Threadfin

Threadfin
Atlantic threadfin, Polydactylus octonemus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Perciformes
Family:	Polynemidae
Genera
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 the Indian threadfish (family Carangidae).

Ranging in length from 20 centimeters (7.9 in) in the black-finned threadfin (Polydactylus nigripinnis) to 200 centimeters (79 in) 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.

Physical description

Their bodies are elongate and fusiform, with spinous and soft dorsal fins widely separate. Their tail fins are large and deeply forked; indicating 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. Their most distinguishing feature is their pectoral fins: they are composed of two distinct sections, the lower of which consists of between 3-7 long, thread-like independent rays. In Polynemus species there may be up to 15 of these modified rays.

Sixfinger threadfins, Polydactylus sexfilis

In some species, such as the royal threadfin (Pentanemus quinquarius), the thread-like rays may extend 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.

Range and habitat

Threadfins frequent open, shallow water in areas with muddy, sandy or silty bottoms; they are rarely glimpsed at reefs. Their pectoral rays are thought to serve as tactile structures, helping 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.

Reproduction

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.

Cuisine

Threadfin has been used as a co-ingredient in creating Crab stick.

Mariculture

In Hawaii, sixfinger threadfins (Polydactylus sexfilis) are the subject of commercial open-ocean cage mariculture.

Species

Indian Salmon (Eleutheronema tetradactylum)

There are about forty species in eight genera:

• Genus Eleutheronema
  • Indian Salmon or East Asian Fourfinger threadfin, Eleutheronema tetradactylum (Shaw, 1804)(replaced by Eleutheronema rhadinum (Jordan & Evermann, 1902)).
  • 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.

Timeline of genera

References

• Froese, Rainer, and Daniel Pauly, eds. (2006). "Polynemidae" in FishBase. January 2006 version.
• Sepkoski, Jack (2002). "A compendium of fossil marine animal genera". Bulletins of American Paleontology 364: p.560. http://strata.ummp.lsa.umich.edu/jack/showgenera.php?taxon=611&rank=class. Retrieved 2011-05-19. 

External links

• The Paradise threadfin, Polynemus paradiseus
• Offshore aquaculture project yields a traditional Hawaiian delicacy