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Issue: October 2010

Into the Deep: A Survey of Freshwater Sharks (Full Article)

Author: Phil Purser

PURS T 1010
Photographer: Ed Wong
Unlike their namesake marine predators, freshwater sharks make interesting additions to reasonably sized tanks.

For as far back as I can remember, I’ve always been fascinated by sharks. Sleek, swift, and unchallenged among predators of the deep, these lithe hunters have never failed to strike me with a curious mingling of horror and awe. All through my childhood, I yearned to house a captive shark in my 15-gallon freshwater aquarium. Of course, I knew this was impossible—not only is a 15-gallon tank far too small to adequately house even the tiniest of sharks, sharks are primarily saltwater animals.

So there I sat, a boy of nine years old who was glad to wile away the hours watching my minute world of mollies and platies, but still had a flair for the savage, and who desired more than anything to own a pet shark. You can imagine my surprise, then, when during a trip to the local fish shop some weeks later, I spied a tank bearing the label “Red Tailed Sharks: $2.99 each.”

Racing up to the aquarium, I wrapped my fingers around the metal rim of the tank and pressed my palms against the glass. Inside the tank swam two fish. Midnight black with tails of the deepest crimson red, these two fish had all the hallmarks of true sharks: tall, rigid dorsal fins; torpedo body shape; sleek, powerful caudal fins; and flat, knife-edged pectoral fins. But how could this be—miniature sharks living in a freshwater aquarium?

Well, the truth of the matter is that these fish were sharks in name only. The fish I encountered that day was one of several species known in the freshwater aquarium industry as sharks. Actually closer kin to the carps than any true seagoing predator, the freshwater sharks can make aesthetically attractive, long-lived, and highly active additions to the freshwater aquarium, but great care should be taken when approaching any of these species for the first time, as they may be wary, flighty, or downright hostile to their tankmates should any or all of their captive requirements go unmet. If maintained in a habitat that offers a balance of food, territory, and hiding spaces, however, most of the freshwater sharks can be maintained by even the most moderately skilled hobbyists.

Natural History

Those of us who have been keeping them for a while may remember the red-tail sharks as members of the genus Labeo, but taxonomists have since deemed otherwise and several species are now classified under the genus Epalzeorhynchos. Many freshwater sharks still belong to the genus Labeo,a huge genus boasting far more species than you’ll ever encounter in your local fish shop. A third genus of freshwater sharks is Balantiocheilos. Because of their similar habitat requirements and generally similar dispositions, diets, and care in captivity, this article will only detail species in these three genera.

Hailing from the warm, sluggish streams and waterways of southern Asia, these freshwater sharks have adapted in ways that are advantageous to a semi-predatory lifestyle. These fish are physically built for both speed and slow cruising: Powerful caudal fins can provide impressive thrust to aid a shark in making shorts bursts forward in pursuit of fleeing prey, or in long, continued marathons of swift escape from a would-be predator.

Easily the fastest among these sharks is the very popular yet nervy bala shark Balantiocheilos melanopterus. Their long, flat, rigid pectoral and dorsal fins offer superb maneuverability and agility underwater: Should a partially hidden morsel of food catch their eye, or should a predator’s shadow loom suddenly over the waters, these fishes’ swift pectoral fins will angle upward and, with a swift flick of the caudal fin, steer them very quickly in the appropriate direction. Because of this adaptation, most freshwater sharks will often adopt a fast zig-zagging escape route to elude and confuse a predator.

Feeding

All these sharks are omnivores. Employing their down-turned, carp-like mouths to rasp algae and detritus off underwater structures, these predominately bottom-feeders can survive on a diet of plant life and minuscule invertebrates. Their survival tactics do not end there, however, for many species of freshwater sharks—in particular, the red-tail shark Epalzeorhynchos bicolor and the black shark Labeo chrysophekadion—are also competent hunters of larger crustaceans and vertebrates such as fry, minnows, and small amphibians.

When food is particularly scarce, red-tail sharks and black sharks have even been known to consume the scales, fins, and skin of other fish. A hungry shark will swim beside its intended victim and lunge suddenly sideways, inflicting a rapid series of bites on the side or finnage of the victim fish and swallowing any bits of flesh that rasp off into its mouth. Slow-moving or docile fish species may repeatedly find themselves victims of a series of eat-and-run attacks, for the slower fish make easy targets for the lithe and swift sharks.

Most species of freshwater sharks, as might be expected, are staunch loners that only school briefly as fry for matters of safety from predation. As they mature, most shark species become increasingly territorial; males will violently defend their underwater domains (usually a submerged tree stump or tangle of rotting roots jutting from a stream bank) from all comers. During the mating season, females may travel from one male’s domain to the next, and competition between rival males can rise to a fever pitch. Combat between any two fish (regardless of the sex of the combatants) is also very common during feeding; two fish that happen upon the same morsel of food will often become so entrenched in battling one another that the food item either sinks or swims away uneaten by either shark. Adult sharks of many species (the red-tail shark, in particular) will also greedily consume the fry and eggs of their own species, and individuals often kill or consume their own progeny.

Red-Tail Shark

Perhaps the most attractive of all the freshwater sharks available in the pet trade today is the red-tail shark Epalzeorhynchos bicolor. As its common name suggests, this shark bears a vibrant crimson caudal fin and caudal peduncle, which offers stark contrast to the coal- to ebony-black body and fins. The only other color on this species, which does not occur in all specimens, is a white tip on the top of the dorsal fin. Average adult length is 4¾ inches, with specimens 5 inches and larger being especially handsome and impressive. Captive longevity may exceed seven years.

Males are distinguishable from females only by the curvature of the dorsal fin: The male’s fin forms a sharper point at the top, and the posterior edge of the female’s dorsal fin forms a right angle with her body. A close examination of numerous fish should be enough to train the eye to recognize males from females. Both males and females become sexually viable around 15 months of age or 2½ inches in length.

Red-tail sharks require moderately large aquaria that afford plenty of hideouts and foliage the shark can claim as its own. Small aquaria or those overpopulated with too many fish will not make the best homes for E. bicolor, as such conditions only stimulate the shark’s aggressive nature. Expect your shark to attack and kill several of its tankmates under such circumstances.

This is problematic for two reasons. First of all, no hobbyist wants to see his other livestock perish under the rasping jaws of the shark (not a quick or pretty death, I assure you), and secondly, the shark will not always stop the attacks when the tank population is thinned-out to its liking. Once a shark has asserted itself as the dominant fish in a community tank, it may well bully all other inhabitants to death.

If, however, the shark has ample habitat to call its own (I have found that driftwood snarls, hollow logs or caves, and/or dense groves of living or artificial plants are favorite haunts) from the beginning, it may never give much trouble to any of its tankmates. I do, however, recommend against housing this species with elaborately finned species such as bettas, fancy guppies, angels, or sailfin mollies. Red-tail sharks also fare very poorly with other members of their own kind; multiple sharks housed in the same tank will almost always fight to the death over a course of weeks or months.

Superior water conditions include pH of 6.5 to 7.4, dH to 15°, and temperatures ranging from 72° to 80°F. These sharks do best when introduced into mature aquariums that have active cultures of nitrifying bacteria and are not prone to radical swings in water chemistry, which can be highly detrimental to any species of freshwater shark.

Bear in mind that your red-tail shark may nibble more than a little on any live plants in your tank; Amazon swords and other large-leafed varieties are particularly at risk. While one shark—assuming it is well fed on a diet of flakes, shrimp pellets, and freeze-dried tubifex or bloodworms—will not usually inflict a fatal level of damage on a plant, serious nibbling can cause brown edges and unsightly discoloration.

First described by Smith in 1931, the red-tail shark is, sadly, almost certainly extinct in the wild. Hundreds of thousands of these gorgeous fish are supplied to the pet industry every year by hatcheries and fish farms in Thailand and Malaysia. These sharks are so popular and so aesthetically pleasing that they have been immortalized on postage stamps in Vietnam (1984), the Ivory Coast (1981), and the Philippines (1993).

Rainbow Shark

A close cousin of the red-tail shark is the rainbow shark E. frenatus. The torpedo-like bodies of these two species are very much alike, as are the red colors of the caudal fin, the black of the body, and the sharp-edged finnage. They look so similar, in fact, that one is very frequently sold as the other in pet shops. Don’t be fooled, however. Although these two sharks look very much alike, the difference in their dispositions is like night and day. A sure-fire way to distinguish a rainbow shark from a red-tail shark is to look at the coloration of the pectoral, dorsal, and anal fins, which are all red in the rainbow shark and black in the red-tail shark.

Naturally occurring in the warm, sluggish backwaters of the Chao Phraya, Mekong, and Xe Bang Fai Basins of southern Asia, these sharks are omnivores in the truest sense of the word: They will graze on algae and plant matter, they will scavenge on detritus and animal carcasses, and they will prey upon small invertebrates, fry, and amphibian eggs.

Not nearly as hostile or aggressive as its red-tail cousin, the rainbow shark fares well in community aquariums that do not have occupants boasting excessive finnage, such as bettas and fancy guppies, as those elaborately finned and slow-moving fish are ripe targets for occasional fin nipping. Naturally a secretive and retiring species, the rainbow shark also needs an adequate amount of hiding places, caves, and dark retreats in which it can escape the bustle and flow of the general aquarium.

Rainbow sharks also seem to take more vegetative matter than some other shark species, so a diet rich in plant matter is definitely in order. Algae wafers and flakes may be supplemented with zucchini. Prepare zucchini by cutting it into slices, then either freezing it for 24 hours and thawing it, or microwaving it until tender. Both freezing and microwaving the zucchini will cause the thick cell walls to explode (thereby making the plant feel mushy to the touch) and render it digestible by the sharks.

Anchor the zucchini by rubber-banding it to a large stone or some other firmly seated underwater decoration. Bear in mind that uneaten zucchini will add a considerable amount of plant detritus to the tank, so superior filtration is paramount.

Appropriate water conditions for the rainbow shark consist of moderate temperatures from 73° to 80°F and a stable pH ranging from 6.6 to 7.4. This fish is particularly sensitive to radical swings in pH, so regular water changes to keep the water parameters stable are definitely in order.

Maxing out at just over 6 inches long as an adult (though it remains much more slender than an adult red-tail shark), a rainbow shark needs appropriately sized quarters with a photoperiod consisting of equal periods of light and dark. As is true with the red-tail shark, this species’ coloration may vary depending on the amount of lighting it receives: Conditions that are perpetually too dark will result in the shark taking on a bleached or washed-out appearance, while superior lighting conditions will bring out rich red and deep black in the shark’s skin and scales.

If given ample hiding places and kept with calm, docile tankmates, the rainbow shark can make an excellent and long-lived addition to the community tank. Despite its docile nature toward other fishes, however, this shark is best kept one per tank.

Bala Shark

The most popular of all the freshwater sharks is the unmistakable bala shark Balantiocheilos melanopterus. Initially described by Bleeker in 1851, the bala shark is native to the lakes and clear streams of Malaysia and the Philippines, where it may live for several years and grow to an impressive 14 inches long. As is true of the red-tail shark, the bala shark is rapidly becoming extinct throughout many of the waterways in which it originally thrived.

Wearing a silvery coat of metallic-sheen scales and tall black-edged fins, the bala shark has enlarged eyes and reduced barbels (which are pronounced in the red-tail, rainbow, and black sharks), which are tell-tale signs that this animal is a visual, midlevel hunter that preys upon fish fry, swimming invertebrates, and small amphibians. These fish seldom take vegetative matter.

Unlike most other species of freshwater sharks, B. melanopterus is a semi-schooling species that tends to congregate much more so than the previously described species. Balas are also much more tolerant of other members of their own species in captivity, and it is even advisable that these animals be kept in small schools of three or five in the home aquarium.

Sometimes referred to as the tri-color shark, the bala shark can tolerate a much wider range of water conditions than other sharks. A stable pH of 6.0 to 8.0 is acceptable, as are temperatures ranging from 72° to 82°F. Bala sharks seem particularly sensitive to spikes in ammonia and other nitrogenous wastes, so regular water changes are highly recommended.

A captive diet of flakes, freeze-dried worms, and other protein-rich fare is best. Because these animals can be extremely flighty and nervous in captivity, they are best housed in overly large environs. Smaller tanks restrict these fish too much, causing stress and injury from the shark ramming its head into the glass wall of the tank. Care should also be taken in approaching a tank with adult bala sharks, for these animals scare easily and can injure or even kill themselves from dashing headlong into the glass or leaping out of the tank. Establishing a regular routine (turning the lights on or off at a certain time and feeding on a routine basis) with these fish is an excellent way to help them adapt to the captive conditions in your house. Tanks devoted to bala sharks are best kept in rooms that receive minimal traffic.

Black Shark

By far the largest of the commonly available freshwater sharks is the black shark. Measuring in at a thunderous 24 inches long as an adult, this is truly a heavyweight species, and the eventual size of this fish must always be kept in mind before considering purchase.

Native to the Mekong Basin as well as the Malay Peninsula, Sumatra, Java, and Borneo, the black shark is an important species both as an export for the aquarium hobby and as a food fish for market trade and consumption by locals. Unfortunately, the black shark has a very low fecundity, so it takes a long time for damaged or depleted wild populations to regain their numbers.

The head of the black shark is bulky and solid, and the scales are large. The finnage is pronounced, and the stocky body morphology is indicative of a powerful, distance-swimming fish. This body form works great for swimming in the lakes, reservoirs, and powerful rivers in which the shark occurs.

Maintain black sharks in mature aquariums with stable pH levels from 6.0 to 7.4, and ammonia and other nitrogenous waste levels as close to zero ppm as possible. A photoperiod of roughly equal durations of light and dark should be regulated—like the rainbow shark, this animal’s coloration may fade to a burnt-charcoal gray when kept in overly dark conditions, but it will deepen to a rich and even black when exposed to ample light conditions.

Young specimens also tend to be richer in coloration than older adults, which may take on a perpetual gray or silvery hue that no longer varies based on lighting conditions. Indeed, this is a common fate of black sharks. Purchased when young due to their stark aesthetic appeal, these fish are often discarded when, with age, they fade into dull coloration and sluggish movements (when kept in cramped conditions and excessively small aquaria).

A solid captive diet for the black shark should include equal parts plant and animal matter: flakes and shrimp pellets supplemented with algae wafers and prepared zucchini slices. This shark is perhaps the most dangerous to house in the company of smaller fish, as it has a bit of the red-tail shark’s tenacity and, simply by virtue of its size and appetite, may swallow its tankmates whole! That being said, it’s also common practice to feed large adult black sharks feeder guppies and minnows, as well as crustaceans such as crayfish and small shrimp.

A suitable aquarium setup for the black shark includes large quarters of not less than 125 gallons for a medium-sized adult fish, superior mechanical and biological filtration (I recommend powerheads and large canister filters), and plenty of robust plants and decorations. Because a large black shark can take some pretty hefty bites out of their leaves, living plants can be hard to maintain in such a tank. Artificial plants and hunks of sunken wood work well, as do acrylic or ceramic hideaways.

Unlike most of its smaller cousins, however, the black shark also enjoys large amounts of open water, as well as dense tangles of cover. I recommend that at least half the tank be dedicated to open water and that pockets or islands of dense plant/wood structure be situated at intervals throughout the tank. This shark also seems to enjoy a sandy substrate or one composed of very fine pebbles.

A Shark for Everyone

At the end of the day, it seems that the freshwater sharks have something to offer fishkeepers at virtually all points on the experience continuum, from the beginner keeping the hardy, benevolent rainbow shark, to the seasoned veteran who maintains a large adult black shark. Of course, the four species described here are a mere sampling of the interesting and beautiful species of freshwater fish called “sharks,” many of which make good aquarium specimens.

See the full article on TFH Digital http://www.tfhdigital.com/tfh/201010/#pg77

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