Threadfin belongs to the family Polynemidae, which are represented in Indian waters by eight species. They derive their name from the thread-like free rays of the pectoral (shoulder) find projecting out under the shoulder. They have a prominent snout, and a mouth on the lower side beset with small teeth. Adipose eyelids, two dorsal fins, pelvic fins below the shoulder; somewhat rounded body and large forked tail are other characteristics. Three species, Polynemus tetradactylus, P.indicus and P.plebeius are more common on the west coast while others which are of smaller size are numerous on the east.
P.tetradactvlus, because of some similarity between its head and body form with that of the Atlantic salmon is known as Indian salmon, though they have no taxonomic relationship. It has four free pectoral finrays and is a most popular food fish on both coasts, growing to a length of 1.2 metres and weighing 10 kilograms. P.indicus has five free pectoral fin rays and is the largest of the family, growing to two metres and weighing more than sixteen kilograms. It is more frequent near the bottom, feeding on small sciaenids (Dhoma), hemipterids, prawns, Crabs and Bombay Duck. P.tetradactylus has similar feeding habits but comes closer to the shore and even enters estuaries and prefers rocky environment where it obtains its favourite food more plentifully. It breeds near the shore and young ones also enter estuaries for feeding.
The bodies of threadfins are fusiform and elongate, with soft and spinous dorsal fins which are widely separate. The tail fins of these fishes are very large and are deeply forked; this indicates their agility and speed. Their mouth is inferior and large; there is a blunt snout which projects far ahead. The palate and jaws bear bands of villiform or fibrous teeth. The major characteristic feature of these fishes is their pectoral fins, which are made of two different sections; the lower section, which contains from three to seven long independent rays looking like threads. In the species Polynemus, there are up to fifteen modified rays.
The threadfins are found more frequently in the open, shallow water in places with sandy, muddy or silty bottoms; they are scantily viewed at reefs. The filamentous pectoral rays of these fishes are believed to act as tactile structures, which help the fish to get its prey within the sediments. Known well for being euryhaline, the threadfins are capable of tolerating huge levels of salinity. This character helps them to get into the estuaries at times into the rivers. They are presumed to be pelagic spawners and probably release a number of very small buoyant eggs in the water column. These eggs them become a significant part of the plankton and the eggs float in water freely until they hatch.
