giant clam

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.{{Rp|page=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.{{Cite journal |last=Rosewater |first=Joseph |date=1965 |title=The family Tridacnidae in the Indo-Pacific. |journal=Indo-Pacific Mollusca |volume=1 |pages=347}} 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.{{Cite journal |last=Yan |first=Hong |last2=Shao |first2=Da |last3=Wang |first3=Yuhong |last4=Sun |first4=Liguang |date=July 2013 |title=Sr/Ca profile of long-lived Tridacna gigas bivalves from South China Sea: A new high-resolution SST proxy |url=http://dx.doi.org/10.1016/j.gca.2013.03.007 |journal=Geochimica et Cosmochimica Acta |volume=112 |pages=52–65 |doi=10.1016/j.gca.2013.03.007 |issn=0016-7037}}{{Cite journal |last=Gannon |first=M. E. |last2=Pérez-Huerta |first2=A. |last3=Aharon |first3=P. |last4=Street |first4=S. C. |date=2017-01-06 |title=A biomineralization study of the Indo-Pacific giant clam Tridacna gigas |url=http://dx.doi.org/10.1007/s00338-016-1538-5 |journal=Coral Reefs |volume=36 |issue=2 |pages=503–517 |doi=10.1007/s00338-016-1538-5 |issn=0722-4028}} The isotopic ratio of oxygen in carbonate and the ratio between Strontium and Calcium together may be used to determine historical sea surface temperature.

The mantle border itself is covered in several hundred to several thousand pinhole eyespots approximately {{convert|0.5|mm|abbr=on}} in diameter.{{Cite journal |last=Wilkens |first=Lon A. |date=May 1984 |title=Ultraviolet sensitivity in hyperpolarizing photoreceptors of the giant clam Tridacna |url=http://dx.doi.org/10.1038/309446a0 |journal=Nature |volume=309 |issue=5967 |pages=446–448 |doi=10.1038/309446a0 |issn=0028-0836}} 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. 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.{{cite journal|author1=Wilkens, L. A.|title=The visual system of the giant clam Tridacna: behavioral adaptations.|journal=Biological Bulletin|volume=170|issue=3|year=1986|pages=393–408|doi=10.2307/1541850|jstor=1541850|url=https://www.biodiversitylibrary.org/part/13022|access-date=25 June 2022|archive-date=4 June 2023|archive-url=https://web.archive.org/web/20230604041114/https://www.biodiversitylibrary.org/part/13022|url-status=live}} The optical system forms an image by sequential, local dimming of some eyes using pigment from the aperture.{{cite journal|author=Land M.F.|name-list-style=amp |title=The spatial resolution of the pinhole eyes of giant clams|year=2002|journal= Proc. R. Soc. Lond. B|volume=270|issue=1511 |pages=185–188|doi=10.1098/rspb.2002.2222 |pmid=12590758 |pmc=1691229}}

The largest known T. gigas specimen measured {{convert|137|cm|ftin}}, 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.{{Rp|page=31}}{{Cite journal|last1=McClain|first1=Craig R.|last2=Balk|first2=Meghan A.|last3=Benfield|first3=Mark C.|last4=Branch|first4=Trevor A.|last5=Chen|first5=Catherine|last6=Cosgrove|first6=James|last7=Dove|first7=Alistair D.M.|last8=Gaskins|first8=Lindsay C.|last9=Helm|first9=Rebecca R.|date=2015-01-13|title=Sizing ocean giants: patterns of intraspecific size variation in marine megafauna|journal=PeerJ|language=en|volume=3|pages=e715|doi=10.7717/peerj.715|issn=2167-8359|pmc=4304853|pmid=25649000 |doi-access=free}}

A heavier giant clam was found in 1956 off the Japanese island of Ishigaki. The shell's length was {{convert|115|cm|ftin}}, and it weighed {{convert|333|kg|lb}} dead and estimated {{convert|340|kg|lb}} alive.{{Rp|page=32}}

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File:Giant Clam (Tridacna gigas) (6058446919).jpg|Giant clam in Bunaken Island, Sulawesi, Indonesia

File:Tridacna gigas 01 by Line1.JPG|Empty giant clam shell in the French National Museum of Natural History

File:Tridacna gigas.001 - Aquarium Finisterrae.JPG|Empty shell from the Aquarium Finisterrae in Spain

File:Living giant clam (Tridacna gigas), Waikiki Aquarium.JPG|Giant clam in Waikiki Aquarium, Honolulu, Hawaii, United States

Giant clams are filter-feeders, yet 65-70 percent of their nutritional needs are supplied by zooxanthellae.{{Cite web |url=https://www.asianscientist.com/2019/09/in-the-lab/giant-clam-poop-algae/ |title=Giant Clams' Poop Hosts Symbiotic Algae |date=5 September 2019 |access-date=4 September 2023 |archive-date=4 September 2023 |archive-url=https://web.archive.org/web/20230904235813/https://www.asianscientist.com/2019/09/in-the-lab/giant-clam-poop-algae/ |url-status=live}} This enables giant clams to grow as large as one meter in length even in nutrient-poor coral-reef waters. The clams cultivate algae in a special circulatory system that enables them to keep a substantially higher number of symbionts per unit of volume. The mantle's edges are packed with symbiotic zooxanthellae, which presumably use carbon dioxide, phosphates, and nitrates supplied by the clam.

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

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.{{Rp|page=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.{{Rp|page=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 {{convert|100|micrometre|in}}. Egg release initiates the reproductive process. An adult T. gigas can release more than 500 million eggs at a time.{{Rp|page=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.

File:Behaviours associated with different stages of the giant clam’ life cycle.webp

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 {{convert|160|micrometre|in}}. Soon it develops a "foot," which is used to move on the ground. Larvae also can swim to search for appropriate habitat.{{Rp|page=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 {{Convert|20|cm||0|abbr=on}}.{{Rp|page=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 {{Convert|12|cm||abbr=on}} a year.

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.{{Cite journal |last=Lucas |first=John S. |date=January 1994 |title=The biology, exploitation, and mariculture of giant clams (Tridacnidae) |url=http://dx.doi.org/10.1080/10641269409388557 |journal=Reviews in Fisheries Science |volume=2 |issue=3 |pages=181–223 |doi=10.1080/10641269409388557 |issn=1064-1262}} 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.{{Cite journal |last=Yonge |first=C. M. |date=1981-10-31 |title=Functional morphology and evolution in the Tridacnidae (Mollusca: Bivalvia: Cardiacea) |url=http://dx.doi.org/10.3853/j.0067-1975.33.1981.196 |journal=Records of the Australian Museum |volume=33 |issue=17 |pages=735–777 |doi=10.3853/j.0067-1975.33.1981.196 |issn=0067-1975}} 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.{{Cite journal |last=Stasek |first=Charles R. |date=May 1963 |title=Orientation and form in the bivalved Mollusca |url=http://dx.doi.org/10.1002/jmor.1051120302 |journal=Journal of Morphology |volume=112 |issue=3 |pages=195–214 |doi=10.1002/jmor.1051120302 |issn=0362-2525}}

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Image:Jean-Baptiste Pigalle bénitier.jpgs of the Église Saint-Sulpice in Paris, carved by Jean-Baptiste Pigalle]]

File:Egyptian paint holder HARGM10425.JPG 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.{{Rp|page=33}}

File:Tridacna gigas.jpg

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.{{Rp|page=11}}

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

As is often the case historically with uncharacteristically large species, the giant clam has been misunderstood.{{Cite journal |last=Lucas |first=John S. |title=Quick Guide: Giant Clams |journal=Current Biology |volume=24 |issue=5 |pages=183–184}}

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.{{Cite web |last=Barnett |first=Cynthia |date=2021-07-06 |title=The History, Myth, and Future of the Giant Clam |url=http://www.atlasobscura.com/articles/giant-clams-sound-of-the-sea-excerpt |access-date=2023-11-18 |website=Atlas Obscura |language=en |archive-date=18 November 2023 |archive-url=https://web.archive.org/web/20231118083751/https://www.atlasobscura.com/articles/giant-clams-sound-of-the-sea-excerpt |url-status=live}} 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. 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.[http://www.pearlforpeace.org/cobb.html Accounts by Wilburn Dowell Cobb] {{webarchive|url=https://web.archive.org/web/20070701194925/http://www.pearlforpeace.org/cobb.html |date=1 July 2007}}. pearlforpeace.org 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.

Other myths focus on the huge size of giant clams being associated with long age. 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.

Mass culture of giant clams began at the Micronesian Mariculture Demonstration Center in Palau (Belau).{{cite journal|title=Mass culture of giant clams (F. Tridacnidae) in Palau|doi=10.1016/0044-8486(84)90266-7|year=1984|last1=Heslinga|first1=Gerald A.|last2=Perron|first2=Frank E.|last3=Orak|first3=Obichang|journal=Aquaculture|volume=39|issue=1–4|pages=197–215}} 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.Copland, J. W. and J. S. Lucas (Eds.) 1988. Giant Clams in Asia and the Pacific. ACIAR Monograph No. 9{{cite journal|author=Braley, R.D. |year=1988|title= Farming the Giant Clam|journal= World Aquaculture |volume=20|issue=1|pages=7–17}}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 Seven of the ten known species of giant clams in the world are found in the coral reefs of the South China Sea.

File:Tridacna giant clam.jpg]]

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. 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.

T. gigas has been reported as locally extinct in peninsular Malaysia, while T. derasa and Hippopus porcellanus are restricted to Eastern Malaysia. These recent local extinctions have motivated the introduction of giant clams to Hawaii and Micronesia following maricultural advancements.{{Cite web |date=1996-08-01 |title=Tridacna gigas: Wells, S. |url=http://dx.doi.org/10.2305/iucn.uk.1996.rlts.t22137a9362283.en |access-date=2024-04-06 |website=IUCN Red List of Threatened Species}} Restocked individuals in the Philippines have successfully dispersed their own spawned larvae to at least several hundred meters away after only ten years.{{Cite journal |last=Cabaitan |first=Patrick C. |last2=Conaco |first2=Cecilia |date=2017-02-16 |title=Bringing back the giants: juvenile Tridacna gigas from natural spawning of restocked giant clams |url=http://dx.doi.org/10.1007/s00338-017-1558-9 |journal=Coral Reefs |volume=36 |issue=2 |pages=519–519 |doi=10.1007/s00338-017-1558-9 |issn=0722-4028}}

• Platyceramus, the largest bivalve in the fossil record
• Alatoconchidae, large-bodied extinct bivalves
• Plicatostylidae, large-bodied extinct bivalves

{{reflist|1=30em|refs=

{{cite journal|doi=10.1016/0044-8486(81)90040-5|title=Cultivation, spawning, and growth of the giant clams Tridacna gigas, T. Derasa, and T. Squamosa in Palau, Caroline Islands|year=1981|last1=Beckvar|first1=N.|journal=Aquaculture|volume=24|pages=21–30}}

{{cite journal|doi=10.1007/BF00288258|title=Reproduction in the giant clams Tridacna gigas and T. Derasa in situ on the north-central Great Barrier Reef, Australia, and Papua New Guinea|year=1984|last1=Braley|first1=Richard D.|journal=Coral Reefs|volume=3|issue=4|pages=221–227|bibcode=1984CorRe...3..221B|s2cid=39673803}}

Dame, Richard F. (1996) [https://books.google.com/books?id=98zeNrvTWuEC Ecology of marine bivalves an ecosystem approach] {{Webarchive|url=https://web.archive.org/web/20230604041114/https://books.google.com/books?id=98zeNrvTWuEC |date=4 June 2023}}. Boca Raton: CRC. p. 51. {{ISBN|1-4398-3909-3}}.

Gosling, Elizabeth (2003). Bivalve Molluscs Biology, Ecology and Culture. Grand Rapids: Blackwell Limited. p. 23. {{ISBN|978-0-85238-234-9}}

{{cite journal|author1=Jeffrey, S. W. |author2=F. T. Haxo |jstor=1539622 |title=Photosynthetic Pigments of Symbiotic Dinoflagellates (Zooxanthellae) from Corals and Clams |journal=Biological Bulletin |volume=135 |issue=1 |year=1968 |pages=149–65 |doi=10.2307/1539622 |url=https://www.biodiversitylibrary.org/part/13145}}{{dead link|date=January 2017 |bot=InternetArchiveBot |fix-attempted=yes}}

{{cite journal|author=Klumpp, D.W.|author2=Bayne, B.L.|author3=Hawkins, A.J.S.|name-list-style=amp |doi=10.1016/0022-0981(92)90030-E|title=Nutrition of the giant clam, Tridacna gigas (L). 1. Contribution of filter feeding and photosynthesis to respiration and growth|year=1992|journal=Journal of Experimental Marine Biology and Ecology|volume=155|pages=105}}

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

{{cite journal|author=Norton, J. H.|author2=M. A. Shepherd|author3=H. M. Long|author4=W. K. Fitt|name-list-style=amp|url=http://www.biolbull.org/cgi/content/abstract/183/3/503|title=The Zooxanthellal Tubular System in the Giant Clam|journal=The Biological Bulletin|volume=183|issue=3|pages=503–506|year=1992|doi=10.2307/1542028|jstor=1542028|pmid=29300506|access-date=24 November 2009|archive-date=16 October 2008|archive-url=https://web.archive.org/web/20081016144811/http://www.biolbull.org/cgi/content/abstract/183/3/503|url-status=live}}

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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. [https://web.archive.org/web/20111014000211/http://www.aslo.org/lo/toc/vol_56/issue_5/ Limnol. Oceanogr. 56(5)]: 1940–1951 (Open access)
• {{Wikicite|id=idYonge1936|reference=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}}

{{Commons category|Tridacna gigas}}

• ARKive – [https://web.archive.org/web/20040529203943/http://www.arkive.org/species/GES/invertebrates_marine/Tridacna_gigas/ images and movies of the giant clam (Tridacna gigas)]
• Tridacna gigas entry on [http://animaldiversity.ummz.umich.edu/site/accounts/information/Tridacna_gigas.html Animal Diversity Web]
• [https://web.archive.org/web/20050209052147/http://www.usm.my/cemacs/Webpg%20Mariculture%20Research.htm Giant clam conservation research project] at Universiti Sains Malaysia
• [http://www.richard-seaman.com/Underwater/Australia/GiantClams/ Giant Clams of the Great Barrier Reef]
• [http://www.stanford.edu/group/microdocs/ Microdocs] {{Webarchive|url=https://web.archive.org/web/20110727042839/http://www.stanford.edu/group/microdocs/sucks.html |date=27 July 2011 }}: [http://www.stanford.edu/group/microdocs/solarclams.html The solar powered clam] {{Webarchive|url=https://web.archive.org/web/20110730161929/http://www.stanford.edu/group/microdocs/solarclams.html |date=30 July 2011 }} & [http://www.stanford.edu/group/microdocs/growing.html Growing a giant clam] {{Webarchive|url=https://web.archive.org/web/20131109212751/http://www.stanford.edu/group/microdocs/growing.html |date=9 November 2013 }}
• [http://molluscan-eye.epoc.u-bordeaux1.fr/index.php?rubrique=accueil&lang=en MolluSCAN eye project] {{Webarchive|url=https://web.archive.org/web/20161113173444/http://molluscan-eye.epoc.u-bordeaux1.fr/index.php?rubrique=accueil&lang=en |date=13 November 2016 }}, a website dedicated to the in situ study of bivalve molluscs around the world
• {{SealifePhotos|207670}}

{{Taxonbar|from=Q751784}}

Category:Bivalves described in 1758

Category:Taxa named by Carl Linnaeus

Category:Tridacna