The type locality on Trangan Island lies in a luxuriant rainforest on the downstream side of a spectacular 75 m-long limestone tunnel. The stream has about 10-12 m width, 0.5-2.0 m depth and relatively clear and slow-flowing water with a rocky limestone bottom and abundant cover mainly in the form of submerged fallen tree limbs. Another location on Kobroor Island where the species occurs is also lined by luxuriant forest and has 15-20 m width, 1-3 m depth, and clear and slow flowing water. The margins of the channel are composed of abundant, partially submerged terrestrial plants, which offered abundant shelter for small fishes .
Dorsal spines (total): 5 - 7; Dorsal soft rays (total): 10 - 13; Anal spines: 1; Anal soft rays: 20 - 23. Melanotaenia aruensis is distinguished by the following combination of characters: V to VII-I,10-13 dorsal-fin rays; I,20-23 anal-fin rays; 14-15 pectoral-fin rays; 34-36 scales on lateral line, 15-18 predorsal scales; 13-19 cheek scales; 17-18 total gill rakers on first arch; greatest body depth of adult male 35.1 % SL; least depth of caudal peduncle 9.5-11.1 % SL; colour in life grey-brown dorsally on head, nape, and adjacent anterodorsal portion of body; posterior two-thirds of upper half of body bluish grey with narrow bronze margin around each scale and bronze stripe between each horizontal scale row; dorsal and caudal fins semitranslucent grey, second dorsal fin with relatively broad whitish margin .: fusiform / normal.
Behavioural data for Aru rainbowfish is currently being reviewed.
Diet information for Aru rainbowfish is currently being compiled.
Reproduction data for Aru rainbowfish is currently being compiled.
Freshwater; benthopelagic; pH range: 8.0 - ?. Tropical; 27°C - ?
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