Threadfin

Taxobox
name = Threadfin


image_caption = Sixfinger threadfin, "Polydactylus sexfilis"
regnum = Animalia
phylum = Chordata
classis = Actinopterygii
ordo = Perciformes
familia = Polynemidae
subdivision_ranks = Genera
subdivision = "Eleutheronema"
"Filimanus"
"Galeoides"
"Leptomelanosoma"
"Parapolynemus"
"Pentanemus"
"Polydactylus"
"Polynemus" See text for species.

Threadfins are silvery grey perciform marine fish of the family Polynemidae. Found in tropical to subtropical waters throughout the world, the threadfin family contains nine genera and 33 species. An unrelated species sometimes known by the name threadfin, "Alectis indicus", is properly known as the Indian threadfish (family Carangidae).

Ranging in length from 20 centimetres in the black-finned threadfin ("Polydactylus nigripinnis") to 200 centimetres in fourfinger threadfins ("Eleutheronema tetradactylum") and giant African threadfins ("Polydactylus quadrifilis"), threadfins are both important to commercial fisheries as a food fish, and popular among anglers. Their habit of forming large schools makes the threadfins a reliable and economic catch.

Their bodies are elongate and fusiform, with spinous and soft dorsal fins widely separate. Their tail fins are large and deeply forked; this is an indication of their speed and agility. The mouth is large and inferior; a blunt snout projects far ahead. The jaws and palate possess bands of "villiform" (fibrous) teeth. The most distinguishing feature of the threadfins are their pectoral fins: they are composed of two distinct sections, the lower of which consisting of between 3-7 long, thread-like independent rays. In "Polynemus" species there may be up to 15 of these modified rays.

In some species, such as the royal threadfin ("Pentanemus quinquarius"), the thread-like rays may extent well past the tail fin. This feature explains both the common name "threadfin" and the family name "Polynemidae", from the Greek "poly" meaning "many" and "nema" meaning "filament." Similar species, such as the mullets (family Mugilidae) and milkfish (family Chanidae) can be easily distinguished from threadfins by their lack of filamentous pectoral rays.

Threadfins frequent open, shallow water in areas with muddy, sandy or silty bottoms; they are rarely glimpsed at reefs. Their filamentous pectoral rays are thought to serve as tactile structures, helping the fish to find prey within the sediments. Noted for being euryhaline, threadfins are able to tolerate a wide range of salinity levels. This attribute allows threadfins to enter estuaries and even rivers. They feed primarily on crustaceans and smaller fish.

Presumed to be pelagic spawners, threadfins probably release many tiny buoyant eggs into the water column which then become part of the plankton. The eggs float freely with the currents until hatching. In Hawaii, sixfinger threadfins (Polydactylus sexfilis) are the subject of experimental aquaculture; they may soon alleviate pressures on wild stocks and lessen the state's dependence on imported fish.

pecies

There are about forty species in eight genera:
* Genus "Eleutheronema"
** East Asian fourfinger threadfin, "Eleutheronema rhadinum" (Jordan & Evermann, 1902).
** Fourfinger threadfin, "Eleutheronema tetradactylum" (Shaw, 1804).
** Threefinger threadfin, "Eleutheronema tridactylum" (Bleeker, 1849).
* Genus "Filimanus"
** Sevenfinger threadfin, "Filimanus heptadactyla" (Cuvier, 1829).
** Javanese threadfin, "Filimanus hexanema" (Cuvier, 1829).
** Splendid threadfin, "Filimanus perplexa" Feltes, 1991.
** Eightfinger threadfin, "Filimanus sealei" (Jordan & Richardson, 1910).
** "Filimanus similis" Feltes, 1991.
** Yellowthread threadfin, "Filimanus xanthonema" (Valenciennes, 1831).
* Genus "Galeoides"
** Lesser African threadfin, "Galeoides decadactylus" (Bloch, 1795).
* Genus "Leptomelanosoma"
** Indian threadfin, "Leptomelanosoma indicum" (Shaw, 1804).
* Genus "Parapolynemus"
** Dwarf paradise fish, "Parapolynemus verekeri" (Saville-Kent, 1889).
* Genus "Pentanemus"
** Royal threadfin, "Pentanemus quinquarius" (Linnaeus, 1758).
* Genus "Polydactylus"
** Blue bobo, "Polydactylus approximans" (Lay & Bennett, 1839).
** Slender fivefinger threadfin, "Polydactylus bifurcus" Motomura, Kimura & Iwatsuki, 2001.
** Long-limb threadfin, "Polydactylus longipes" Motomura, Okamoto & Iwatsuki, 2001.
** King threadfin, "Polydactylus macrochir" (Günther, 1867).
** River threadfin, "Polydactylus macrophthalmus" (Bleeker, 1858).
** African blackspot threadfin, "Polydactylus malagasyensis" Motomura & Iwatsuki, 2001.
** Small-mouthed threadfin, "Polydactylus microstomus" (Bleeker, 1851).
** "Polydactylus mullani" (Hora, 1926).
** Australian threadfin, "Polydactylus multiradiatus" (Günther, 1860).
** Black-finned threadfin, "Polydactylus nigripinnis" Munro, 1964.
** Atlantic threadfin, "Polydactylus octonemus" (Girard, 1858).
** Littlescale threadfin, "Polydactylus oligodon" (Günther, 1860).
** Yellow bobo, "Polydactylus opercularis" Seale & Bean, 1907.
** Persian blackspot threadfin, "Polydactylus persicus" Motomura & Iwatsuki, 2001.
** Striped threadfin, "Polydactylus plebeius" (Broussonet, 1782).
** Giant African threadfin, "Polydactylus quadrifilis" (Cuvier, 1829).
** Sixfinger threadfin, "Polydactylus sexfilis" (Valenciennes, 1831).
** Blackspot threadfin, "Polydactylus sextarius" (Bloch & Schneider, 1801).
** "Polydactylus siamensis" Motomura, Iwatsuki & Yoshino, 2001.
** Barbu, "Polydactylus virginicus" (Linnaeus, 1758).
* Genus "Polynemus"
** Northern paradise fish, "Polynemus aquilonaris" Motomura, 2003.
** "Polynemus bidentatus" Motomura & Tsukawaki, 2006.
** Eastern paradise fish, "Polynemus dubius" Bleeker, 1854.
** Hornaday's paradise fish, "Polynemus hornadayi" Myers, 1936.
** Elegant paradiseus fish, "Polynemus multifilis" Temminck & Schlegel, 1843.
** Paradise threadfin, "Polynemus paradiseus" Linnaeus, 1758.

References

*

External links

* [http://www.practicalfishkeeping.co.uk/pfk/pages/show_article.php?article_id=111 The Paradise threadfin, Polynemus paradiseus]
* [http://www.oar.noaa.gov/spotlite/archive/spot_hawaii.html Offshore aquaculture project yields a traditional Hawaiian delicacy]