Tridacna

''Tridacna'' is a genus of large saltwater clams, 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 is often brightly coloured. They inhabit shallow waters of coral reefs in warm seas of the Indo-Pacific 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, 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.

All species in the genus ''Tridacna'' are protected under CITES Appendix II.

Etymology

The name Tridacna arises from Greek words '' tri'', meaning three, and '' dacno,'' meaning bite''.'' In the Ancient Roman text ''Natural History,'' Pliny the Elder explained the nomenclature comes from the fact that "they are so large as to require three bites in eating them.”

List of Species, 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'' (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 -- Mauritius
* '' 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.

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 through which light penetrates.

Ecology and behaviour

''Tridacna'' clams are common inhabitants of Indo-Pacific coral reef benthic communities in shallower waters. They live in symbiosis with photosynthetic dinoflagellate algae (''Symbiodinium'') that grow in the mantle tissues. Light penetrates the mantle through small lens-like structures called ocelli. They are sessile 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. 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 trade.

''Tridacna'' clams can produce large white pearls with an undulating, porcelain-like surface, which may be described as "non- nacreous 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 ''Tridacna gigas'' 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. 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, ''Tridacna gigas''
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