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| Melanotaenia nigrans [Scotts Creek, NT] - photo© Günther Schmida |
(Richardson, 1843)
Black-Banded Rainbowfish
Description
Melanotaenia nigrans is the type species of the genus Melanotaenia and was collected by John Gilbert, who later perished somewhere in the Australian wilderness with the famous German explorer, Ludwig Leichhardt. In 1863 the American, Thomas Gill, created the genus Melanotaenia for this species; the name being inspired by the typical black mid-lateral band.
Colouration includes a continuous distinct black band in the mid-lateral position. Above the lateral line the colouration is generally an olive-grey brown colouration and silvery-white below. Colour variations can be found in the different geographically located populations. Specimens from the Kimberley region have several lines of dots below the lateral line and red in the fins. Blue colouration in the body and fins is also found in some populations. When spawning the males generally take on an even more brilliant colouration with the forehead nape area and pectorals fins becoming an orange shade, while the dorsal and caudal fins become yellowish. Above the mid-lateral stripe, the body becomes a dark grey but behind the dorsal fins it assumes a golden colour. Below the lateral line the body is iridescent mauve. Females are much paler in colour than the males.
Distribution & Habitat
Melanotaenia nigrans has a discontinuous distribution across northern Australia, from the Kimberley region in Western Australia, across the northern part of the Northern Territory to Cape York Peninsula in northern Queensland, including a number of offshore islands such as Groote Eylandt in the Gulf of Carpentaria and Prince of Wales Island in Torres Strait. Discontinuous distribution of fish species appears to be a feature of much of the northern Australian fish fauna. A species found in one river system may not necessarily exist in an adjoining system. Most specimens have been collected from the lower reaches of streams within about 50 km of the coast; however, they have been collected in the upper South Alligator River some 130 km upstream. In Western Australia, Melanotaenia nigrans have so far only been collected in Dominic Creek; however, they probably occur elsewhere. Dominic Creek is a small isolated stream between the Drysdale and King Edward Rivers. Unlike other nigrans varieties, they have a row of reddish-orange dots below the black stripe and may prove to be genetically different.
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| Melanotaenia nigrans [Dominic Creek] - photo© Bruce Hansen |
Melanotaenia nigrans have been found in a variety of freshwater environments but seem to prefer slow-flowing clear water streams, billabongs, and swamps with abundant aquatic vegetation. A temperature range of 23-35°C has been recorded in their natural environment and pH 3.9 to 6.7; hardness levels usually below 50 ppm; conductivity 4 to 180 µS/cm. They are most frequently found in clear waters with sandy substrates, followed by rocks, leaves and mud. They are usually found around sub-surface vegetation, submerged logs, or branches. They are often found in streams with Melanotaenia australis and/or Melanotaenia splendida inornata or both.
Biology
Melanotaenia nigrans are sexually dimorphic. In males, the spines of the first dorsal are usually extended and may lie well past the origin of the second dorsal when not erect. The posterior rays of the second dorsal and anal fins are extended caudally and may extend past the origin of the caudal fin. In females, the first dorsal spines are short, not reaching the origin of the second dorsal. The posterior rays of the anal and second dorsal fin are not extended. The spines and outer rays of the ventral fins of some males are also extended and may reach past the vent and the origin of the anal fin. They can be recognised by a rather slender body than most other rainbowfishes and may reach a maximum size of 12 cm, but usually less than 7 cm.
Very little is known, however, about the biology of this species in their natural environment. Most information on Melanotaenia nigrans is mainly based on aquarium observations. Small (less than 20 mm) juvenile fish have been collected in all seasons. Therefore it is difficult to define their breeding season. However, a peak in reproductive activity was recorded during the early-wet season. Melanotaenia nigrans appears to breed in small streams that contain deep shaded pools with roots and submerged vegetation around the edges. Eggs may be attached by threads to such vegetation. Spawning is possibly continuous, with a few eggs laid at a time, or opportunistic whenever conditions are favourable. In one study of ovaries, the number of eggs ranged from 220 to 500 (mean = 344); egg diameters were not measured. These fish were not mature, so numbers only indicate developing eggs within the ovary, not how many might actually ripen during spawning.
Melanotaenia nigrans is an omnivore feeding opportunistically across substrates and in surface waters, with possibly less emphasis on mid-water areas. The main food items are aquatic insects, algae and terrestrial insects. The diet varies in relation to the habitat they occupy. In the mainchannel waterbodies they eat mainly aquatic insects, with small amounts of terrestrial insects, plant material and algae. In perennial streams, algae and terrestrial plant material are less important, while aquatic insects and, to a lesser extent, oligochaetes and microcrustaceans, are consumed. The diet in the lowland sandy creekbeds had much larger algal and terrestrial insect components. Specimens examined from the floodplains feed mainly on aquatic arachnids and aquatic insects, and a small amount of algae.
Remarks
From the very beginning of its introduction to the aquarium hobby, until around the mid 1960's, Melanotaenia duboulayi was mistakenly identified in both Australian and International hobby publications as Melanotaenia nigrans. Even in the mid 1980's it was still being incorrectly identified in some hobby publications. The real Melanotaenia nigrans never entered the International hobby until around 1976, when specimens were sent to Germany.
© Copyright Adrian R. Tappin
Updated April, 2007.
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