Simply because many of the fish called characins or tetras are commonly traded, and kept by beginners to fishkeeping, people can be dismissive of small tetras on the whole. Either they are dismissed as purely for the novices, or they are mainly there to add dither to a lushly planted, underwater garden. Yet such a rich evolutionary radiation can scarcely be dull, and there are no other fishes on earth that so resemble the freshwater hatchetfishes. The remarkable thing is that these are among the most surprising fishes to have ever evolved, yet they have also remained perfect examples of what a typical tetra is - for the aquarist these are pleasantly peaceful, active, schooling community fishes.
Carnigiella strigata, the marbled hatchetfish, is very often traded in aquarium shops, and it could well stand to represent the entire hatchetfish clade. This is a 3 and a 1/2 centimeter, or 1 and a 1/2 inch schooling fish, that is a native of the blackwater rivers and streams in northeastern South America. These fish are insectivorous and swim near the water surface, close to the bank and in association with overhanging vegetation above; these supply the C. strigata below with a supply of prey, and this species is considered to be dependent upon the flooded forests within which it has evolved. Their mouths are upturned towards the water surface, because this is where they seize floating food items from below. It is likely that C. strigata is, in fact, a confusing species complex; and if it is not, there are a few geographical races that have evolved within C. strigata.
Hatchetfishes have always amazed zoologists because, unlike the marine flying fishes or exocets - which are related to halfbeaks - the hatchetfishes can beat their fins so as to create lift. This ability places them as one of a select few clades of animals - together with winged insects, bats, birds, and the extinct pterosaurs - to be capable of powered flight. They are also thought yo be the only animals that have evolved true flight from an underwater starting point, as opposed to an arboreal one. This the flight of hatchetfishes was and is a great novelty to science, but one that is too often ignored, simply because it feels irrelevant to discussions like 'trees down or ground up'.
Broadly speaking flight refers to the movement of any object, living or otherwise, through the air or water without contact with a solid substrate. A number of animals possess the ability to either glide horizontally, or to parachute safely downwards from above; even humans, when we fall a long distance, instinctively attempt to parachute by positioning our limbs out for a safe landing. Most living and extinct flying fishes merely glide themselves far, after leaping into the air; however to the zoologist this is not the same as powered flight, in which the actions of the airborne animal work to generate lift, that keeps it airborne.
Although hatchetfishes are not as proficient in the air as are birds or bats, they are nonetheless able, once they have launched themselves, to keep themselves above the water surface by quickly beating their paired pectoral fins. Normally this behavior allows them to skim along the water surface, or hydroplane; but they are authentically capable of true flight, without contacting a firmer substrate below. The initial launch is caused when the fish suddenly thrusts its enlarged pectoral fins downwards, in combination with thrust from the tail; by timing these two actions the hatchetfish predicts and controls its leaps. The tail motion in question is obviously evolved from the motions of a fish that is startled, confirming that hatchetfish flight first evolved for evasive reasons. Whereas beats of the same, long pectoral fins when underwater, are used by the fishes to propel themselves fast towards food items.
The pectoral fins of freshwater hatchetfishes are not just hydrofoils that have become aerofoils - a simple shift from he medium of water to that of air; they are truly functional as beating wings. To enable this the hatchetfish evolved an impressive musculature, which in turn required they evolve a well developed skeletal support, akin to the keeled breastbone of birds. Thus their pectoral fins or wings, and the bones and muscles of their thorax, have become tightly integrated for the purposes of flight, as they have in the birds. In fact a quarter of the weight of the hatchetfish is its flight-related anatomy, and this is why these peculiar looking tetras have such a unique, deep bodied appearance. Normally they swim with their pectoral fins or wings held close to vertically upright, and they swim when they are underwater like other tetas, by using horizontal movements of their tails.
When C. strigata are observed in the wild, their natural environments feature leaf litter, tree roots, and fallen branches. Therefore in the aquarium also, an apt aquascape for C. strigata should include wood and leaves. The water in their habitat is rich in tannins, soft, and naturally on the low end of the pH scale. The pH might well be very low in the wild, to a degree that would impede normal aquarium biofiltration; fortunately these fish fo well at a higher pH, even up to around neutral. But I still recommend the addition of tannins to their water by some means, and to that of all blackwater fishes; this is because the blackwater conditions of their natural habitats, suppress pathogens that are thus unfamiliar to such fishes. I am convinced that it is tannins and not the pH per se, that is critical to 'difficult' blackwater fishes. Where these hatchetfishes are found the water temperature is reported as 27 to 29 degrees centigrade, although they are found where overhanging vegetation provides them cool shade, just as it does their food; indeed they become chronically ill when the water is only a little warmer than this.
It should go without saying that these fish are likely to leap out of the aquarium, unless it is fitted with a lid. They might also injure themselves against the glass when they leap, and older aquarium literature ferks paranoid about this possibility; but as hatchetfishes are more normalized and even beginners have kept them, it is clear that such tragedies are a rarity. But of course the possibility of an accident should not be discounted - in the wild they would feel safe among hanging roots and branches. In most regards, these are a wonderful fish for an ordinary community tank, and especially a well planted one; they will school per their natural habits near the surface, not starting far into the water column below, and adding liveliness to an often overlooked region of the aquarium. As insectivores hatchetfishes are easy to feed on proprietary preparations, and on tiny defrosted 'prey'; because it floats, and old fashioned flake of correct composition seems particularly appropriate, and hatchetfishes consume it with gusto.
