''Polydactylus sextarius'', the blackspot threadfin, is a species of marine ray-finned fish, a threadfin from the family

Polynemidae Threadfins are silvery grey perciform fish of the family Polynemidae. Found in tropical to subtropical waters throughout the world, the threadfin family contains eight genera and about 40 species. An unrelated species sometimes known by the name ...

which is native to the western Pacific and eastern Indian Oceans.

Distribution

''Polydactylus sextarius'' is a small- to medium-sized threadfin which attains a maximum total length of but is more common. It has a pointed snout with the dorsal profile of the head being almost straight. There are two separate dorsal fins, the first with 8 similar spines and the second dorsal fin has a single spine and 12 or 13 soft rays. The

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

has 3 spines and 11 to 13 soft rays, with the anal fin base being roughly the same length as the base of the second dorsal fin. The pectoral fin contains 13 to 15 rays most if which are unbranched, with the exception of the highest 1 or 2, the tip of the pectoral fin does not extend as far as the tip of the

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two en ...

. There are six pectoral filaments with the first being the shortest and the sixth being the longest, at more than a quarter of the

standard length Fish measurement is the measuring of individual fish and various parts of their anatomies. These data are used in many areas of ichthyology, including taxonomy and fisheries biology. Overall length * Standard length (SL) is the length of a fish m ...

but not reaching to the tip of the pectoral fin. The caudal fin is deeply forked, with its long lobes lacking filaments, the length of both lobes reaching a maximum of 41% of standard length. The simple

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

has 45 to 51 pored scales and extends from the upper end of the gill slit to the upper end of lower caudal fin lobe. The head and upper flanks are dark with a silver tinge paler on the lower flanks. It has a semi transparent snout. Both dorsal fins and the caudal fin are translucent with a sooty rear margin. The anla fin is blackish with a whitish tip. The pectoral fin membrane is white with scattered black spots. The pectoral filaments are whitish. The front of the pelvic fin is yellowish white, paler towards the rear, There is a large black spot on lateral line just behind the upper part of the gill opening.

Distribution

''Polydactylus sextarius'' is found in the Eastern Indian Ocean and Western Pacific. Its range extends from southwestern India to Papua New Guinea, north to Japan. It is rare in the Philippines, eastern Indonesia, and Papua New Guinea.

Habitat and biology

''Polydactylus sextarius'' is found where there are sandy and muddy bottoms on the continental shelf and it appears to favour depths of . This carnivorous species has its diet dominated by shrimps of less than length and

amphipod Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods range in size from and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far descr ...

s but it has also been recorded feeding on crabs, mysids, fishes and polychaetes. Sampling of the population in the Bay of Bengal off India found that a large proportion of the fish are hermaphrodites and that these fell within the range of standard length of , with a smaller proportion of females with standard lengths between . The hermaphrodites with comparatively large testes were counted as mature males and these measured standard length) while the fish with granulated ova of diameter were considered to be mature females and these fell within the standard length range of . The species undergoes a

protandrous Sequential hermaphroditism (called dichogamy in botany) is a type of hermaphroditism that occurs in many fish, gastropods, and plants. Sequential hermaphroditism occurs when the individual changes its sex at some point in its life. In particular, ...

sex change starting as juveniles, becoming hermaphrodites and ending as females progression. They first reach sexual maturity as hermaphrodites which function as males and there is no evidence which suggests that females develop directly from juveniles.

Fisheries

''Polydactyus sextarius'' is an important species for commercial fisheries in the Bay of Bengal and off Thailand where it is caught using trawls.

Species description

''Polydactylus sextarius'' was first formally described as ''Polynemus sextarius'' in 1801 by Marcus Elieser Bloch and John Gottlob Schneider with the type locality given as Tranquebar in India.

References

{{Taxonbar, from=Q2314917 Fish of Thailand Fish of Japan Taxa named by Marcus Elieser Bloch Taxa named by Johann Gottlob Theaenus Schneider Fish described in 1801 Fish of the Pacific Ocean Fish of the Indian Ocean Fish of the Philippines

sextarius The ancient Roman units of measurement were primarily founded on the Hellenic system, which in turn was influenced by the Egyptian system and the Mesopotamian system. The Roman units were comparatively consistent and well documented. Length T ...