The Dusky Narrow Hatchetfish is native to river channels of much of the Amazon Basin, including in Ecuador, Bolivia, Peru, Venezuela and Brazil.

The hatchet shape with deep keel has powerful chest muscles which allow the fish to leap well out of the water and glide several metres, both to evade predators and also, especially in flooded forests, to pick fruit, seeds and insects/spiders from the trees or to catch insects in flight. It also feeds on those which fall to the water surface.

Most, if not all, of the Triportheus species look very similar to the Dusky Narrow Hatchetfish above. Most also follow their Latin epiphet, e.g. angulatus above is sharply angled, elongatus (not shown) is slimmer and elongated. Looking at those above, most show the typical hatchet shape with a flat top, deep keel and narrow caudal "wrist". Four are slightly more "fish-shaped" (the first one and the last three). These may actually be rotundatus.

However, the collection, which does hold rotundatus together with angulatus, claims that the Brycon-looking species immediately above is Triportheus rotundatus, which has a variety of common names varying with the source but which is said for these to be Flagtail Butterfly Tetra rather than a Narrow Hatchetfish. Other sources show a very different fish, one looking like those further above, for the T. rotundatus, by whatever common name.

The above does have a somewhat deepened chest/keel and may be rotundatus or may be a Brycon or something other. I am still trying to confirm its identity.