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Phylum Mollusca > Class Bivalvia > Family Cardiidae
Giant clams
Subfamily Tridacninae
updated May 2020
if you learn only 3 things about them ...
They are among the largest clams to have existed on Earth!
They 'farm' algae in their bodies. The algae provides the clams food.
They are globally endangered due to overcollection as a delicacy.

Where 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 rubble.

What are giant clams? Giant clams belong to Family Cardiidae (True cockles), subfamily Tridacninae. Previously, it was placed in Family Tridacnidae.

The Fluted giant clam is usually
firmly attached to a hard surface.
Sisters Island, Jan 04

Shell is anchored by a large byssus mass
that emerges from a gap at the bottom.
Seen here in this dead clam.
Pulau Semakau (East), Aug 21
Photo shared by Vincent Choo on facebook.

The Burrowing giant clam
are often embedded inside corals.
Pulau Hantu, Mar 05
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. The shell is 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.

Burrowing giant clam.
Pulau Hantu, Feb 06

Fluted giant clam.
Pulau Jong, Nov 08

Burrowing giant clam.
Cyrene, Aug 18

Fluted giant clam.
Big Sisters Island, Jul 13

Fluted giant clam.
Terumbu Hantu, Jun 13
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 flesh.

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.

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 fertilisation.
Fluted giant clam (Tridacna squamosa) and Scallop (Family Pectinidae)
Giant myth: 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 Nature blog.

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.

Video clips of Singapore Giant clams from links shared by Neo Mei Lin on her blog.
An animal behavior film project in partial fulfilment for NUS LSM4253 Animal Behaviour AY2008/09.
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

Some Giant clams on Singapore shores

Fluted giant clam

Subfamily Tridacninae recorded for Singapore
from Tan 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.
^from WORMS

  ^Subfamily Tridacninae (previously Family Tridacnidae)
  Hippopus hippopus (EN: Endangered)

Tridacna crocea
(Burrowing giant clam) (EN: Endangered)
Tridacna maxima
(EN: Endangered)
Tridacna squamosa
(Fluted giant clam) (EN: Endangered)

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