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Articles on Characiform families, subfamilies...: The Larger Pencilfishes, Family Anostomidae, Characid/Tetra Fishes, Alestiine Characid Fishes, Characinine/Tetra FishesPiranhas and Relatives, subfamily Serrasalminae, Tetragonopterine/Tetra Fishes, Cardinal Tetras, Distichodus and More, Family Citharinidae, Pike-Characoids, Family Ctenoluciidae, Curimatidae, Trahiras, Family Erythrinidae, Hatchetfishes, Family GasteropelecidaeHemiodus "Sharks" and More, Family HemiodontidaeThe Pike-Like Hepsetid, Family HepsetidaeSmaller Pencilfishes, Splashing Tetras & More, Family Lebiasinidae, Prochilodus/ontids

Survey Articles on: Freshwater Fishes

Extreme Characins 

Part 1: Hatchets, pikes, and other lethal weapons


© Neale Monks


It's easy to assume that characins and tetras are one and the same thing. In fact, the characins are a very diverse group of fish, running the full range of types from small, goby-like species that inhabit leaf litter through to enormous open-water predators that can fairly be described as freshwater sharks! So why do aquarists so frequently assume that all characins are tetras? Perhaps because it's the tetras that dominate the hobby -- tetras are, almost always, small, peaceful, and sociable fish that have bright colours and lively personalities. They consequently make excellent community tank residents. Many species also happen to be inexpensive and relatively hardy, making them good fish for beginners, too. But for the more experienced fishkeeper after oddball species, characins also have plenty to offer, and that's the focus of this series of articles.

But what is a characin? They are relatives of the catfish, carps, loaches, and knifefish, collectively known as the Ostariophysi, which all possess a structure known as Weber's Apparatus, a series of bones that connect the ear to the swim bladder. The swim bladder acts as a 'sounding board', something that amplifies sounds. The hollow bodies of violins and guitars, for example, are sounding boards. Thanks to Weber's Apparatus, the Ostariophysi have unparalleled sensitivity to sounds and vibrations. More specifically, while catfish lack scales, characins usually have them. Characins also lack the whiskers typical of catfish. They are obviously different to knifefish and loaches in shape, and unlike loaches, they do not have barbels around the mouth. Characins and carps are easily confused, but broadly speaking, characins have a small fin in between the dorsal fin and the tail fin, known as an adipose fin, that carps lack. This isn't a hard and fast rule though, as a few characins don't have an adipose fin either, such as pencilfish and hatchetfish. The biggest difference between characins and carps is that while characins have mouths filled with teeth, carps only have teeth on the pharyngeal bones in the throat; the mouth itself lacks teeth entirely.

Hatchetfish, family Gasteropelecidae

Hatchetfish are among the most widely traded oddball characins, presumably because they are peaceful and not particularly difficult to keep. They are basically tetra-like in temperament, but what makes them special is their ability to fly. Hatchetfish have massively expanded pectoral fin muscles attached to a very deep sternum, a striking parallel with the similar development of the pectoral muscles and breastbone in birds, which also rely on those muscles for powered flight. Viewed from the side, hatchets have a very distinctive, semi-circular shape quite unlike anything else in the aquarium. If viewed from above, they are just as peculiar, as instead of being short and flat, the pectoral fins are very long and slightly curled, which makes them much more useful as 'wings'. So while there are many fish that can glide if they jump out of the water at speed, not least of all the celebrated marine flying fish, only the hatchets have true, powered flight. By rapidly fluttering their pectoral fins, they can extend their jumps out of the water to cover several meters. For fish that live close to the surface and in open water, this is an excellent defence mechanism as it quickly gets them out of the range of large predatory fish. However, in an aquarium flying is something that can lead to an untimely death dried up on the carpet, so it is important to keep an aquarium containing these species covered at all time.

Hatchets range in size from small species about 2.5 cm/1" in length like Carnegiella myersi up to about 9 cm/3.5" in the case of Thoracocharax securis. None of the hatchets are especially brightly coloured, though they are all attractive enough in their way. The marble hatchetfish, Carnegiella strigata, is perhaps the standout species in terms of overall prettiness. Although barely 4 cm/1.5" in length, their deep body shape and chocolate brown markings mean that these fish can look very dramatic in the right aquarium. As with hatchetfish generally, open water with a plenty of current is preferred, and while rockwork, ornaments, rooted plants, and other such things are unnecessary, a few floating plants will be appreciated. These fish stay very close to the surface, and any food offered to them should be of the floating variety. Flake will be eaten, but this should be augmented with suitable live or frozen foods, such as bloodworms, mosquito larvae, and wingless fruit flies. Marbled hatchetfish do have a preference for soft and acidic water conditions, but they can be kept successfully in slightly alkaline, moderately hard water provided their other needs are met. As with all the smaller hatchetfish, these fish are skittish and easily bullied, so combine only with peaceful tankmates. Neons, cardinals, pencilfish, and other small, peaceful Amazonian fish would make ideal tankmates.

Typical of the larger hatchetfish is the silver hatchetfish, Gasteropelecus sternicula. The German aquarist Florian Krieger has referred to these delightful animals as 'grey mice', a reference to their tendency to be overlooked in comparison to the jewels of the aquarium. Certainly, these aren't immediately eye-catching fish: they are basically silvery-grey in colour with a blue-grey stripe running from behind the pectoral fins to the base of the tail. But what does make these fish worth keeping is their liveliness. Silver hatchets squabble among themselves almost constantly, and will frequently engage in chases around the tank. Though no harm seems to be done, it's a good idea to make sure you keep more than two specimens so that one fish cannot constantly bully another. In fact all hatchets are best kept in schools of six or more, especially the smaller species. Silver hatchets are fairly robust animals and will adapt well to most conditions. Neutral, not too hard water seems to be best, and as far as tankmates go, pretty much anything not overtly aggressive or nippy will be tolerated. Unlike the smaller hatchetfish, these larger species will swim at all levels of the aquarium, and feeding presents no problems at all: they will eat anything small and meaty, including livebearer fry. Flake is also enjoyed. Silver hatchets appear to have insatiable appetites. They are rather active animals, and it might well be they need a little more food than comparably sized tetras. Healthy, well-fed hatchetfish should have a chunky shape when viewed from the front. Starved specimens often have concave flanks, and may need special care in a quarantine tank before they can be mixed in with ordinary community fish. Otherwise, once settled in, these fish are not particularly difficult to keep.

The spotted hatchetfish, Gasteropelecus maculatus, is one of the biggest species and can reach up to 9 cm/3.5" in length. Again, it is a silvery fish, though perhaps a shade more brilliant than the silver hatchet, and it is immediately recognisable thanks to the pattern of small blue-grey spots on the back half of the body. Given its size and shape, this fish can be very impressive when kept in a decent sized group, and while not as commonly traded as marble and silver hatchets, it is not too difficult to obtain. This species looks superb kept with things like angelfish and discus, but it could equally easily be kept with peaceful gouramis, climbing perch such as Ctenopoma acutirostre, clown loaches, or any other non-aggressive tankmate of suitable size. The spotted hatchetfish needs fairly soft, slightly acidic to neutral water conditions and plenty of oxygen. It is a bit less forgiving than the silver hatchetfish, and good filtration and plenty of water changes seem to be essential to long-term success with this species. A superb fish, to be sure, but one best suited to the more experienced aquarist.

Other fish not belonging to this family are sometimes called hatchetfish, including a great many deep-sea fishes. The only ones of importance to the aquarists are the minnows of the genus Chela, sometimes sold as 'Indian hatchetfish' or 'Asian hatchetfish'. While danio-like in terms of overall care, their body shape is strikingly similar to that of the characin hatchetfish discussed above. Chela caeruleostigmata in particular has the same deep body and extended pectoral fins as its namesakes. Unsurprisingly, these fish are adept jumpers as well, and need to be kept in a securely covered aquarium; in fact, one common name for these fish the 'leaping barb', which underlines nicely just how similar they are to the South American hatchetfish.

Pike-characins, family Ctenoluciidae

Like the hatchetfish, the pike-characins are peaceful South American characins that inhabit open waters and stay close to the surface at all times; but there the similarities end. These fish are schooling predators that view smaller fish as food. They are fairly frequently offered for sale as oddball fish though, and in the right aquarium, can make excellent community fish. As is often the case with specialised predators, these fish are not aggressive at all, and actually want pass their time unnoticed by other fish so that they can more easily creep up on their prey.

Just as with hatchetfish, when scared, these open water fish instinctively jump or dash about the tank. While that may be helpful in a river or lake, in an aquarium these fish can end up damaging themselves. Injuries to the snout in particular are common and often become infected, leading to the death of this fish. Keep pike-characins either among their own kind or in a community tank with peaceful tankmates such as plecs or large barbs. Besides being easily frightened by aggressive tankmates, pike-characins will also panic if the lights come on too suddenly over the aquarium, especially if the room is dark. On the other hand, floating plants seem to calm these fish down, and tall plants around the edge of the tank, such as Vallisneria and Amazon swords, help to keep these fish from slamming into the glass when alarmed. If you're going to keep these fish, you do need to plan ahead, and choose plants and tankmates with care.

There are two genera in the Ctenoluciidae, Ctenolucius and Boulengerella, between which seven species are known. Boulengerella lateristriga is perhaps the one most frequently sold, though under a variety of names including striped pike-characin and striped gar. It is usually sold at an appealingly small size, around 15 cm/6" being typical, but this fish can reach a maximum size of 26 cm/10" when fully grown (reports of larger specimens appear to be misidentifications). None of the pike characins are 'easy fish' and the striped pike-characin is no exception. It needs a large, mature aquarium with excellent filtration and frequent water changes must be carried out to ensure that the nitrate levels are kept low. Soft, slightly acidic water is preferred, and likely essential to long-term success. Certainly, very hard and alkaline water should be avoided.

Feeding these fish is another headache. In the wild these fish are predators, taking insects when young and small fish as they mature. Since all the fishes sold are wild-caught, weaning them off live foods and onto frozen substitutes will be a major hurdle. Earthworms, mealworms, crickets, and other such foods will work as either a stopgap or a permanent diet, and it is definitely important to get your specimens feeding properly as soon as they are introduced to the aquarium. Once settled in, silversides and lancefish can be used effectively. These fish are crepuscular hunters, meaning that they are most likely to accept food at dawn and dusk. As with many other fish of this type, they hunt by sight, lunging at flashes of silver or white. Tossing a defrosted fish into a strong current of water within the tank usually does the trick, by fooling the pike-characin that the target is alive. Starving the pike-characin for a day or two does no harm if you want to whet its appetite, but do bear in mind newly imported specimens will likely not have eaten a proper meal for several weeks.

The spotted pike-characin, Boulengerella maculata, is broadly similar but a bit bigger, getting to about 30cm/12" in length.

Ctenolucius hujeta is known variously as the hujeta, the rocket gar, and the blunt-nosed gar (this latter name on account of its slightly upturned snout). Aquarium specimens can reach a trifle over 20 cm/8", though wild fish are said to get to more than twice that size (though as with Boulengerella lateristriga, some of these giant specimens may in fact be misidentifications). Ctenolucius hujeta is silvery-green in colour with a prominent dark spot on the base of the tail. Water chemistry and quality need to be similar to those required by Boulengerella spp, but on the whole Ctenolucius hujeta has proved to be a bit more amenable to life in captivity, and has even been bred in aquaria.

A closely related species, Ctenolucius beani, is superficially similar but larger species, getting up to 28 cm/11" in length. It is distinguished from Ctenolucius hujeta by being covered in dark blotches when young and small dark spots arranged in neat rows as an adult. Care is otherwise identical. All the pike-characins are sometimes sold as freshwater barracudas, though in fact that name is more properly applied to a completely different family of characins, the Acestrorhynchidae, a fascinating group of characins of which more will be said next month!

Sidebar: The blind cave tetra, Astyanax mexicanus

Blind cave tetras are a peculiar variety of Mexican tetra, Astyanax mexicanus, that inhabit limestone caves. Living in complete and perpetual darkness, they have no eyes and no skin pigmentation, and for food they depend very largely on bat droppings, which contain large amounts of undigested insect remains. Because they cannot see their food, as would Mexican tetras living above ground, they instead use a heightened sensitivity to sounds and vibrations. When something hits the surface of the water, they will swim towards it. They can also use their sensitivity to vibrations as a sort of pathfinding sense, allowing them to locate other individuals in the group as well as the physical boundaries of their environment. 

Cave tetras are a bit pushy and can be nippy, so while they can be kept in an ordinary community tanks with robust species, they are most rewarding kept their own aquarium. Black sand and upright pieces of slate will work very nicely, and for illumination consider a moonlight or Grolux tube instead of normal aquarium lights. Catfish such as plecs and Synodontis will work perfectly in such an aquarium and make excellent companions for these fish. Hard, alkaline water is preferred but these fish are very adaptable, and they will eat practically anything, including flakes, pellets, bloodworms, and most small live foods. Cave tetras get to about 8-10 cm/3-3.5" in length and are relatively easy to breed if the water is cooled down to about 18Ë°C (64Ë°F). Surprisingly perhaps, the fry have normal eyes, and only lose them as they mature.

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