''Tridacna'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial no ...

of large saltwater

clam Clam is a common name for several kinds of bivalve molluscs. The word is often applied only to those that are edible and live as infauna, spending most of their lives halfway buried in the sand of the seafloor or riverbeds. Clams have two sh ...

s, marine

bivalve 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. As a group, biva ...

mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is est ...

s 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 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 groups. ...

s in warm seas of 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 ...

region.Huelsken, T., Keyse, J., Liggins, L., Penny, S., Treml, E.A., Riginos, C. (2013) A Novel Widespread Cryptic Species and Phylogeographic Patterns within Several Giant Clam Species (Cardiidae: Tridacna) from the Indo-Pacific Ocean. PLoS ONE
DOI: 10.1371/journal.pone.0080858
These clams are popular in marine aquaria, and in some areas, such as the

Philippines The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no), * bik, Republika kan Filipinas * ceb, Republika sa Pilipinas * cbk, República de Filipinas * hil, Republ ...

, members of the genus are farmed for the marine aquarium trade. They live in symbiosis with photosynthetic

algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular m ...

(

zooxanthella 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 genus ''Symbi ...

e). Some species are eaten by humans. All species in the genus ''Tridacna'' are protected under

CITES Appendix II CITES (shorter name for the Convention on International Trade in Endangered Species of Wild Fauna and Flora, also known as the Washington Convention) is a multilateral treaty to protect endangered plants and animals from the threats of interna ...

Systematics and phylogeny

The genus contains the following species: * '' Tridacna crocea'' Lamarck, 1819 -- Western Pacific * '' Tridacna derasa'' (Röding, 1798) -- Western Pacific * '' Tridacna elongatissima'' Bianconi, 1856 * ''

Tridacna gigas 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'', t ...

'' (Linnaeus, 1758) -- Tropical Indo-Pacific * '' Tridacna maxima'' Röding, 1798 ( =''Tridacna elongata'') -- Tropical Indo-Pacific * '' Tridacna mbalavuana'' Ladd, 1934 -- Fiji, Tonga * '' Tridacna noae'' (Röding, 1798) -- China sea * '' Tridacna rosewateri'' Sirenho & Scarlato, 1991 -- Mascarene region * '' Tridacna squamosa'' Lamarck, 1819 -- Tropical Indo-Pacific * '' Tridacna squamosina'' Sturany, 1899 (= '' Tridacna costata'' Roa-Quiaoit, Kochzius, Jantzen, Al-Zibdah & Richter 2008) -- Indo-Pacific ;Synonyms: * ''Tridacna acuticostata'' G. B. Sowerby III, 1912: synonym of ''Tridacna maxima'' (Röding, 1798) * ''Tridacna compressa'' Reeve, 1862: synonym of ''Tridacna (Chametrachea) maxima'' (Röding, 1798) represented as ''Tridacna maxima'' (Röding, 1798) (junior subjective synonym) * ''Tridacna costata'' Roa-Quiaoit, Kochzius, Jantzen, Zibdah & Richter, 2008: synonym of ''Tridacna squamosina'' Sturany, 1899 * ''Tridacna cumingii'' Reeve, 1862: synonym of ''Tridacna (Chametrachea) crocea'' Lamarck, 1819 represented as ''Tridacna crocea'' Lamarck, 1819 (junior subjective synonym) * ''Tridacna detruncata'' Bianconi, 1869: synonym of ''Tridacna maxima'' (Röding, 1798) (junior subjective synonym) * ''Tridacna elongata'' Lamarck, 1819: synonym of ''Tridacna maxima'' (Röding, 1798) (junior subjective synonym) * ''Tridacna ferruginea'' Reeve, 1862: synonym of ''Tridacna (Chametrachea) crocea'' Lamarck, 1819 represented as ''Tridacna crocea'' Lamarck, 1819 (junior subjective synonym) * ''Tridacna fossor'' Hedley, 1921: synonym of ''Tridacna maxima'' (Röding, 1798) * ''Tridacna glabra'' Link, 1807: synonym of ''Tridacna derasa'' (Röding, 1798) (junior subjective synonym, synonym) * ''Tridacna imbricata'' (Röding, 1798): synonym of ''Tridacna (Chametrachea) maxima'' (Röding, 1798) represented as ''Tridacna maxima'' (Röding, 1798) * Tridacna lamarcki Hidalgo, 1903: synonym of Tridacna squamosa Lamarck, 1819 (synonym - pars) * Tridacna lanceolata G. B. Sowerby II, 1884: synonym of Tridacna (Chametrachea) maxima (Röding, 1798) represented as Tridacna maxima (Röding, 1798) (unaccepted > junior subjective synonym) * ''Tridacna lorenzi'' Monsecour, 2016 -- Mascarene region: synonym of ''Tridacna lorenzi'' K. Monsecour, 2016: synonym of ''Tridacna (Chametrachea) rosewateri'' Sirenko & Scarlato, 1991 represented as ''Tridacna rosewateri'' Sirenko & Scarlato, 1991 * ''Tridacna mutica'' Lamarck, 1819: synonym of ''Tridacna (Chametrachea) maxima'' (Röding, 1798) represented as ''Tridacna maxima'' (Röding, 1798) (synonym - pars) * ''Tridacna ningaloo'' Penny & Willan, 2014: synonym of ''Tridacna noae'' (Röding, 1798) * ''Tridacna obesa'' G. B. Sowerby III, 1899: synonym of ''Tridachnes derasa'' Röding, 1798: synonym of ''Tridacna derasa'' (Röding, 1798) (junior subjective synonym) * ''Tridacna reevei'' Hidalgo, 1903: synonym of ''Tridacna (Chametrachea) maxima'' (Röding, 1798) represented as ''Tridacna maxima'' (Röding, 1798) (junior subjective synonym, synonym) * ''Tridacna rudis'' Reeve, 1862: synonym of ''Tridacna (Chametrachea) maxima'' (Röding, 1798) represented as ''Tridacna maxima'' (Röding, 1798) (junior subjective synonym) * ''Tridacna serrifera'' Lamarck, 1819: synonym of ''Tridacna derasa'' (Röding, 1798) * ''Tridacna tevoroa'' Lucas, Ledua & Braley, 1990: synonym of ''Tridacna mbalavuana'' Ladd, 1934 * ''Tridacna troughtoni'' Iredale, 1927: synonym of ''Tridacna maxima'' (Röding, 1798) (junior subjective synonym) An alternative older classification recognises a third subgenus ''Persikima'' containing ''T. derasa'' and ''T. mbalavuana''. Recent biochemical studies have suggested that there may exist morphologically indistinct

cryptic species In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...

Anatomy

Compared to other clams, the soft mantle that secretes the shell is greatly expanded. The clams even have small lens-like structures called

ocelli A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-le ...

through which light penetrates.

Ecology and behaviour

''Tridacna'' clams are common inhabitants of

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning ...

communities in shallower waters. They live in

symbiosis Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasi ...

with photosynthetic

dinoflagellate The dinoflagellates ( Greek δῖνος ''dinos'' "whirling" and Latin ''flagellum'' "whip, scourge") are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered algae. Dinoflagellates ...

algae (''

Symbiodinium : ''This is about the genus sometimes called Zoox. For the company, see Zoox (company)'' ''Symbiodinium'' is a genus of dinoflagellates that encompasses the largest and most prevalent group of endosymbiotic dinoflagellates known. These unicellul ...

'') that grow in the mantle tissues. Light penetrates the mantle through small lens-like structures called

. They are

sessile Sessility, or sessile, may refer to: * Sessility (motility), organisms which are not able to move about * Sessility (botany), flowers or leaves that grow directly from the stem or peduncle of a plant * Sessility (medicine), tumors and polyps that ...

in adulthood. By day, the clams spread out their mantle so that the algae receive the sunlight they need to photosynthesize, whereas the colour pigments protect the clam against excessive light and UV radiation. Adult clams can get most (70–100%) of their nutrients from the algae and the rest from

filter feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

. When disturbed, the clam closes its shell. The popular opinion that they pose danger to divers who get trapped or injured between the closing sharp-edged shell is not very real, as the closing reaction is quite slow. Their large size and easy accessibility has caused overfishing and collapse of the natural stocks in many places and extirpation in some of the species. They are being sustainably farmed in some areas, both for the seafood market in some Asian countries and for the

aquarium An aquarium (plural: ''aquariums'' or ''aquaria'') is a vivarium of any size having at least one transparent side in which aquatic plants or animals are kept and displayed. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, aq ...

trade. ''Tridacna'' clams can produce large white

pearl A pearl is a hard, glistening object produced within the soft tissue (specifically the mantle) of a living shelled mollusk or another animal, such as fossil conulariids. Just like the shell of a mollusk, a pearl is composed of calcium ca ...

s with an undulating,

porcelain Porcelain () is a ceramic material made by heating substances, generally including materials such as kaolinite, in a kiln to temperatures between . The strength and translucence of porcelain, relative to other types of pottery, arises mai ...

-like surface, which may be described as "non-

nacreous Nacre ( , ), also known as mother of pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer; it is also the material of which pearls are composed. It is strong, resilient, and iridescent. Nacre is ...

pearls". The " Pearl of Lao Tzu", also known as the "Pearl of Allah", is the world's largest pearl weighing 6.4 kilogrammes; it was said to have been found inside a ''

'' by a Filipino diver in 1934.

Artistic use

Over a hundred examples of carved ''Tridacna'' shells have been found in archaeological expeditions from Italy to the Near East. Similar in artistic style, they were probably produced in the mid-seventh century, made or distributed from the southern coast of

Phoenicia Phoenicia () was an ancient thalassocratic civilization originating in the Levant region of the eastern Mediterranean, primarily located in modern Lebanon. The territory of the Phoenician city-states extended and shrank throughout their hist ...

. The backs and interior perimeters of the shells show animal, human, and floral motifs, while the interiors typically show recumbent sphinxes. The umbo of the shell is in the shape of a human female or bird's head. They were probably used to store eye cosmetics.Markoe, Glenn. ''Phoenicians.'' British Museum Press (2000).

Images

Image:Tridacna crocea recorte.jpg, '' Tridacna crocea'' Image:Tridacna derasa.001 - Aquarium Finisterrae.JPG, '' Tridacna derasa'' Image:Tridacna gigas.jpg, ''

'' Image:Tridacna maxima Thiladhoo.JPG, '' Tridacna maxima'' Image:Tridacna squamosa Réunion.jpg, '' Tridacna squamosa'' Image:Tridacna costata 159b.jpg, '' Tridacna squamosina''

Notes

References

*
How to Care for Tridacnid Clams
FishChannel.com {{Taxonbar, from=Q2387255 Bivalve genera Taxa named by Jean Guillaume Bruguière Extant Miocene first appearances