Giant clam

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Giant clam
Giant clam or Tridacna gigas.jpg
A live individual of Tridacna gigas, with the mantle showing (Great Barrier Reef, Australia)
Scientific classification Red Pencil Icon.png
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Subclass: Heterodonta
Order: Cardiida
Family: Cardiidae
Genus: Tridacna
Species:
T. gigas
Binomial name
Tridacna gigas
Synonyms [2]

Chama giganteaPerry, 1811

Mantle of giant clam with light-sensitive spots which detect danger and cause the clam to close Giant clam detail.jpg
Mantle of giant clam with light-sensitive spots which detect danger and cause the clam to close

The giant clams are the members of the clam genus Tridacna that are the largest living bivalve mollusks. There are actually several species of "giant clams" in the genus Tridacna , which are often misidentified for Tridacna gigas, the most commonly intended species referred to as “the giant clam”.

Contents

Tridacna gigas is one of the most endangered clam species. Antonio Pigafetta documented these in his 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 can 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 over 100 years. [3] They are also found off the shores of the Philippines and in the South China Sea in the coral reefs of Sabah (Malaysian Borneo).

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). [4] 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. [5] Its rapid growth rate is likely due to its ability to cultivate algae in its body tissue. [4]

Although larval clams are planktonic, they become sessile in adulthood. 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.

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. [6]

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. [6] As with massive deposition of coral matrices composed of calcium carbonate, the bivalves containing zooxanthellae have a tendency to grow massive calcium carbonate shells. [7] The mantle's edges are packed with symbiotic zooxanthellae that presumably utilize carbon dioxide, phosphates, and nitrates supplied by the clam. [8]

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. [9] 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. [10]

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. The weight of the two shells was 230 kilograms (510 lb). This suggests that the live weight of the animal would have been roughly 250 kilograms (550 lb). Today these shells are on display in a museum in Northern Ireland. [11] [12]

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). [11]

Ecology

Feeding

Algae provide giant clams with a supplementary source of nutrition. [8] These plants consist of unicellular algae, whose metabolic products add to the clam's filter food. [4] As a result, they are able to grow as large as one meter in length even in nutrient-poor coral-reef waters. [8] The clams cultivate algae in a special circulatory system which enables them to keep a substantially higher number of symbionts per unit of volume. [13] [14]

In small clams—10 milligrams (0.010 g) dry tissue weight—filter feeding provides about 65% of total carbon needed for respiration and growth; large clams (10 g) acquire only 34% of carbon from this source. [15] A single species of zooxenthellae may be symbionts of both giant clams and nearby reef–building (hermatypic) corals. [8]

Reproduction

Tridacna gigas reproduce sexually and are hermaphrodites (producing both eggs and sperm). Self-fertilization is not possible, but this characteristic does allow them to reproduce with any other member of the species. This reduces the burden of finding a compatible mate, while simultaneously doubling the number of offspring produced by the process. As with all other forms of sexual reproduction, hermaphroditism ensures that new gene combinations be passed to further generations. [16]

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, which transmit the information directly to the cerebral ganglia, a simple form of brain. [17]

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. [18]

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. [19]

Development

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

The fertilized egg floats in the sea for about 12 hours until eventually a larva (trocophore) 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. [21]

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) . [22] 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. [23]

Human relevance

One of the two clam stoups of the Eglise Saint-Sulpice in Paris, carved by Jean-Baptiste Pigalle Jean-Baptiste Pigalle benitier.jpg
One of the two clam stoups of the Église Saint-Sulpice in Paris, carved by Jean-Baptiste Pigalle
Piece of giant clam shell used as an ancient Egyptian paint holder Egyptian paint holder HARGM10425.JPG
Piece of giant clam shell used as an ancient Egyptian paint holder

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. [24]

The giant clam is considered a delicacy in Japan (known as himejako), France, South East Asia and many Pacific Islands. Some Asian foods include the meat from the muscles of clams. On the black market, giant clam shells are sold as decorative accoutrements. At times large amounts of money were paid for the adductor muscle, which Chinese people believed have aphrodisiac powers. [25]

Legend

As is often the case with uncharacteristically large species, the giant clam has been historically misunderstood. It was known in times past as the "killer clam" and "man-eating clam", and reputable scientific and technical manuals 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. [26]

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. [27]

Today the giant clam is considered neither aggressive nor particularly dangerous. While it is certainly capable of gripping a person, the shell's closing action is defensive, not aggressive, and the shell valves close too slowly to pose a serious threat.[ citation needed ] Furthermore, many large individuals are unable to completely close their shells.

Aquaculture

Mass culture of giant clams began at the Micronesian Mariculture Demonstration Center in Palau (belau). [28] 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. [29] [30] [31] Recent developments in aquaculture, specifically at Harbor Branch Oceanographic Institute in Fort Pierce, Florida, and in the Marshall Islands, have succeeded in tank-raising T. gigas, both for use in home aquariums and for release into the wild.

Seven of the ten known species of giant clams in the world are found in the coral reefs of the South China Sea. A programme to propagate endangered giant clams for release into the wild has been ongoing since 2007. Undertaken by the Marine Ecology Research Centre (www.merc-gayana.com) based in Gaya Island just west of Sabah’s capital, Kota Kinabalu, the programme successfully nurtured all seven species of the giant clams found in Malaysian waters to sufficient maturity for them to be placed in an ocean nursery for the first time during an awareness month from 22 March until 22 April 2012 in Maloham Bay. This marine awareness month had been planned to highlight and celebrate MERC's success in raising the giant clam larvae (called "spats") to juvenile stage, to highlight the importance of the giant clams and to raise awareness and support with the general public on the threats that are faced by the giant clams within the sea. During this marine awareness month, the coral restoration programme entered its final stage and attachment of 1000 one-year-old coral fragments grown at MERC's ocean nursery onto the coral reef were done throughout the month. The coral restoration programme is aimed to provide the giant clams with a suitable home surroundings when they are big enough in the future to be placed onto the reef.

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.

See also

Related Research Articles

Coral Marine invertebrates of the class Anthozoa

Corals are marine invertebrates within the class Anthozoa of the phylum Cnidaria. They typically live in compact colonies of many identical individual polyps. Coral species include the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.

Oyster Variety of families of Mollusc

Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not all oysters are in the superfamily Ostreoidea.

Bivalvia Class of molluscs

Bivalvia, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. Bivalves as a group have no head and they lack some usual molluscan organs like the radula and the odontophore. They include the clams, oysters, cockles, mussels, scallops, and numerous other families that live in saltwater, as well as a number of families that live in freshwater. The majority are filter feeders. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment where they are relatively safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. Some bivalves, such as the scallops and file shells, can swim. The shipworms bore into wood, clay, or stone and live inside these substances.

Zooxanthellae Dinoflagellates in symbiosis with coral, jellyfish and nudibranchs

Zooxanthellae is a colloquial term for single-celled dinoflagellates that are able to live in symbiosis with diverse marine invertebrates including demosponges, corals, jellyfish, and nudibranchs. Most known zooxanthellae are in the family Symbiodiniaceae, but some are known from the genus Amphidinium, and other taxa, as yet unidentified, may have similar endosymbiont affinities. The true Zooxanthella K.brandt is a mutualist of the radiolarian Collozoum inerme and systematically placed in Peridiniales. Another group of unicellular eukaryotes that partake in similar endosymbiotic relationships in both marine and freshwater habitats are green algae zoochlorellae.

<i>Tridacna</i> Genus of bivalves

Tridacna is a genus of large saltwater clams, marine bivalve molluscs in the subfamily Tridacninae, the giant clams. They have heavy shells, fluted with 4 to 6 folds. The mantle is brightly coloured. They inhabit shallow waters of coral reefs in warm seas of the Indo-Pacific region. These clams are popular in marine aquaria, and in some areas, such as the Philippines, members of the genus are farmed for the marine aquarium trade. They live in symbiosis with photosynthetic algae (zooxanthellae). Some species are eaten by humans.

<i>Tridacna squamosa</i> Species of bivalve

Tridacna squamosa, known commonly as the fluted giant clam and scaly clam, is one of a number of large clam species native to the shallow coral reefs of the South Pacific and Indian Oceans. It is distinguished by the large, leaf-like fluted edges on its shell called 'scutes' and a byssal opening that is small compared to those of other members of the subfamily Tridacnindae. Normal coloration of the mantle ranges from browns and purples to greens and yellows arranged in elongated linear or spot-like patterns. Tridacna squamosa grows to 40 centimetres (16 in) across.

Maxima clam Species of bivalve

The maxima clam, also known as the small giant clam, is a species of bivalve mollusc found throughout the Indo-Pacific region. They are much sought after in the aquarium trade, as their often striking coloration mimics that of the true giant clam; however, the maximas maintain a manageable size, with the shells of large specimens typically not exceeding 20 centimetres (7.9 in) in length.

<i>Hippopus hippopus</i>

Hippopus Hippopus, also known as the Horse Hoof clam and Strawberry clam, is a species of Giant Clam in the Family Tridacna and the family Hippopus. Hippopus Hippopus is delicacy in many southeast asian countries due to its high quality meat.

<i>Tridacna crocea</i> Species of bivalve

Tridacna crocea, the boring clam, crocus clam, crocea clam or saffron-coloured clam, is a species of bivalve in the family Cardiidae. It is native to the Indo-Pacific region. It is occasionally found in the aquarium trade where it is often simply referred to as crocea.

<i>Tridacna derasa</i> Species of bivalve

Tridacna derasa, the southern giant clam or smooth giant clam, is a species of extremely large marine clam in the family Cardiidae.

Freshwater bivalves are one kind of freshwater mollusc, along with freshwater snails. They are bivalves which live in freshwater, as opposed to saltwater, the main habitat type for bivalves.

Tridacninae Subfamily of bivalves

Tridacninae, common name, the giant clams, is a taxonomic subfamily of very large saltwater clams, marine bivalve molluscs in the family Cardiidae, the cockles.

Bivalve shell

A bivalve shell is part of the body, the exoskeleton or shell, of a bivalve mollusk. In life, the shell of this class of mollusks is composed of two hinged parts or valves. Bivalves are very common in essentially all aquatic locales, including saltwater, brackish water, and freshwater. The shells of bivalves commonly wash up on beaches and along the edges of lakes, rivers, and streams. Bivalves by definition possess two shells or valves, a "right valve" and a "left valve", that are joined by a ligament. The two valves usually articulate with one another using structures known as "teeth" which are situated along the hinge line. In many bivalve shells, the two valves are symmetrical along the hinge line—when truly symmetrical, such an animal is said to be equivalved; if the valves vary from each other in size or shape, inequivalved. If symmetrical front-to-back, the valves are said to be equilateral, and are otherwise considered inequilateral.

Aquaculture of coral

Coral aquaculture, also known as coral farming or coral gardening, is the cultivation of corals for commercial purposes or coral reef restoration. Aquaculture is showing promise as a tool for restoring coral reefs, which are dying off around the world. The process protects young corals while they are most at risk of dying. Small corals are propagated in nurseries and then replanted on the reef.

<i>Corculum cardissa</i> Species of bivalve

Corculum cardissa, the heart cockle, is a species of marine bivalve mollusc in the family Cardiidae. It is found in the Indo-Pacific region. It has a symbiotic relationship with dinoflagellates (zooxanthellae), which live within its tissues.

<i>Crassadoma</i> Genus of bivalves

Crassadoma is a genus of rock scallops, marine bivalve molluscs in the family Pectinidae. It is monotypic, the only species being Crassadoma gigantea, the rock scallop, giant rock scallop or purple-hinge rock scallop. Although the small juveniles are free-swimming, they soon become sessile, and are cemented to the substrate. These scallops occur in the eastern Pacific Ocean.

<i>Bartholomea annulata</i> Species of sea anemone

Bartholomea annulata is a species of sea anemone in the family Aiptasiidae, commonly known as the ringed anemone or corkscrew anemone. It is one of the most common anemones found on reefs in the Caribbean Sea.

<i>Fragum erugatum</i> Species of bivalve

Fragum erugatum is a small species of cockle, a marine bivalve mollusc in the family Cardiidae. It is found in the shallow seas off the coast of Western Australia. It is commonly known as the Hamelin cockle, cardiid cockle or heart cockle.

<i>Pocillopora verrucosa</i> Species of coral

Pocillopora verrucosa, commonly known as cauliflower coral, rasp coral, or knob-horned coral, is a species of stony coral in the family Pocilloporidae. It is native to tropical and subtropical parts of the Indian and Pacific Oceans.

<i>Tridacna noae</i> Species of bivalve

Tridacna noae, also known as Noah’s Giant Clam or the Teardrop clam, is a species of giant saltwater clam. Up until recently, T. noae was mistaken as being part of the giant clam species Tridacna maxima, but is now known to be its own independent species. It has a broad distribution, but is mainly found in the Indo-Pacific.

References

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Cited sources

Further reading