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Amongst the archaic "living-fossil" show specimens for aquariums, some of the bony-tongue fishes rank supreme. One, the arapaima ("air-ah-pee-mah") holds the title as largest freshwater fish period; some relatives that share it's geography are mere snacks at a 3-4 feet in length!
These "oddball" fishes are the domain of aquarists with huge tanks and feeding budgets that appreciate the wonder and awe of friends and family. After graduating from juvenile size all species prove hardy and generally long-lived, given decent filtration, water changes, lots of food... and that their tank top is secure (boy do they jump!).
The arapaima, handful of Arowana species and Featherback Knifefishes are definitely not for everyone; but for the advanced freshwater enthusiast who has the time, money and tank space, here are my notions of how to select, rear, and maintain these "tank-busters".
Classification: Taxonomy, Relation With Other Groups
The bony-tongue fishes make up one of the lower Orders of true bony fishes, the Osteoglossiformes, roughly translated as "bony tongue forms". The common and scientific names reference the presence of teeth on the parasphenoid and tongue bones that make up the principal bite of these primitive fishes.
Numerous other internal characteristics tie together the six families, and 29 genera of about 217 species of osteoglossiform fishes; we will list all the family groups here though only covering the two in depth.
Most of the "other" bony-tongue fish families should be familiar to you; the one species of freshwater Butterflyfish, Pantodon buchholzi, Family Pantodontidae; the Elephantfishes like the baby whale and Elephantnose, Family Mormyridae, with a surprisingly large number of 18 genera and 198 species; the monotypic Family Gymnarchidae, with the lone species Gymnarchus niloticus. Only the systematic aquarists and freshwater fishermen of North America will know the two species of mooneyes, Family Hiodontidae.
Our focus here are the two remaining families the Osteoglossidae and Notopteridae. So many of their species are of interest to aquarists that we'll list them all.
The family of osteoglossids are noted for their possession of an epibranchial organ that they utilize in breathing atmospheric air. There are two subfamilies, of four genera with seven living species (and many more extinct ones).
The subfamily Heterotidinae lacks the "feelers" (mandibular barbels) of the Arowana subfamily. There are two species, the gargantuan arapaima or pirarucu (Arapaima gigas) of South America; up to 3 meters length, and the one meter Heterotis niloticus, sometimes sold as the "African Arowana", of western Africa.
The subfamily Osteoglossinae are the Arowanas/aruanas/arawanas with mandibular barbels. There are two South American members, the silver Osteoglossum bicirrhosum and handsome black Osteoglossum ferrarai; two Australian Scleropages, Scleropages jardinii from the north and New Guinea, and Scleropages leichardti of the Fitzroy River, Queensland. Lastly the "Asian Arowana", gold, red or plain silver, Scleropages formosus of Southeast Asia, Sumatra and Borneo rounds out our list.
The Old World, Featherback or Featherfin Knifefishes come to us from Southeast Asia to Africa. They're characterized by having a long anal fin confluent with their small caudal, a small or absent dorsal. Four genera of eight species. Chitala with four species has been raised to generic rank; it formerly was a subgenus of Notopterus, which now is monotypic. The "African Knifefish" Xenomystus nigri, is the Featherfin without a dorsal, the "odd-fish out" amongst those here, great even for larger-fish community tanks. The remaining genus, West African Papyrocranus has the two species of West African Notopterids that lack pelvic fins altogether.
Species of Use/Availability to Aquarists:
All the above species make their way into markets in some number, with only a few of Papyrocranus making their way all the way to the U.S. (Papyrocranus afer, the Mottled African Featherfin used to be labeled Notopterus afer). I have only seen Papyrocranus congicus once. The most common Notopterid is easily, the Clown Knifefish, Chitala chitala, with its numerous emarginated ventral eye-spots, though the gorgeous Royal Knife, Chitala blanci, with many more markings would outsell it if it were more available.. Notopterus notopterus, the Asiatic Knifefish, though being a somber gray overall is infrequently seen.
Trade in Scleropages formosus is restricted/controlled in some countries; all members of the genus are considered endangered by the U.S. and German authorities, as is Arapaima. All South American and Australian Arowana species are seasonal items as young. Featherfin Knifefishes also show annual flux in availability, though mid-size pond-kept individuals can be had pretty much year round, and large ones as "trade-ins" abound.
The arapaima is available in the hobby from time to time, but should only be taken on by the serious, informed individual. It is a touchy feeder (a filter type as very young, less than a foot), and grows very large, quickly, necessitating the largest of aquariums.
The "tiny" member of the group is Xenomystus nigri, the African knife at about eight inches maximum. The other Featherfin Knifes get BIG, Chitala chitala and Chitala lopis to 1.5 meter; yep, about five feet, most of the others to three.
We've mentioned the arapaima, in the wild caught at near ten feet long. Heterosis and the Scleropages Arowanas top out at three feet or so, and the two South Americans at about four.
A comment here and later regarding the urban myth of "Oh, they'll grow to the size of their aquarium". No, this isn't the whole or real story. You probably won't "bonsai" a bony tongue by confining it to a too-small space. Out of fright, frustration (yes, I'll use that word), physical trauma, yours will jump out, dash itself into oblivion. Count on these fishes approaching 50-75 % of their wild growth over their effective lifespans in captivity, and be able/willing to commit to their concomitant system requirements... try one of the smaller Notopterus species, or Xenomystus... or just visit them in huge public aquariums.
Selection: General to Specific
We are covering these seemingly disparate species not only because of their taxonomic affinities, but their shared physiology (e.g. aerial respiration), temperaments (i.e. bad), and related husbandry; such as selection & rearing.
1) Size: is of critical importance to survivability. Smallish (under 2 1/2"), starved specimens die in droves every year. Slightly larger individuals are tremendously heartier.
2) Feeding: Make sure yours is feeding at whatever size you acquire it AND that it continues to do so. For Arowanas at sac-size, still sporting a nutritive egg-sac, keep an eye out that they're not going thin from high activity and immoderate food intake. A great "trick" to use in the trade or home hobby is to lower the water level in their system to optimize food availability and stimulation to feed. Brine shrimp, daphnia, mosquito, glassworm, bloodworm other live insect larvae are superb.
Similarly for Notopterids, a shallow bottom glass worm feeder with continuous Tubificid offerings will ensure a robust specimen.
Arapaima Arapaima and Heterotis at zooplanktivorous juvenile phases are very difficult to rear; endorse their proper collection by buying them only at larger sizes.
Keep these fishes plump when young and lean as adults.
3) Scrapings: are a bad sign, as they're indicative of beating... by other fishes, netting... In particular they show up as "eye fungus", a bacterial infection that can lead to appetite and outright fish loss. Therefore an important note here regarding moving these fishes. Unless they're tiny, skip actually using a net as such, instead using the net to direct the specimen into a submersed bag (or two thick) and lift the fish out in the bag.
Big, dark, and open describes the ideal space for these animals. Suitable floating plants, like Riccia or the water fern Ceratopteris provide good cover and do most everything live plants do for a captive system. Rooted varieties need to be tough and securely positioned to put up with the thrashing movements during food searching and seizure, as well as the digging habits of Notopterids.
Water of mid-pH (6.5-7.5), temperature (70's, low eighties) and medium hardness is fine. The emphasis with these fishes as most captive aquatics should be on stability, keeping your water near consistent through filtration and water changing, not intentionally fooling with any given set of values.
These fishes bite! I'm not kidding. Isn't it bad enough that they're so jumpy and disagreeable with each other? Keep your hands clear of them in cleaning and feeding. Perhaps this is the origin of the term "finger food"? Definitely not a good idea to hand feed.
Two mutually exclusive aims exist in providing filtration of these aquatic animals; high volume coupled with quiet circulation. Intakes and discharges should be as widely spaced as possible, and arranged to disallow discontinuity by bumping. For processing such large amounts of food/waste, you will want a good two or more turns per hour through your filtration, located outside the system.
As large, big, humongous system that you can muster; replete with some hiding spaces for the knifes, entirely open surface for the Arowanas and Pirarucu... with a VERY SECURE TOP.
Lighting should be subdued, providing just enough energy for your live plants (floating forms do nicely), and arranged on a system of timers, rheostatic control such that changes from darkness to light are gradual so as to not spook your fishes. For Notopterids which are nocturnally active, some small red lighting or low illumination can be utilized for your viewing.
Likewise, you want to have their tank located out of main traffic where the least amount of walk-by or tank-bumping occurs.
With the exception of Xenomystus all these fishes are extremely antisocial as adults, fighting and damaging members of their own family's to the point of death. As such they should be displayed in "species" tanks, one member of the family per set-up... unless you have HUGE space available (hundreds, thousands of gallons).
Though they can be individual differences, most specimens tolerate other fishes too large to swallow. Allow me a moment to illustrate. Ages back when I did service work, taking care of aquariums in restaurants, hospitals and rich folks homes, we had an account of about 125 gallons in a pediatrics waiting room. This tank had an 18 inch silver Arowana, and some fast-moving silver dollar-type fishes (Myleus, Mylossoma). Good for those characins too, as this silver had quite a "snappy" personality. Well, thinking the tank needed "something" for the bottom, I added a ten inch shovel nose catfish (Sorubim lima). Sure as heck, coming in one night to do the weekly maintenance I glanced at the tank, and not seeing the catfish, looked feverishly around the outside of the system. Well, you guessed it, hanging out from the Arowanas mouth? The tail of one expensive Sorubim meal. Live and learn from my errors.
This is an extremely critical time for your new charge. Take care to turn the lights off, inside the tank and even outside if it's very bright.
If it's not too much trouble, I'd even lower the water level down a few to several inches to alleviate injuries due to jumping, facilitate feeding training, and slowly raise it back up over the next few days.
Due to trauma-cost, this is one group of fishes that I discourage utilizing a quarantine operation with. Instead, utilize a dip/bath procedure (in the transport bag if you're dealing with an adult specimen), and pour the new fish into it's new home. If the tank has inadequate dark spaces, make at least one good temporary one available with a clay pot, plastic pipe, etc. to provide solace.
For very small (a few inches) specimens, only shy and retiring tankmates should be attempted; others will intimidate and out-compete them for food.
Larger, semi- to adult forms will do best placed after other fishes have become established, to reduce territorial fighting, as long as you make sure they're getting food.
Rest assured, all but the little Xenomystus will attempt and likely inhale other fishes if they are slow or small enough, even armored catfishes. Choose tankmates well.
Reproduction, Sexual Differentiation:
Amongst the fishes here, only the Arowanas may be sexed externally; Notopterids and the arapaima display no distinguishing characteristics.
Featherfin Knifes practice male brood care, guarding fertilized eggs for about two weeks, fanning them with their pectoral fins. P. afer has been bred in captivity (Ong Kay Yong, TFH 1965), and Notopterus notopterus (Van Pixteren, DATZ 27, 364-369)
Heterotis Heterotis is a nest builder, with the female assuming guard duty following spawning for 3-4 weeks. Young reportedly hatch out in about a day. The arapaima has a similar spawning routine, but with the male taking on protection of the nest and young for 2-4 weeks.
Osteoglossids and Notopterids use their long unpaired fins and sinusoidal body musculature to make dramatic, or shall I state "spectacular" bursts of forward propulsion, reserving the pectorals for turning and rising. Again, I bring up the propensity for jumping. They can and will literally leap out a few feet of an unguarded top space, or if spooked into the top or side wall with as much force. My advice? Utilize low light, floating plant material, and slow precise movements when working in or around their systems.
Feeding/Foods/Nutrition: Types, Frequency, Amount, Wastes
Most of these fishes are thoroughly omnivorous; as carnivores Osteoglossum are known to even consume little river bats. As juveniles bony-tongues including Notopterids may only consume moving live foods (small fish, brine and other shrimp, insect larvae, worms, fishes), surface types for the species that live there and demersal for bottom types. As adults they consume anything meaty that is small enough to fit in their capacious maws. Cut meat like fat-trimmed beef heart, cut or whole fish, crustaceans, tankmates. A note regarding the second and last; take care to avoid spiny foods and the ingestion of other livestock that can choke or puncture the gut of your bony-tongue. This is probably the second most cause of death of adult fishes (after jumping out). Cut up intentional foodstuffs, eliminating spines and other sharp processes, and make sure their "buddies" are more than tempting mouth-size.
The exceptions to the "anything meaty" rule in feeding bony-tongues are the arapaima as young and the African Heterotis niloticus which are filter feeders throughout life. They inhale detritus and plankton, concentrating it in the roof of the mouth with their gill rakers and mucus.
As juveniles these fishes are quite susceptible to white-spot disease (Ichthyophthiriasis) and "fungal" (actually bacterial) secondary infections due to physical trauma. The former is best treated with careful manipulation of temperature (raising and maintaining in the mid-80's), the addition of non-iodized salt (about a teaspoon per gallon added over a few days time as other livestock/plants allow), AND the correct dosage of dye-based medication. Take especial precaution to not overdose the Notopterids, as their fine scalature allows easy toxification.
Signs of tearing of fins, reddening or whitish outgrowths can best be administered to with regular water changing and addition of the antibiotic erythromycin (sold as Maracyn, et al.)
Feel the need for a "living fossil", have the really big tank space, bricks and C-clamps on your top to keep one in, full permission from significant other(s) to house and feed a "monster"? Okay, maybe a bony-tongue fish is for you.
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