturbed
Southern shores. Some burrow into coral rubble or among live coral
and are thus easily overlooked. Others lie above but attached to coral
What are giant clams? Giant clams
belong to Family Cardiidae
(True cockles), subfamily Tridacninae. Previously, it was placed in
Features: 15-40cm. Giant clams
are among the largest bivalves to have ever existed on our planet!
The two-part shell is thick and usually has a wavy opening that never
closes completely. The shell opening faces the sunlight, while the
hinged side is at the bottom, attached to a hard surface by a large
byssus mass that emerges from a gap between the valves near the hinge.
Some giant clams burrow into coral, with most of the shell hidden
and only the shell opening facing sunlight.
What do they eat? Unlike most
other bivalves, the giant clam harbours symbiotic zooxanthellae (a
kind of single-celled algae) in its fleshy body. The zooxanthellae
produce food through photosynthesis which it shares with the clam.
To maximise the productivity of its "farm", the clam faces
the shell opening (and thus the body containing the algae) to sunlight.
The shell opening never closes completely even at low tide, and the
body is exposed. The body expands under water and appears like colourful
thick lips in between the wavy shell opening. The brightly coloured
spots in the body protect against excessive sun. The clam has transparent
lenses that focus sunlight onto the algae that are found deeper in
The giant clam has an extensive digestive system to extract the nutrients
produced by the symbiotic algae. And enlarged excretory organs to
deal with the large load of by-products of the algae. Although giant
clams are highly dependent on the symbiotic algae, they are still
able to filter feed like other bivalves.
It is a mistaken belief that divers can be trapped underwater if the
giant clam closes over their foot or hand. Many of these peaceful
clams can't even close their shells completely. They certainly don't
eat people! More about this on the Psychedelic
Giant clam babies: Giant clams
mature first as males then eventually become hermaphrodites, producing
both eggs and sperm. Sperm is released first, probably to avoid self
Human uses: Giant clams are considered
a delicacy and in some places, an aphrodisiac. The large shells of
these magnificent creatures are often turned into tacky souvenirs
like ash-trays. There are efforts to cultivate giant clams on a commercial
basis so as to reduce over-collection of wild clams.
Status and threats: Giant clams
have been listed in CITES Appendix II since 1985. The Fluted
giant clam (Tridacna squamosa) is listed as 'Endangered'
on the Red List of threatened animals of Singapore. According to the
Singapore Red Data Book: "Large specimens have virtually disappeared
from our shores. Young specimens are occasionally but infrequently
seen". Like other creatures of the intertidal zone, they are
affected by human activities such as reclamation and pollution. Trampling
by careless visitors and over-collection can also affect local populations
of young clams.
giant clam is usually
firmly attached to a hard surface.
Sisters Island, Jan 04
When submerged, the fleshy body
expands like thick lips!
Pulau Hantu, Feb 06
Pulau Semakau, Mar 05
of Singapore Giant clams from links shared by Neo Mei Lin on her
An animal behavior
film project in partial fulfilment for NUS LSM4253 Animal Behaviour
Done by: Neo Meilin Pamela Soo Daniel Storisteanu Nicholas Yap
The Secrets Of The Giant Clam part 1: Introduction and Larval Movement
The secrets of
the Giant clam part 2: Righting
The secrets of
the Giant clam part 3: Aggregation
The Secrets of
the Giant Clam part 4: Squirting and conservation
Tridacninae recorded for Singapore
Siong Kiat and Henrietta P. M. Woo, 2010 Preliminary Checklist
of The Molluscs of Singapore.
in red are those listed among the threatened
animals of Singapore from Davison, G.W. H. and P. K. L. Ng
and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened
plants and animals of Singapore.
Tridacninae (previously Family Tridacnidae)
the Bivalves section by J.M. Poutiers in the FAO Species Identification
Guide for Fishery Purposes: The Living Marine Resources of the
Western Central Pacific Volume
1: Seaweeds, corals, bivalves and gastropods on the Food and
Agriculture Organization of the United Nations (FAO) website.