Gigas is derived from Greek word γίγας (gígantes), meaning “giants”.

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[Clam Queen] Meeting a wild Tridacna gigas is rare treat, as they are widely locally extinct through most of their geographic range.

Tridacna gigas, also affectionately known as the “True Giant Clam”, is the only truly gigantic species in the subfamily Tridacninae! The species has an intermediate geographic range, which spans from Myanmar (Burma) to the Republic of Kiribati (but not Cook Islands), and the Ryukyus (southern Japan) to Queensland (Australia).

I was very privileged (and super excited) to get an opportunity to meet a wild individual on a leisure dive trip around Malaysia recently. This individual is found within a marine protected area in Sabah. Meeting a truly wild Tridacna gigas is a rare treat, as natural stocks are widely locally extinct throughout its geographic range. 😦

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Description

The largest recorded individual was 137 cm long, while the heaviest known clam (106 cm long) weighed approximately 500 kg! Tridacna gigas usually lives in coral reefs with good light penetration, and is usually free-living on either sand or hard reef substrata.

The species is easily identified by its size and distinctive elongate and triangular projections on the upper shell margins. Mantle colours are mostly dull brown and olive green, and the mantle edge bears numerous iridescent blue-green circles. Unlike the other Tridacna species, the incurrent siphon of T. gigas bears no tentacles.

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Current Status

Currently, there were at least 31 localities with natural wild populations of T. gigas, but at 26 of them this species is severely depleted, locally extinct or data deficient. The Great Barrier Reef in Australia is the most extensive area within the natural distribution of T. gigas that still supports relatively undisturbed populations, with evidence of natural recruitment.

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Tridacna gigas shells harvested for the ornamental trade.

Populations typically face high levels of exploitation pressure and habitat deterioration. Tridacna gigas remains a valuable coastal resource for both domestic and commercial markets, as it is highly favoured for its meat as food and large shells for the ornament trade.

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[Fangirl moment with the True Giant Clam]

Conservation Efforts

On the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, T. gigas is listed as ‘Vulnerable’ due to the rate of decline of remaining wild stocks. This status, however, requires updating as previous assessment in 1996 may be outdated.

To assist its conservation, T. gigas has been extensively cultivated and reintroduced (albeit in some areas, sometimes limited to a couple of individuals) to Peninsular Malaysia, Sabah, Philippines, Fiji, Northern Mariana Islands, Vanuatu and Tonga, as well as introduced to American Samoa, the Cook Islands, Hawaii (USA) and Samoa.

The oldest known maricultured T. gigas individual was 34 years old and was produced at Palau’s Micronesian Mariculture Demonstration Center in 1982. It used to be on display at the Waikiki Aquarium in Honolulu. Such efforts require long-term commitment as the giant clam is long-lived with late reproductive maturity (at least 10 years).

Unfortunately, there is also little information available regarding the outcomes of restocking in these areas (with a notable exception of the Philippines). To the best of my knowledge, there has only been one report where the restocked populations of T. gigas in the Philippines contributed to natural recruitment of juvenile clams. And this process took almost 25 years long!

Coupled with restocking efforts, there has also been parallel efforts to raise public awareness about the species by the Bolinao Marine Laboratory, UP-MSI. They also held an ‘Clam Adoption Drive’ during World Oceans Day!

What can I do?

References:

  • Cabaitan PC & C Conaco (2017) Bringing back the giants: juvenile Tridacna gigas from natural spawning of restocked giant clams. Coral Reefs, 36(2); 519.
  • Neo ML, CCC Wabnitz, RD Braley, GA Heslinga, C Fauvelot, S Van Wynsberge, S Andréfouët, C Waters, AS-H Tan, ED Gomez, MJ Costello & PA Todd (2017) Chapter 4. Giant clams (Bivalvia: Cardiidae: Tridacninae): A comprehensive update of species and their distribution, current threats and conservation status. Oceanography and Marine Biology: An Annual Review, Volume 55. Pp. 87–388.
  • TED Talk on “The fascinating secret lives of giant clams
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