Giant clam
Giant clam | |
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T.gigas, Michaelmas Cay , Queensland, Australia
Great Barrier Reef | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Mollusca |
Class: | Bivalvia |
Order: | Cardiida |
Family: | Cardiidae |
Genus: | Tridacna |
Species: | T. gigas
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Binomial name | |
Tridacna gigas | |
Synonyms[3] | |
Chama gigantea Perry, 1811 |
Tridacna gigas, the giant clam, is the most well-known species of the giant clam
Known to indigenous peoples in the area for thousands of years,
The giant clam lives in flat coral sand or broken coral and can be found at depths of as much as 20 m (66 ft).[5] Its range covers the Indo-Pacific, but populations are diminishing quickly, and the giant clam has become extinct in many areas where it was once common. The maxima clam has the largest geographical distribution among giant clam species; it can be found off high- or low-elevation islands, in lagoons or fringing reefs.[6] Its rapid growth rate is likely due to its ability to cultivate algae in its body tissue.[5]
Although
Anatomy
Young T. gigas are difficult to distinguish from other species of Tridacninae. Adult T. gigas are the only giant clams unable to close their shells completely. Even when closed, part of the mantle is visible, unlike the very similar T. derasa. However, this can only be recognized with increasing age and growth. Small gaps always remain between shells through which retracted brownish-yellow mantle can be seen.[7]
Tridacna gigas has four or five vertical folds in its shell; this is the main characteristic that separates it from the similar shell of T. derasa, which has six or seven vertical folds.
The mantle border itself is covered in several hundred eyespots about .5mm in diameter. Each one consists of a small cavity containing a pupil-like aperture and a base of one hundred or more photoreceptors. These receptors allow T. gigas to respond to sudden dimming of light by withdrawing their mantles and partially closing their shells, presumably to protect from potential predators. They do not retract their mantles in response to increased illumination, but it has been observed that a change in the direction of light results in a shift in mantle orientation. In addition to a dimming response, T. gigas also responds to the movement of an object before a shadow has been cast.[10] In order for this to happen, an image forming optical system is required as the response is based on the local dimming of one part of the generated image relative to the rest. This sequential dimming of receptors caused by the movement of a dark object allows enough time for the mantle to be retracted before a potential predator is directly overhead and casting a shadow.[11]
Largest specimens
The largest known T. gigas specimen measured 137 centimetres (4 ft 6 in). It was discovered around 1817 on the north western coast of Sumatra, Indonesia, and its shells are now on display in a museum in Northern Ireland. The joint weight of the two shells is 230 kilograms (510 lb), which suggests that the live weight of the animal would have been roughly 250 kilograms (550 lb).[12][13]
Another unusually large giant clam was found in 1956 off the Japanese island of Ishigaki. However, it was not examined scientifically before 1984. The shell's length was 115 centimetres (3 ft 9 in) and the weight of the shells and soft parts was 333 kilograms (734 lb). Scientists estimated the live weight to be around 340 kilograms (750 lb).[12]
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Empty giant clam shell in the French National Museum of Natural History
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Empty shell from the Aquarium Finisterrae in Spain
Ecology
Feeding
Giant clams are filter-feeders, yet 65-70 percent of their nutritional needs are covered by symbiotic unicellular algae (zooxanthellae).[14] This enables giant clams to grow as large as one meter in length even in nutrient-poor coral-reef waters.[9] The clams cultivate algae in a special circulatory system which enables them to keep a substantially higher number of symbionts per unit of volume.[15][16]
In very small clams—10 milligrams (0.010 g) dry tissue weight—filter feeding provides about 65% of total carbon needed for
Reproduction
Tridacna gigas reproduce sexually and are
Since giant clams cannot move themselves, they adopt broadcast spawning, releasing sperm and eggs into the water. A transmitter substance called spawning induced substance (SIS) helps synchronize the release of sperm and eggs to ensure fertilization. The substance is released through a
Detection of SIS stimulates the giant clam to swell its mantle in the central region and to contract its
Spawning seems to coincide with incoming tides near the second (full), third, and fourth (new) quarters of the
Development
The fertilized egg floats in the sea for about 12 hours until eventually a larva (trochophore) hatches. It then starts to produce a calcium carbonate shell. Two days after fertilization it measures 160 micrometres (0.0063 in). Soon it develops a "foot," which is used to move on the ground; it can also swim to search for appropriate habitat.[23]
At roughly one week of age, the clam settles on the ground, although it changes location frequently within the first few weeks. The larva does not yet have symbiotic algae, so it depends completely on plankton. Free floating zooxanthellae are also captured while filtering food. Eventually the front adductor muscle disappears and the rear muscle moves into the clam's center. Many small clams die at this stage. The clam is considered a juvenile when it reaches a length of 20 cm (8 in) .[24] It is difficult to observe the growth rate of T. gigas in the wild, but laboratory-reared giant clams have been observed to grow 12 cm (4.7 in) a year.[25]
The ability for Tridacna to grow to such large sizes with fleshy
Human relevance
The main reason that giant clams are becoming endangered is likely to be intensive exploitation by bivalve fishing vessels. Mainly large adults are killed, since they are the most profitable.[29]
The giant clam is considered a delicacy in Japan (known as himejako), France,
Legend
As is often the case with uncharacteristically large species, the giant clam has been historically misunderstood. Even in countries where giant clams are easily seen, stories depict giant clams as aggressive beings. For instance, a Polynesian folk tale involves a monkey's hand being bitten off, and a Maori legend involves an attack on a canoe.[31]
Starting from the 18th century, claims of danger had surfaced to the western world. In 1920s, a reputable science magazine Popular Mechanics once claimed that the great mollusc had caused deaths; versions of the U.S. Navy Diving Manual even gave detailed instructions for releasing oneself from its grasp by severing the adductor muscles used to close its shell.[31]
In an account of the discovery of the
In practice, their abductor muscle's slow speed leaves little room for unpleasant surprises.[4]
Aquaculture
Mass culture of giant clams began at the Micronesian Mariculture Demonstration Center in Palau (Belau).[33] A large Australian government-funded project from 1985 to 1992 mass-cultured giant clams, particularly T. gigas at James Cook University's Orpheus Island Research Station, and supported the development of hatcheries in the Pacific Islands and the Philippines.[34][35][36] Seven of the ten known species of giant clams in the world are found in the coral reefs of the South China Sea.[citation needed]
Conservation status
There is concern among
See also
- Platyceramus, the largest bivalve in the fossil record
References
- ^ a b "Appendices | CITES". cites.org. Archived from the original on 3 February 2007. Retrieved 14 January 2022.
- ^ Bouchet, P.; Huber, M. (2013). "Tridacna gigas (Linnaeus, 1758)". WoRMS. World Register of Marine Species. Retrieved 9 April 2014.
- ^ a b "Giant Clam: Tridacna gigas". National Geographic Society. Archived from the original on 15 April 2021. Retrieved 19 November 2023.
- ^ a b Knop, p. 10.
- ^ Munro, John L. (1993) "Giant Clams." Nearshore marine resources of the South Pacific information for fisheries development and management. Suva [Fiji]: Institute of Pacific Studies, Forum Fisheries Agency, International Centre for Ocean Development. p. 99
- ^ a b Knop, p. 32.
- ISBN 1-4398-3909-3.
- ^ ISBN 978-0-85238-234-9
- from the original on 4 June 2023. Retrieved 25 June 2022.
- PMID 12590758.
- ^ a b Knop, p. 31.
- PMID 25649000.
- ^ "Giant Clams' Poop Hosts Symbiotic Algae". 5 September 2019. Archived from the original on 4 September 2023. Retrieved 4 September 2023.
- JSTOR 1539622.[permanent dead link]
- from the original on 16 October 2008. Retrieved 24 November 2009.
- .
- ^ Knop, p. 46.
- ^ Knop, p. 47.
- ^ Knop, p. 48.
- S2CID 39673803.
- PMID 25414524.
- ^ Knop, p. 49.
- ^ Knop, p. 53.
- .
- ISSN 1064-1262.
- ISSN 0067-1975.
- ISSN 0362-2525.
- ^ Knop, p. 33.
- ^ Knop, p. 11.
- ^ a b Barnett, Cynthia (6 July 2021). "The History, Myth, and Future of the Giant Clam". Atlas Obscura. Archived from the original on 18 November 2023. Retrieved 18 November 2023.
- ^ Accounts by Wilburn Dowell Cobb Archived 1 July 2007 at the Wayback Machine. pearlforpeace.org
- .
- ^ Copland, J.W. and J.S. lucas (Eds.) 1988. Giant Clams in Asia and the Pacific. ACIAR Monograph No. 9
- ^ Braley, R.D. (1988). "Farming the Giant Clam". World Aquaculture. 20 (1): 7–17.
- ^ Fitt W.K (Ed.) 1993. Biology and Mariculture of Giant Clams; a workshop held in conjunction with the 7th International Coral Reef Symposium, 21–26 June 1992, Guam, USA
Cited sources
- Knop, Daniel. Giant Сlams: A Comprehensive Guide to the Identification and Care of Tridacnid Clams. Ettlingen: Dähne Verlag, 1996, ISBN 3-921684-23-4
Further reading
- Schwartzmann C, G Durrieu, M Sow, P Ciret, CE. Lazareth and J-C Massabuau. (2011) In situ giant clam growth rate behavior in relation to temperature: a one-year coupled study of high-frequency non-invasive valvometry and sclerochronology. Limnol. Oceanogr. 56(5): 1940–1951 (Open access)
- Yonge, C.M. 1936. Mode of life, feeding, digestion and symbiosis with zooxanthellae in the Tridacnidae, Sci. Rep. Gr. Barrier Reef Exped. Br. Mus., 1, 283–321
External links
- ARKive – images and movies of the giant clam (Tridacna gigas)
- Tridacna gigas entry on Animal Diversity Web
- Giant clam conservation research project at Universiti Sains Malaysia
- Giant Clams of the Great Barrier Reef
- Microdocs Archived 27 July 2011 at the Wayback Machine: The solar powered clam Archived 30 July 2011 at the Wayback Machine & Growing a giant clam Archived 9 November 2013 at the Wayback Machine
- MolluSCAN eye project Archived 13 November 2016 at the Wayback Machine, a website dedicated to the in situ study of bivalve mollusks around the world
- Photos of Giant clam on Sealife Collection