Giant Clam
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''Tridacna gigas'', the giant clam, is the best-known species of the giant clam
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
''
Tridacna ''Tridacna'' is a genus of large saltwater clams, Marine (ocean), marine bivalve molluscs in the subfamily Tridacninae, the giant clams. Many Tridacna species are threatened. They have heavy shells, fluted with 4 to 6 folds. The Mantle (mollus ...
''. Giant
clams Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...
are the largest
living Living or The Living may refer to: Common meanings *Life, a condition that distinguishes organisms from inorganic objects and dead organisms ** Living species, one that is not extinct *Personal life, the course of an individual human's life * ...
bivalve Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...
s. Several other species of "giant clam" in the genus ''
Tridacna ''Tridacna'' is a genus of large saltwater clams, Marine (ocean), marine bivalve molluscs in the subfamily Tridacninae, the giant clams. Many Tridacna species are threatened. They have heavy shells, fluted with 4 to 6 folds. The Mantle (mollus ...
'' 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 reef A coral reef is an underwater ecosystem characterized by reef-building corals. Reefs are formed of colonies of coral polyps held together by calcium carbonate. Most coral reefs are built from stony corals, whose polyps cluster in group ...
s of the South
Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is bounded by the cont ...
and Indian oceans, they may weigh more than , measure as much as across, and have an average lifespan in the wild of more than 100 years. They also are found off the shores of the
Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...
and in the
South China Sea The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by South China, in the west by the Indochinese Peninsula, in the east by the islands of Taiwan island, Taiwan and northwestern Philippines (mainly Luz ...
in the coral reefs of
Malaysia Malaysia is a country in Southeast Asia. Featuring the Tanjung Piai, southernmost point of continental Eurasia, it is a federation, federal constitutional monarchy consisting of States and federal territories of Malaysia, 13 states and thre ...
. The giant clam lives in flat
coral Corals are colonial marine invertebrates within the subphylum Anthozoa of the phylum Cnidaria. They typically form compact Colony (biology), colonies of many identical individual polyp (zoology), polyps. Coral species include the important Coral ...
sand or broken coral and may be found at depths of as great as 20 m (66 ft). Its range covers the
Indo-Pacific The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...
, 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 may be found off high- or low-elevation islands, in
lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...
s or
fringing reef A fringing reef is one of the three main types of coral reef. It is distinguished from the other main types, barrier reefs and atolls, in that it has either an entirely shallow backreef zone (lagoon) or none at all. If a fringing reef grows direc ...
s. Its rapid growth rate is likely due to its ability to cultivate
algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
in its body tissue. Although
larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
l clams are
plankton Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
ic, they become sessile in adulthood. The creature's mantle tissues act as a
habitat In ecology, habitat refers to the array of resources, biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species' habitat can be seen as the physical manifestation of its ...
for the
symbiotic Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...
single-celled
dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered protists. Dinoflagellates are mostly marine plankton, but they are also commo ...
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
photosynthesis Photosynthesis ( ) is a system of biological processes by which photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical energy necessary to fuel their metabo ...
e. This method of algal farming is under study as a model for highly efficient bioreactors.


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, allowing part of the brownish-yellow mantle to remain visible. ''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. Similar to coral matrices composed of
calcium carbonate Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...
, giant clams grow their shells through the process of
biomineralization Biomineralization, also written biomineralisation, is the process by which living organisms produce minerals, often resulting in hardened or stiffened '' mineralized tissues''. It is an extremely widespread phenomenon: all six taxonomic kingd ...
, which is very sensitive to seasonal temperature. The isotopic ratio of oxygen in
carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...
and the ratio between
Strontium Strontium is a chemical element; it has symbol Sr and atomic number 38. An alkaline earth metal, it is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is exposed to ...
and
Calcium Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to it ...
together may be used to determine historical
sea surface temperature Sea surface temperature (or ocean surface temperature) is the ocean temperature, temperature of ocean water close to the surface. The exact meaning of ''surface'' varies in the literature and in practice. It is usually between and below the sea ...
. The mantle border itself is covered in several hundred to several thousand pinhole eyespots approximately in diameter. Each one consists of a small cavity containing a
pupil The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black becau ...
-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 Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...
. 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. The optical system forms an image by sequential, local dimming of some eyes using
pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...
from the aperture.


Largest specimens

The largest known ''T. gigas'' specimen measured , 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 Sumatra () is one of the Sunda Islands of western Indonesia. It is the largest island that is fully within Indonesian territory, as well as the list of islands by area, sixth-largest island in the world at 482,286.55 km2 (182,812 mi. ...
,
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...
, and its shells are now on display in a museum in
Northern Ireland Northern Ireland ( ; ) is a Countries of the United Kingdom, part of the United Kingdom in the north-east of the island of Ireland. It has been #Descriptions, variously described as a country, province or region. Northern Ireland shares Repub ...
. A heavier giant clam was found in 1956 off the Japanese island of Ishigaki. The shell's length was , and it weighed dead and estimated alive. File:Giant Clam (Tridacna gigas) (6058446919).jpg, Giant clam in
Bunaken Bunaken is an island of 8 km2, part of the Bunaken National Park, Bunaken National Marine Park. Bunaken is located at the northern tip of the island of Sulawesi, Indonesia. It belongs administratively to the municipality of Manado. Scuba div ...
Island,
Sulawesi Sulawesi ( ), also known as Celebes ( ), is an island in Indonesia. One of the four Greater Sunda Islands, and the List of islands by area, world's 11th-largest island, it is situated east of Borneo, west of the Maluku Islands, and south of Min ...
,
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...
File:Tridacna gigas 01 by Line1.JPG, Empty giant clam shell in the French
National Museum of Natural History The National Museum of Natural History (NMNH) is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. With 4.4 ...
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 Honolulu ( ; ) is the List of capitals in the United States, capital and most populous city of the U.S. state of Hawaii, located in the Pacific Ocean. It is the county seat of the Consolidated city-county, consolidated City and County of Honol ...
,
Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...
, United States


Ecology


Feeding

Giant clams are filter-feeders, yet 65-70 percent of their nutritional needs are supplied by zooxanthellae. 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 In vertebrates, the circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the body. It includes the cardiovascular system, or vascular system, that consists of the heart ...
that enables them to keep a substantially higher number of symbionts per unit of volume. The mantle's edges are packed with
symbiotic Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...
zooxanthellae, which presumably use
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
,
phosphate Phosphates are the naturally occurring form of the element phosphorus. In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...
s, and
nitrate Nitrate is a polyatomic ion with the chemical formula . salt (chemistry), Salts containing this ion are called nitrates. Nitrates are common components of fertilizers and explosives. Almost all inorganic nitrates are solubility, soluble in wa ...
s supplied by the clam. In very small clams— dry tissue weight—filter feeding provides approximately 65% of total carbon needed for respiration and growth; comparatively larger clams () 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.


Reproduction

''Tridacna gigas'' reproduce sexually and are hermaphrodites (producing both eggs and
sperm Sperm (: sperm or sperms) is the male reproductive Cell (biology), cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile 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. 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 A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance ( endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorece ...
situated close to the incurrent syphon that transmit the information directly to the cerebral
ganglia A ganglion (: ganglia) is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system, this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system, there a ...
, a simple form of brain. 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 . Egg release initiates the reproductive process. An adult ''T. gigas'' can release more than 500 million eggs at a time. 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.


Development

The fertilized egg floats in the sea for approximately 12 hours until eventually a
larva A larva (; : larvae ) is a distinct juvenile form many animals undergo before metamorphosis into their next life stage. Animals with indirect development such as insects, some arachnids, amphibians, or cnidarians typically have a larval phase ...
(trochophore) hatches. It then starts to produce a calcium carbonate shell. Two days after fertilization it measures . Soon it develops a "foot," which is used to move on the ground. Larvae also can swim to search for appropriate habitat. 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 Plankton are the diverse collection of organisms that drift in Hydrosphere, water (or atmosphere, air) but are unable to actively propel themselves against ocean current, currents (or wind). The individual organisms constituting plankton are ca ...
. 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 . It is difficult to observe the growth rate of ''T. gigas'' in the wild, but laboratory-reared giant clams have been observed to grow 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 Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...
development and morphology. 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. Stasek proposed instead that the growth occurs primarily in a posterior direction instead of the more typical direction of
ventral Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
in most bivalves, which is reflected in the transitional stages of alternative ways of growing that juveniles undergo.


Human relevance

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. The giant clam is considered a delicacy in Japan (known as ''himejako''), France,
Southeast Asia Southeast Asia is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern region of Asia, consisting of the regions that are situated south of China, east of the Indian subcontinent, and northwest of the Mainland Au ...
, 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 An aphrodisiac is a substance that increases libido, sexual desire, sexual attraction, sexual pleasure, or sexual behavior. These substances range from a variety of plants, spices, and foods to synthetic chemicals. Natural aphrodisiacs, such as ...
powers. On the
black market A black market is a Secrecy, clandestine Market (economics), market or series of transactions that has some aspect of illegality, or is not compliant with an institutional set of rules. If the rule defines the set of goods and services who ...
, giant clam shells are sold as decorative accoutrements.


Legend

As is often the case historically with uncharacteristically large species, the giant clam has been misunderstood. 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
Polynesia Polynesia ( , ) is a subregion of Oceania, made up of more than 1,000 islands scattered over the central and southern Pacific Ocean. The indigenous people who inhabit the islands of Polynesia are called Polynesians. They have many things in ...
n 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. Starting from the eighteenth century, claims of danger had been related to the western world. In the 1920s, a reputable science magazine ''
Popular Mechanics ''Popular Mechanics'' (often abbreviated as ''PM'' or ''PopMech'') is a magazine of popular science and technology, featuring automotive, home, outdoor, electronics, science, do it yourself, and technology topics. Military topics, aviation an ...
'' 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 The Pearl of Lao Tzu was once considered the world's largest known pearl. The pearl was found by a Filipino diver in the Palawan sea, which surrounds the island of Palawan in the Philippines. It is not considered a gemstone pearl, but is instead ...
, Wilburn Cobb said he was told that a Dyak diver was drowned when the ''Tridacna'' closed its shell on his arm. In reality, the slow speed of their adductor 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.


Aquaculture

Mass culture of giant clams began at the Micronesian Mariculture Demonstration Center in
Palau Palau, officially the Republic of Palau, is an island country in the Micronesia subregion of Oceania in the western Pacific Ocean. The Republic of Palau consists of approximately 340 islands and is the western part of the Caroline Islands ...
(Belau). A large
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
n 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. Seven of the ten known species of giant clams in the world are found in the coral reefs of the South China Sea.


Conservation status

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 Convention may refer to: * Convention (norm), a custom or tradition, a standard of presentation or conduct ** Treaty, an agreement in international law ** Convention (political norm), uncodified legal or political tradition * Convention (meeting ...
(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 Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...
and
Micronesia Micronesia (, ) is a subregion of Oceania, consisting of approximately 2,000 small islands in the Northwestern Pacific Ocean. It has a close shared cultural history with three other island regions: Maritime Southeast Asia to the west, Poly ...
following maricultural advancements. Restocked individuals in the
Philippines The Philippines, officially the Republic of the Philippines, is an Archipelagic state, archipelagic country in Southeast Asia. Located in the western Pacific Ocean, it consists of List of islands of the Philippines, 7,641 islands, with a tot ...
have successfully dispersed their own spawned larvae to at least several hundred meters away after only ten years.


See also

*''
Platyceramus ''Platyceramus'' was a genus of Cretaceous bivalve molluscs belonging to the extinct inoceramid lineage. It is sometimes classified as a subgenus of ''Inoceramus''. Size The largest and best known species is ''P. platinus''. Individuals of this ...
'', the largest bivalve in the fossil record * Alatoconchidae, large-bodied extinct bivalves * Plicatostylidae, large-bodied extinct bivalves


References


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) *


External links

* ARKive
images and movies of the giant clam ''(Tridacna gigas)''
* ''Tridacna gigas'' entry o



at Universiti Sains Malaysia
Giant Clams of the Great Barrier ReefMicrodocs





MolluSCAN eye project
, a website dedicated to the ''in situ'' study of bivalve molluscs around the world * {{Taxonbar, from=Q751784 Bivalves described in 1758 Taxa named by Carl Linnaeus Tridacna