Clarias nigricans is a species of clariid catfish

Catfish

Catfishes are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the heaviest and longest, the Mekong giant catfish from Southeast Asia and the second longest, the wels catfish of Eurasia, to detritivores...

. It is only known from the Mahakam River

Mahakam River

The Mahakam River is in Indonesia. It flows 980 km from the district of Long Apari in the highlands of Borneo, to its mouth at the Makassar Strait....

 in eastern Borneo

Borneo

Borneo is the third largest island in the world and is located north of Java Island, Indonesia, at the geographic centre of Maritime Southeast Asia....

. The eel

Eel

Eels are an order of fish, which consists of four suborders, 20 families, 111 genera and approximately 800 species. Most eels are predators...

-like catfish of this region were long assumed to belong to the widespread species Clarias nieuhofii

Clarias nieuhofii

Clarias nieuhofii is a species of clariid catfish. It has a wide distribution in Southeast Asia including peninsular Malaysia, Java, Sumatra, Borneo and the Philippines....

but examination of 6 specimens purchased from a fish market in Samarinda

Samarinda

Samarinda is the capital of the Indonesian province of East Kalimantan on the island of Borneo. The city lies on the banks of the Mahakam River. It is the most populous city in East Kalimantan with a population of 726,223...

 in 1999 and preserved specimens from the same location revealed consistent and distinctive differences indicating a separate species.

This is an elongated catfish up to 32 cm standard length

Fish measurement

Fish measurement refers to the measuring of the length of individual fish and of various parts of their anatomy. These data are used in many areas of ichthyology, including taxonomy and fisheries biology.-Overall length:...

. Compared to C. nieuhofii it has a "neck" distinctly narrower than the head, giving the head an egg-shaped appearance when viewed from above or below and the overall colour is much darker: purplish grey with much fainter white spots arranged in one row below the lateral line

Lateral line

The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

instead of two.