The Definitive Guide: Natural History & Captive Care
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Co-Authors: Alfon76 (Ivan), Aquafan2001 (Rich), E_americanus (Solomon)
Photos by: Alfon76, Aquafan2001, E_americanus
Edited, Arranged, and Referenced by Solomon David
*** All Captive Care Information & Images Copyright 2005 Solomon David OR used with permission. No material (listed above) may be reproduced without permission.
Introduction -
The African arowana, or perhaps more accurately, the African bonytongue (since H. niloticus is in fact not a true arowana), is one of the most primitive extant fishes and a true oddball in the aquarium hobby.
These fish are not nearly as colorful as their true arowana cousins, such as Osteoglossum bicirrhosum (silver arowana) or Scleropages formosus (Asian arowana), but have a personality all their own and have slowly increased in popularity among 'oddball' fish enthusiasts over the recent past. They are even oddballs among the oddballs, growing to a very large size (up to 100cm, 3' in the wild), yet they are peaceful filter-feeders, a unique trait amongst the primitve fishes (the paddlefish is one of the only others). Here we will consider the natural history and focus on the captive care of this ancient African fish.
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*Some notes on the organization of this article: Natural history information is provided first, captive care secondly and in more detail. Captive care is divided into general information on the species, followed by divisions based on three stages of development: juvenile, subadult, and adult. Feel free to skip to the sections containing the information you are seeking.
Hn = Heterotis niloticus
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Natural History
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Taxonomy & Classification
H. niloticus is a primitive fish and member of the group Osteoglossomorpha, the most primitive subdivision of Teleostei. H. niloticus belongs to the family Arapaimatidae, which contains only two species, the African arowana and Arapaima gigas, the pirarucu of South America (8). These two species and the African butterfly fish (Pantodon buchholzi) are the only air-breathing members of the order Osteoglossiformes (4).
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Identification
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H. niloticus is characterized by an elongate, relatively thick body with dorsal and anal fins spineless and set far posterior on the body. Pectoral fins are placed low on the body, pelvic fins are nearly mid-body. The caudal peduncle is small and the caudal fin rounded. The head is relatively short and covered with large sensory pits. The lips are large and mouth terminal to subterminal. The body is generally uniform brown, bronze or gray in color with a lighter underbelly; subadults may be gray with a bronze patch on head. Scales are large with a visible lateral line (8).
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Size: 100cm, ~3' wild max; more frequently 80cm. 10.2 kg, max published weight (8).
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Distribution & Habitat
H. niloticus is native to African river drainages such as the Nile and Congo, as well as Lake Turkana. H. niloticus is relatively common throughout its range and has been introduced both for aquaculture/fisheries purposes as well as accidentally in several locations. Although successful introductions for aquaculture and fisheries have occurred, they have been considered problematic in several of these non-native systems(1,3,8).
Young are found in swampy places among aquatic vegetation; adults live in the open water of rivers and lakes, where they can be found in the pelagic (open water) zone as well as the littoral (near-shore) zone (8).
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Biology
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Breathing Air
H. niloticus is an obligate air-breather with a specialized gas bladder. The gas bladder in this species is very similar to that of its South American counterpart, the arapaima. Both H. niloticus and the arapaima have unusually large air breathing organs, such that the kidney is enclosed in the latter 2/3 of the structure (7). Groups of H. niloticus have been observed in the wild performing synchronized breathing, where members of the group will surface and take air at the same time. This modification for breathing air helps H. niloticus survive in relatively anoxic environments such as swamps and other floodplain habitats (8).
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Filter-feeding
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H. niloticus is the only member of the family Osteoglossidae that is a filter feeder. These fish have a specialized spiral structure above the gills on either side of the head through which small food particles (taken in through the mouth) are passed through and mixed with mucus. This mixture is then passed into the pharynx and then swallowed (5). This specialization allows H. niloticus to consume very small particles of food, primarily small invertebrates in the wild. H. niloticus has also been known to consume phytoplankton, and adults in some cases consume small seeds and detritus (1). Because of this filter-feeding or more specifically planktivorous feeding behavior, H. niloticus prefers to eat more frequently over a long period as opposed to consuming large infrequent portions.
H. niloticus will forage for food through the substrate, which is usually sand or mud in the wild; as well as feed midwater (and sometimes at the surface).
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Spawning
H. niloticus adults spawn primarily during the wet season in lake as opposed to river habitats. H. niloticus creates a circular nest in swampy areas with aquatic plants. 5-7 days after hatching, the yolk sac is absorbed and the young forage as a school; at 25-30 days post-hatch the young forage independently (2). Young are guarded by the male and possess gills early in life (5).
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Captive Care
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General notes on captive care of Heterotis niloticus :
H. niloticus is a unique primitive fish that has special requirements compared to other large tropical fishes. Most importantly, H. niloticus is a filter feeder and primarily planktivorous. It will generally consume only small-sized food items and needs to eat more frequently when compared to other large fishes (large predatory fishes do not eat as frequently and can take larger food items). H. niloticus 'captures' food by sifting through the substrate or gulping food directly from various points in the water column. It then 'processes' the food, indicated by rapid movement of the mouth and throat, before swallowing. Food that cannot be processed (usually too large) is ejected out the gill covers or back out through the mouth. Because of this specialized feeding behavior, special attention must be given to H. niloticus food items at all stages of development. Generally particulate or small items are accepted best; these include zooplankton, live black worms, frozen bloodworms, brine shrimp and mysis.
H. niloticus also requires a relatively large amount of swimming room at all stages of life; these fish tend to be skittish at times, and are quite often very active swimmers. Always provide an enclosure that is at least as wide as the the length of the fish.
These fish must also have access to the water's surface to breathe air, therefore leave at least an inch or two between the top cover of the tank and the water. A heavy top or even weighted top is suggested for larger individuals as these fish are very strong jumpers.
Temperature should be set from 78-82 F, and these fish do well around a neutral pH.
Specific Care by Life Stages
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**specific care divisions based on sizes frequently encountered in the aquarium hobby, early life stage sizes set arbitrarily
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Juveniles (3"-7")
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Tank Size
Tank size for H. niloticus at this size should be no smaller than 10g for a single individual at the smaller end of the juvenile size spectrum. These fish require a decent amount of swimming room and good filtration and water quality. As they grow in size the tank size should be increased as well. The ideal tank size for juveniles is as large as possible as long as the fish can successfully and consistently find food. Young juveniles have been successfully reared to the subadult stage in a 180g aquarium with few or no tankmates. This provides the fish with plenty of space for both swimming and foraging. At these early stages a sandy substrate or no substrate at all is preferable. The latter may be ideal because the fish can get directly to the food without having to sift through the substrate. Although the sifting action is natural to them, learning to process prepared foods is also a challenge and therefore making optimal foraging as easy as possible for the juveniles is suggested.
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Feeding & Maintenance
Proper feeding at this stage is integral for H. niloticus. Live blackworms can be provided as a staple at this size and will help the H. niloticus grow quickly to their next stage (subadulthood). A twice a day feeding is suggested for optimal growth. When live blackworms are not available, frozen bloodworms, zooplankton and brine shrimp are acceptable substitutes. When keeping H. niloticus at very small juvenile sizes, it may be necessary to cut up the live blackworms for easier processing. Keep in mind their particulate processing structures are not very large at this size, and live blackworms are a relatively large food item for the juvenile fish. Feeding portions are suggested at approximately ½ - 1 bloodworm cube per feeding, although this may vary depending on desired growth rate. This volume of food may be estimated and applied to other food items They will continue to eat food that is left over from the initial feeding as they are continuous foragers.
Regarding tank maintenance, water changes should be performed regularly to provide optimal conditions for the young H. niloticus. Good filtration and a 25% water change per week or 50% water change every other week is suggested. This will improve growth rates as well.
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Behavior & Tankmates
H. niloticus at this stage will get along with similarly sized tankmates which are unaggressive. It is best to keep H. niloticus alone so it will not have to compete with any other fishes for food. At this stage H. niloticus will get along with others of its own kind, but will need more space in order to diffuse potential aggression. Fighting has been observed between conspecifics, but this will vary with the individual fish as well as with the tank space. If you plan on keeping more than one H. niloticus together provide a large space for these fish; 2 small juveniles in a 30g tank, 3 in a 50g tank. Several have been raised together in a 180g aquarium with few problems concerning aggression.
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Subadults (8"-24")
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Tank Size
subadult H. niloticus will do best in the largest tank one can provide OR in a stepwise tank progression as the fish increases in size. As with the adults, H. niloticus at this stage will require a large amount of swimming room as they are quite active. On the same token make sure to provide them with at least one hiding spot, a cave of rock or driftwood works quite well to provide the fish with security should it get stressed or frightened.
Subadults will range in size greatly, from approximately 8” to 24” when they reach adulthood (wild maturity size - source). With this in mind, it is acceptable to keep the fish in a smaller enclosure, ideally nothing under a 50g aquarium, depending on tankmates (a 30g may be suitable for a brief period of time if the fish is by itself). As the fish grows its enclosure should be increased in size, a 24” H. niloticus would require a 6’ long tank that is preferably wider than 2’ (3’ would work, the wider the better).
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Feeding & Maintenance
Subadults are generally out of their more sensitive stages and are hardier than the juveniles. At this stage they will learn to accept an even wider variety of foods. Feed H. niloticus at the beginning of the subadult stage primarily the foods they were used to as juveniles: live blackworms, frozen bloodworms, zooplankton, etc. While feeding these staples, introduce them to other foods as well, such as frozen mysis, Tetra color bits, even crushed hikari pellets. When first introduced to a new food, they will often suck up the food, process it temporarily and then spit it out in smaller pieces. This process may go on for several feeding periods but they will often learn to accept the other food options which you provide. One of the good things about H. niloticus feeding habits is that it will often go back and consume uneaten food later after feeding time.
As H. niloticus gets larger within the subadult stage, feedings CAN be reduced to twice a day or even once a day. It is still suggested, however, that they be fed twice with moderate amounts, as opposed to once with a large amount of food. Again, this relates back to the fact that they would rather eat somewhat constantly throughout the day as opposed to one large feeding per day. Keep in mind these are planktivorous fish, they do not consume large meals and then fast for long periods, they essentially need to eat ‘continuously’.
In terms of how much to feed, it is suggested that H. niloticus be fed an equivalent of 1-2 cubes of bloodworms per feeding depending on size. This volume of food can be estimated and applied to other foods as well. This should provide the fish with enough to eat immediately as well as a portion of food to eat in between feedings. You may want to experiment with these suggestions and see what works best for you.
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Behavior & Tankmates
H. niloticus are generally peaceful fish, however, they are also very active fish. Their active swimming habits may disturb more sedentary or mellow tankmates;
H. niloticus like to bound quickly back and forth and often surge across the length of the tank to shelter when frightened. Soon enough, tankmates will get used to the behavior of H. niloticus and will coexist peacefully. Since they are not true piscivores (fish-eaters) they will be safe with just about any size fish. It should be noted that small guppies may be consumed by larger individuals; keep this size value in mind when considering very small tankmates with H. niloticus.
H. niloticus will get along with a large diversity of fishes, but what fishes will in turn get along well with H. niloticus? Large, relatively non-aggressive predatory fishes such as gars, bichirs, mormyrids, loaches, various catfishes and datnoids will do well with H. niloticus. The African bonytongue has little way to defend itself other than swimming away from an aggressor or its sheer size (at late subadulthood or adulthood) ; keep this in mind when choosing tankmates. H. niloticus can also be outcompeted by more aggressive feeders, so make sure the fish is getting its share of food when kept in a community setting.
Throughout its life the African bonytongue will do best in clean water conditions. A water change of 25-50% per 1-2 weeks is suggested. Water changes should increase with the amount fed to the tank community and the size of the tank community. Often times more food must be put into a community tank so the H. niloticus can get its fair share, mainly because other tankmates will gladly consume bloodworms and mysis along with their regularly apportioned food items, the reverse does not often occur at this stage for the H. niloticus.
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Adults (25"-30"+)
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Tank Size
The size of the tank should always be as big as possible. Adults (25"+) are best kept in a very large aquarium, such as a 250-400 gallon tank (preferably at least 3' wide); this will hold them only temporarily in many cases and a larger aquarium or pond will eventually be needed. The stepwise progression of the adults in this article are 400g tank - 800g tank - eventually a 20000g pond. Enclosures of this magnitude may not be absolutely necessary, but the more room that can be provided to the fish, the better in any case.
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Feeding & Maintenance
Adult H. niloticus will eat nearly anything that is a smaller morsel for their size. They will avidly consume live blackworms but will also accept frozen bloodworms, krill, pellets, hikari carnivore pellets, colorbits, daphnia and even small feeder guppies. Pretty much anything offered to them will be either consumed right away or after a while when later foraging through the substrate or tank bottom for more food. Their metabolism seems to be very fast and therefore need to eat almost constantly. Even adult H. niloticus can grow thin quickly if not fed at least twice a day.
Water quality is still important at the adult stage. Ammonia and nitrites should be 0 and nitrates as low as possible. Water changes should be performed at least weekly to ensure these water parameters. Water changes of up to 40% have been performed with large adults with no ill effects. Larger water changes are probably reasonable depending on feeding, filtration and number of fishes in the tank.
Effective biological filtration is required to house H. niloticus properly due to the fact that these fish produce large amounts of waste and keep the substrate constantly stirred with their foraging behavior
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Tankmates & Behavior
Adults are also very peaceful towards other fishes but can be quarrelsome with conspecifics and even other arowana species. The three large adults referenced here are kept with each other and also share the tank with a black arowana and an arapaima. No excessive aggression has been noticed and the few episodes of aggression have occurred are limited to chasing around the tank.
Almost any fish can be tankmates with an adult African arowana as they are not piscivores. The adults have been kept with larger fishes such as datnoids, large catfishes, arapaima and high-finned banded shark. All tankmates are basically ignored by H. niloticus and the tankmates in turn do not bother them.
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References:
(detailed references to be added soon; temporary listing of source, author)
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1. Ontogenetic, seasonal, and spatial variation in the diet of Heterotis niloticus (Osteoglossiformes; Osteoglossidae) in the Sô River and Lake Hlan, Benin, West Africa, Adite
2. Population structure and reproduction of Heterotis niloticus (Osteoglossiformes: Osteoglossidae) in the Sô River-floodplain system (Benin, West Africa), Adite
3. The Diversity of Fishes, Helfman
4. Fishes of the World, Nelson
5. Encyclopedia of Fishes, Paxton
6. Jurassic Fishes, Kodera
7. Air-Breathing Fishes, Graham
8. Fishbase.org
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Close:
This is a work in progress that will be continually updated as new information is found, both scientific and hobby-related. Feel free to ask questions or make comments as we are happy to further explain or clarify any of the information here or specifics we may not have covered as in-depth. Thanks to Ivan and Aquafan2001 for contributing their in-depth and expert knowledge on these truly unique oddball fish! We hope you are able to use this resource should you choose to keep H. niloticus, or at the very least have learned a little bit about the fish!--
--solomon
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very informative....i learned so much about this amazing fish 