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Giant clam

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Giant clam
T. gigas, Michaelmas Cay
Great Barrier Reef, Queensland, Australia
CITES Appendix II (CITES)[2]
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Cardiida
Family: Cardiidae
Genus: Tridacna
Species:
T. gigas
Binomial name
Tridacna gigas
Synonyms[3]

Chama gigantea Perry, 1811

Mantle of giant clam with light-sensitive spots, which detect danger and cause the clam to close

Tridacna gigas, the giant clam, is the best-known species of the giant clam genus Tridacna. Giant clams are the largest living bivalve mollusks. Several other species of "giant clam" in the genus Tridacna are often misidentified as Tridacna gigas.

These clams were known to indigenous peoples of East Asia for thousands of years and the Venetian scholar and explorer Antonio Pigafetta documented them in a journal as early as 1521. One of a number of large clam species native to the shallow coral reefs of the South Pacific and Indian oceans, they may weigh more than 200 kilograms (440 lb), measure as much as 120 cm (47 in) across, and have an average lifespan in the wild of more than 100 years.[4] They also are found off the shores of the Philippines and in the South China Sea in the coral reefs of Malaysia.[5]

The giant clam lives in flat coral sand or broken coral and may be found at depths of as great as 20 m (66 ft).[6]: 10  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.[5] The maxima clam has the largest geographical distribution among giant clam species; it may be found off high- or low-elevation islands, in lagoons or fringing reefs.[7] Its rapid growth rate is likely due to its ability to cultivate algae in its body tissue.[6]: 10 

Although larval clams are planktonic, they become sessile in adulthood.[8] The creature's mantle tissues act as a habitat for the symbiotic single-celled dinoflagellate algae (zooxanthellae) from which the adult clams get most of their nutrition. By day, the clam opens its shell and extends its mantle tissue so that the algae receive the sunlight they need to photosynthesise. This method of algal farming is under study as a model for highly efficient bioreactors.

Anatomy

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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, allowing part of the brownish-yellow mantle to remain visible.[6]: 32  Tridacna gigas has four or five vertical folds in its shell, which serves as the main characteristic differentiating it from the similar T. derasa that has six or seven vertical folds.[9] Similar to coral matrices composed of calcium carbonate, giant clams grow their shells through the process of biomineralization, which is very sensitive to seasonal temperature.[10][11] The isotopic ratio of oxygen in carbonate and the ratio between Strontium and Calcium together may be used to determine historical sea surface temperature.[10]

The mantle border itself is covered in several hundred to several thousand pinhole eyespots approximately 0.5 mm (0.020 in) in diameter.[12][13] Each one consists of a small cavity containing a pupil-like aperture and a base of 100 or more photoreceptors sensitive to three different ranges of light, including UV, which may be unique among molluscs.[13] These receptors allow T. gigas to partially close their shells in response to dimming of light, change in the direction of light, or the movement of an object.[14] The optical system forms an image by sequential, local dimming of some eyes using pigment from the aperture.[12]

Largest specimens

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The largest known T. gigas specimen measured 137 centimetres (4 ft 6 in), and it weighed 230 kg (510 lb) dead and was estimated to be 250 kg (550 lb) alive. 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.[6]: 31 [15]

A heavier giant clam was found in 1956 off the Japanese island of Ishigaki. The shell's length was 115 centimetres (3 ft 9 in), and it weighed 333 kilograms (734 lb) dead and estimated 340 kilograms (750 lb) alive.[6]: 32 

Ecology

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Feeding

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Giant clams are filter-feeders, yet 65-70 percent of their nutritional needs are supplied by zooxanthellae.[16] This enables giant clams to grow as large as one meter in length even in nutrient-poor coral-reef waters.[17][18] The clams cultivate algae in a special circulatory system that enables them to keep a substantially higher number of symbionts per unit of volume.[19][20] The mantle's edges are packed with symbiotic zooxanthellae, which presumably use carbon dioxide, phosphates, and nitrates supplied by the clam.[17]

In very small clams—10 milligrams (0.010 g) dry tissue weight—filter feeding provides approximately 65% of total carbon needed for respiration and growth; comparatively larger clams (10 grams (0.35 oz)) acquire only 34% of carbon from this source.[21] A single species of zooxenthellae may be symbionts of both giant clams and nearby reef–building (hermatypic) corals.[17]

Reproduction

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Tridacna gigas reproduce sexually and are hermaphrodites (producing both eggs and sperm by one clam). While self-fertilization is not possible, having both characteristics does allow them to reproduce with any other member of the species as well as hermaphrodically. As with all other forms of sexual reproduction, hermaphroditism ensures that new gene combinations be passed to further generations.[6]: 46  This flexibility in reproduction reduces the burden of finding a compatible mate, while simultaneously doubling the number of offspring produced.

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 syphonal outlet. Other clams can detect SIS immediately. Incoming water passes chemoreceptors situated close to the incurrent syphon that transmit the information directly to the cerebral ganglia, a simple form of brain.[6]: 47 

Detection of SIS stimulates the giant clam to swell its mantle in the central region and to contract its adductor muscle. Each clam then fills its water chambers and closes the incurrent syphon. The shell contracts vigorously with the adductor's help, so the excurrent chamber's contents flows through the excurrent syphon. After a few contractions containing only water, eggs and sperm appear in the excurrent chamber and then pass through the excurrent syphon into the water. Female eggs have a diameter of 100 micrometres (0.0039 in). Egg release initiates the reproductive process. An adult T. gigas can release more than 500 million eggs at a time.[6]: 48 

Spawning seems to coincide with incoming tides near the second (full), third, and fourth (new) quarters of the moon phase. Spawning contractions occur every two or three minutes, with intense spawning ranging from thirty minutes to two and a half hours. Clams that do not respond to the spawning of neighboring clams may be reproductively inactive.[22]

Development

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Behaviours associated with
different stages of the giant clam life cycle [23]

The fertilized egg floats in the sea for approximately 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. Larvae also can swim to search for appropriate habitat.[6]: 49 

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. Also, free-floating zooxanthellae are 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).[6]: 53  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.[24]

The ability for Tridacna to grow to such large sizes with fleshy mantles that extend beyond the edges of their shells is considered to be the result of total reorganization of bivalve development and morphology.[8] Historically, two evolutionary explanations have been suggested for this process. Sir Yonge suggested and maintained for many years that the visceral-pedal ganglia complex rotate 180 degrees relative to the shell, requiring that they develop and evolve independently.[25] Stasek proposed instead that the growth occurs primarily in a posterior direction instead of the more typical direction of ventral in most bivalves, which is reflected in the transitional stages of alternative ways of growing that juveniles undergo.[26]

Human relevance

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One of the two clam stoups of the Église Saint-Sulpice in Paris, carved by Jean-Baptiste Pigalle
Piece of giant clam shell that was used in ancient Egypt as a paint holder

The main reason that giant clams are becoming endangered is likely to be intensive exploitation by bivalve fishers. Mainly large adults are killed because they are the most profitable.[6]: 33 

A giant clam from East Timor of more than one meter in length

The giant clam is considered a delicacy in Japan (known as himejako), France, Southeast Asia, and many Pacific Islands. Some Asian foods include the meat from the muscles of clams. Large amounts of money are paid for the adductor muscle, which Chinese people believe to have aphrodisiac powers.[6]: 11 

On the black market, giant clam shells are sold as decorative accoutrements.

Legend

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As is often the case historically with uncharacteristically large species, the giant clam has been misunderstood.[27]

Even in countries where giant clams are easily seen, stories incorrectly depict giant clams as aggressive beings. For instance, although the clams are unable to close their shells completely, a Polynesian folk tale relates that a monkey's hand was bitten off by one, and even though once past larval stage, the clams are sessile, a Maori legend relates a supposed attack on a canoe by a giant clam.[28] Starting from the eighteenth century, claims of danger had been related to the western world. In the 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.[28] In an account of the discovery of the Pearl of Lao Tzu, Wilburn Cobb said he was told that a Dyak diver was drowned when the Tridacna closed its shell on his arm.[29] In reality, the slow speed of their abductor muscle contraction and the need to force water out of their shells while closing, prevents them from trapping a human.[4][27]

Other myths focus on the huge size of giant clams being associated with long age.[27] While giant clams do live a long time and may serve as a bio-metric for historic climatic conditions, their large size is more likely associated with rapid growth.

Aquaculture

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Mass culture of giant clams began at the Micronesian Mariculture Demonstration Center in Palau (Belau).[30] 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.[31][32][33] Seven of the ten known species of giant clams in the world are found in the coral reefs of the South China Sea.[5]

Conservation status

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Green and blue giant clam from East Timor

There is concern among conservationists about whether those who use the species as a source of livelihood are overexploiting it. The numbers in the wild have been greatly reduced by extensive harvesting for food and the aquarium trade.[8] The species is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES) meaning international trade (including in parts and derivatives) is regulated.[2]

T. gigas has been reported as locally extinct in peninsular Malaysia, while T. derasa and Hippopus porcellanus are restricted to Eastern Malaysia.[5] These recent local extinctions have motivated the introduction of giant clams to Hawaii and Micronesia following maricultural advancements.[34] Restocked individuals in the Philippines have successfully dispersed their own spawned larvae to at least several hundred meters away after only ten years.[35]

See also

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References

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  1. ^ Wells, S. (1996). Tridacna gigas. The IUCN Red List of Threatened Species doi:10.2305/IUCN.UK.1996.RLTS.T22137A9362283.en
  2. ^ a b "Appendices | CITES". cites.org. Archived from the original on 3 February 2007. Retrieved 14 January 2022.
  3. ^ Bouchet, P.; Huber, M. (2013). "Tridacna gigas (Linnaeus, 1758)". WoRMS. World Register of Marine Species. Retrieved 9 April 2014.
  4. ^ a b "Giant Clam: Tridacna gigas". National Geographic Society. Archived from the original on 15 April 2021. Retrieved 19 November 2023.
  5. ^ a b c d Syukri bin Othman, Ahmad; Goh, Gideon H. S.; Todd, Peter A. (28 February 2010). "THE DISTRIBUTION AND STATUS OF GIANT CLAMS (FAMILY TRIDACNIDAE) – A SHORT REVIEW". The Raffles Bulletin of Zoology. 58 (1): 103–111.
  6. ^ a b c d e f g h i j k l Knop, Daniel (1996). Giant clams: a comprehensive guide to the identification and care of Tridacnid clams. Ettlingen: Dähne Verlag. ISBN 978-3-921684-23-8. OCLC 35717617.
  7. ^ 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
  8. ^ a b c Lucas, John S. (January 1994). "The biology, exploitation, and mariculture of giant clams (Tridacnidae)". Reviews in Fisheries Science. 2 (3): 181–223. doi:10.1080/10641269409388557. ISSN 1064-1262.
  9. ^ Rosewater, Joseph (1965). "The family Tridacnidae in the Indo-Pacific". Indo-Pacific Mollusca. 1: 347.
  10. ^ a b Yan, Hong; Shao, Da; Wang, Yuhong; Sun, Liguang (July 2013). "Sr/Ca profile of long-lived Tridacna gigas bivalves from South China Sea: A new high-resolution SST proxy". Geochimica et Cosmochimica Acta. 112: 52–65. doi:10.1016/j.gca.2013.03.007. ISSN 0016-7037.
  11. ^ Gannon, M. E.; Pérez-Huerta, A.; Aharon, P.; Street, S. C. (6 January 2017). "A biomineralization study of the Indo-Pacific giant clam Tridacna gigas". Coral Reefs. 36 (2): 503–517. doi:10.1007/s00338-016-1538-5. ISSN 0722-4028.
  12. ^ a b Land M.F. (2002). "The spatial resolution of the pinhole eyes of giant clams". Proc. R. Soc. Lond. B. 270 (1511): 185–188. doi:10.1098/rspb.2002.2222. PMC 1691229. PMID 12590758.
  13. ^ a b Wilkens, Lon A. (May 1984). "Ultraviolet sensitivity in hyperpolarizing photoreceptors of the giant clam Tridacna". Nature. 309 (5967): 446–448. doi:10.1038/309446a0. ISSN 0028-0836.
  14. ^ Wilkens, L. A. (1986). "The visual system of the giant clam Tridacna: behavioral adaptations". Biological Bulletin. 170 (3): 393–408. doi:10.2307/1541850. JSTOR 1541850. Archived from the original on 4 June 2023. Retrieved 25 June 2022.
  15. ^ McClain, Craig R.; Balk, Meghan A.; Benfield, Mark C.; Branch, Trevor A.; Chen, Catherine; Cosgrove, James; Dove, Alistair D.M.; Gaskins, Lindsay C.; Helm, Rebecca R. (13 January 2015). "Sizing ocean giants: patterns of intraspecific size variation in marine megafauna". PeerJ. 3: e715. doi:10.7717/peerj.715. ISSN 2167-8359. PMC 4304853. PMID 25649000.
  16. ^ "Giant Clams' Poop Hosts Symbiotic Algae". 5 September 2019. Archived from the original on 4 September 2023. Retrieved 4 September 2023.
  17. ^ a b c Gosling, Elizabeth (2003). Bivalve Molluscs Biology, Ecology and Culture. Grand Rapids: Blackwell Limited. p. 23. ISBN 978-0-85238-234-9
  18. ^ Dame, Richard F. (1996) Ecology of marine bivalves an ecosystem approach Archived 4 June 2023 at the Wayback Machine. Boca Raton: CRC. p. 51. ISBN 1-4398-3909-3.
  19. ^ Jeffrey, S. W.; F. T. Haxo (1968). "Photosynthetic Pigments of Symbiotic Dinoflagellates (Zooxanthellae) from Corals and Clams". Biological Bulletin. 135 (1): 149–65. doi:10.2307/1539622. JSTOR 1539622.[permanent dead link]
  20. ^ Norton, J. H.; M. A. Shepherd; H. M. Long & W. K. Fitt (1992). "The Zooxanthellal Tubular System in the Giant Clam". The Biological Bulletin. 183 (3): 503–506. doi:10.2307/1542028. JSTOR 1542028. PMID 29300506. Archived from the original on 16 October 2008. Retrieved 24 November 2009.
  21. ^ Klumpp, D.W.; Bayne, B.L. & Hawkins, A.J.S. (1992). "Nutrition of the giant clam, Tridacna gigas (L). 1. Contribution of filter feeding and photosynthesis to respiration and growth". Journal of Experimental Marine Biology and Ecology. 155: 105. doi:10.1016/0022-0981(92)90030-E.
  22. ^ Braley, Richard D. (1984). "Reproduction in the giant clams Tridacna gigas and T. Derasa in situ on the north-central Great Barrier Reef, Australia, and Papua New Guinea". Coral Reefs. 3 (4): 221–227. Bibcode:1984CorRe...3..221B. doi:10.1007/BF00288258. S2CID 39673803.
  23. ^ Soo, Pamela; Todd, Peter A. (2014). "The behaviour of giant clams (Bivalvia: Cardiidae: Tridacninae)". Marine Biology. 161 (12): 2699–2717. doi:10.1007/s00227-014-2545-0. PMC 4231208. PMID 25414524.
  24. ^ Beckvar, N. (1981). "Cultivation, spawning, and growth of the giant clams Tridacna gigas, T. Derasa, and T. Squamosa in Palau, Caroline Islands". Aquaculture. 24: 21–30. doi:10.1016/0044-8486(81)90040-5.
  25. ^ Yonge, C. M. (31 October 1981). "Functional morphology and evolution in the Tridacnidae (Mollusca: Bivalvia: Cardiacea)". Records of the Australian Museum. 33 (17): 735–777. doi:10.3853/j.0067-1975.33.1981.196. ISSN 0067-1975.
  26. ^ Stasek, Charles R. (May 1963). "Orientation and form in the bivalved Mollusca". Journal of Morphology. 112 (3): 195–214. doi:10.1002/jmor.1051120302. ISSN 0362-2525.
  27. ^ a b c Lucas, John S. "Quick Guide: Giant Clams". Current Biology. 24 (5): 183–184.
  28. ^ 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.
  29. ^ Accounts by Wilburn Dowell Cobb Archived 1 July 2007 at the Wayback Machine. pearlforpeace.org
  30. ^ Heslinga, Gerald A.; Perron, Frank E.; Orak, Obichang (1984). "Mass culture of giant clams (F. Tridacnidae) in Palau". Aquaculture. 39 (1–4): 197–215. doi:10.1016/0044-8486(84)90266-7.
  31. ^ Copland, J. W. and J. S. Lucas (Eds.) 1988. Giant Clams in Asia and the Pacific. ACIAR Monograph No. 9
  32. ^ Braley, R.D. (1988). "Farming the Giant Clam". World Aquaculture. 20 (1): 7–17.
  33. ^ Fitt W.K (Ed.) 1993. Biology and Mariculture of Giant Clams; a workshop held in conjunction with the Seventh International Coral Reef Symposium, 21–26 June 1992, Guam, USA
  34. ^ "Tridacna gigas: Wells, S." IUCN Red List of Threatened Species. 1 August 1996. Retrieved 6 April 2024.
  35. ^ Cabaitan, Patrick C.; Conaco, Cecilia (16 February 2017). "Bringing back the giants: juvenile Tridacna gigas from natural spawning of restocked giant clams". Coral Reefs. 36 (2): 519–519. doi:10.1007/s00338-017-1558-9. ISSN 0722-4028.

Further reading

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  • 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
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