Blackspot threadfin

What it is

This ray-finned fish belongs to the threadfin family. It grows to thirty centimetres at most, though twenty is more typical. The head profile is nearly straight with a pointed snout. Two dorsal fins sit on its back, while the tail fin forks deeply. Six free filaments extend from the pectoral area, with the last one being the longest. A distinct black mark appears on the side line behind the gill cover. The upper body looks dark with silver hints, fading to pale below. Fins are mostly clear or spotted, except for the dark anal fin tip.

Where it lives

These fish inhabit the eastern Indian Ocean and western Pacific waters. Their territory stretches from southwest India up to Japan and across to Papua New Guinea. They are less common in the Philippines, eastern Indonesia, and Papua New Guinea. The species prefers the continental shelf where the seabed consists of sand or mud. They typically stay at depths between sixteen and seventy-three metres.

How it lives

As a carnivore, this species hunts small prey on the sea floor. Shrimps under two centimetres and amphipods make up most of its meals. It also eats crabs, mysids, other fish, and marine worms. Studies in the Bay of Bengal show complex reproductive biology. Many individuals function as males first before changing sex. This protandrous progression means they start life as juveniles, become functional males, and later turn into females.

Breeding and life cycle

Sexual maturity begins when the fish acts as a male hermaphrodite. Those with large testes measure between ninety and one hundred and thirty-four millimetres. Later, they develop granulated eggs and become females. Mature females range from one hundred and five to one hundred and forty-nine millimetres in standard length. There is no sign that young fish skip the male phase to become females directly.

Fish and people

Trawl nets catch this species in significant numbers for commercial sale. It is a key part of the fishing industry in the Bay of Bengal. Thai waters also support important catches of this threadfin. Scientists first described the fish in 1801, naming Tranquebar in India as the type location. The original classification placed it in the genus Polynemus.

Questions people ask about Blackspot threadfin

Good reading and sources

• Polydactylus sextarius on WikipediaEncyclopedia
• More photos on Wikimedia CommonsPhotos
• Wikidata recordOpen data