Apex predators of the deep blue seas, sharks are often perceived as some of the most elusive, dangerous, and aggressive creatures. You might be surprised to find that their kind is hardly restricted to saltwater reefs and the seemingly bottomless depths of our oceans. In fact, there are a handful of sharks that migrate into brackish, coastal waters and freshwater systems like lakes and rivers. Some, such as the bull shark (Carcharhinus leucas), may also regularly migrate between rivers and seas.
Sharks are often portrayed as large, voracious, and fearsome, but there are many that simply grow to the size of sport fishes, maintain remarkably peaceful lives, and are far from dangerous to humans. That being said, most true freshwater sharks are poorly studied. What’s worse, their populations are continuously dwindling due to fishing and various habitat disturbances.
The majority of the extant species of river sharks are members of the genera Glyphis and Carcharhinus. A handful of bony, freshwater fish species – not at all related to true sharks – may also be referred to as “freshwater sharks” or “sharkminnows” in the aquarium trade. Members of both categories are listed and described below.
True Freshwater Sharks (Carcharhinidae Family)
1) Speartooth shark (Glyphis glyphis)
Fairly rare in the wild, the speartooth shark is usually found in near-shore waters. Able to tolerate both freshwater conditions and a wide range of salinity levels, it tends to hide in turbid habitats. Its juveniles and young adults seek shelter and search for prey in large, mangrove-lined rivers and estuaries along the coast. Newborn sharks of this species keep close to muddy substrates in waters with fast currents.
As speartooth sharks age, they tend to move further into coastal zones with higher salinities. Their general position in rivers and estuaries, however, is largely determined by the tides. This is largely due to their sluggish behavior and their ability to swim at slow speeds given their relatively large dorsal fin. Reliant on adaptations for electroreception, they have small eyes with little use in their murky habitats.
The speartooth shark has not been studied extensively because of their scarcity and their elusive nature. Adults are suspected to grow to a maximum length of about 8.5 feet (2.6 meters). Despite their proclivity for minimal movement, their hefty bodies are streamlined. Their mouths are equipped with fine teeth for feeding on crustaceans and fishes found close to the bottom of rivers.
2) Ganges shark (Glyphis gangeticus)
Due to their shared distribution and their similarities in appearance, the Ganges shark and bull shark are often incorrectly identified as one another. Unlike the bull shark, however, G. gangeticus is found only in estuarine and freshwater habitats. It is not known for venturing into marine environments as it favors the low-salinity conditions of the Bay of Bengal, Mahanadi River, and Ganges River.
The feeding habits of this remarkably rare species continue to be uncertain. Critically endangered in the wild, it may rely on electroreception to find its prey in low-visibility waters. Newborn individuals are likely to be found in exclusively freshwater systems, whereas older individuals may migrate into estuaries downstream.
It’s highly probable that less than 250 mature, living individuals of this species continue to exist. Threats to their limited habitats are largely associated with anthropogenic activities, such as the construction of dams and the disposal of pollutants into river systems.
3) Borneo shark (Carcharhinus borneensis)
A relatively small elasmobranch, the Borneo shark grows to just 26 inches (66 cm) long. Remarkably rare in its native habitat, its slate-gray dorsal coloration helps it blend in with its wild environment. Its belly, in contrast, is starkly pale. This white coloration may extend further upward along the length of its flanks. The fins may likewise have pale bands.
Not much is known about the feeding habits and life cycle of this critically endangered species. It unfortunately exists in waters that are heavily exploited for wild-caught fish. A type of requiem shark, it is a viviparous species that gives birth to small batches of live young. Unlike those of the freshwater sharks listed above, its eyes are large relative to the size of its head.
The inshore range of the Borneo shark is suspected to be limited to shallow waters. This is largely due to its recorded specimens having been collected from fishery landing sites. Individuals have yet to be collected anywhere else. Though it is probable that this shark species is occasionally captured and regarded as a source of meat, it is not commercially significant.
4) Northern river shark (Glyphis garricki)
Found in tidal rivers and coastal waters, the northern river shark favors fresh to brackish waters with poor visibility conditions. It also prefers to be associated with areas having large tidal movements and soft bottom substrates. Juveniles are usually found further inland or in estuaries with low salinity levels. Fully matured adults prefer to remain in marine, coastal waters.
Set apart by the high curvature of its back and the generally stout and stocky appearance of its body, the northern river shark grows to a full length of about 8.2 feet (2.5 meters). Due to its extreme rarity, its behavior and its unique preferences continue to be poorly understood. So far, researchers have had to base their knowledge of the species on just a few sampled and tagged individuals.
Based on the mouth anatomy of this species, it is sound to presume that it feeds mostly on smaller fish. Given its small eyes, it likely relies on its ampullae of Lorenzini to detect the presence of prey in murky waters. Tidal movements may significantly determine where and when it feeds.
5) Bull shark (Carcharhinus leucas)
Bull sharks, found in the coastal areas of most tropical to subtropical countries, are well-adapted to both freshwater and marine conditions. Often feared due to their aggressive temperament, they have a reputation for being one of the most dangerous sharks in the world. Attacks involving humans, however, are rare and likely coincidental as they do not deliberately target us as prey.
To survive in freshwater environments, bull sharks have several adaptations for storing salt in their bodies. Their gills are capable of absorbing sodium and chloride from the water they filter for oxygen. Their kidneys also tend to reabsorb salt into their systems as a means of regulating internal salt levels. Moreover, they have evolved to develop specialized rectal glands for secreting excess salt.
Young bull sharks are more likely to move further into freshwater systems than adult sharks. One possible reason for this is they have fewer predators in freshwater environments than they do in marine waters. Thus, they can safely hunt and explore until they grow into their mature sizes (i.e. about 8 feet or 2.4 meters for females and about 7 feet or 2.1 meters for males).
Freshwater Sharks of the Aquarium Industry (Cyprinid and Catfish Families)
1) Redtail shark (Epalzeorhynchos bicolor)
Although the redtail shark is fairly common as an ornamental fish, its wild populations are critically endangered. Past harvests to meet the demands of the aquarium trade have contributed to its decline in the wild. Its sole existing population is found in the Chao Phraya River Basin which flows into the Gulf of Thailand. Today, all stocks in reputable fish shops have been bred in captivity.
As its species epithet suggests, this species is distinguished by two colors – a uniformly black body and a vivid, red-orange tail fin. In suboptimal conditions, unhealthy specimens may lose their red coloration. Temperatures between 22 – 26˚C (72 – 79˚F) and pH levels between 6.8 – 7.5 must be maintained to keep this fish comfortable in tanks. Numerous hiding places are necessary to maintain an agreeable temperament.
2) Iridescent shark (Pangasionodon hypophthalmus)
Juveniles of this ornamental catfish, more commonly known as the iridescent shark due to its shiny appearance, are often sold as aquarium fish. They are not known for being easy to care for, however, as they are easily stressed by external stimuli and by subtle changes in light. In small tanks, this shark-like catfish is prone to developing in a stunted manner and becoming injured.
Wild populations of P. hypophthalmus are frequently exploited as food sources due to the low cost of their meat and their mild flavor profile. They have also been introduced into aquaculture systems. Omnivorous, the iridescent shark can persist in water bodies with minimal choices of live prey. Their adults, which are uniformly grey in color, can measure as much as 4.3 feet (131 cm) long.
3) Columbian shark (Ariopsis seemanni)
The Columbian shark is a type of sea catfish. It develops into its juvenile stage in freshwater, after which it naturally migrates into brackish or marine waters to become sexually mature. Though this species is fairly common as an ornamental “shark” for freshwater aquariums, its dedicated owners eventually increase the salinity levels of their tank to promote normal development.
Due to the swimming habits of the Columbian shark, it also requires a large water volume with a consistently rapid flow rate and high oxygen levels. Though this fish is known for being hardy and able to tolerate mild fluctuations in water conditions, it can develop various ailments due to inadequate salinity levels.
4) Chinese high-fin banded shark (Myxocyprinus asiaticus)
The Chinese high-fin banded shark is a popular addition to freshwater aquariums because of its eye-catching appearance and longevity. Individuals that are well-cared for can live for up to 15 – 25 years in captive conditions. Known for the striped appearance of their juveniles, they have strikingly large dorsal fins that bring sharks to mind.
Over time, however, the banded shark lengthens and its dorsal fin becomes more balanced in size relative to the rest of its body. This same fin can even look quite small on fully-grown individuals! Adults are unfortunately considered undesirable by some aquarists, though they remain biologically fascinating and visually unique in their own right. Fully grown, this fish can measure 3 – 4 feet (91 – 122 cm) long! You can’t expect it to grow to this size in a tank, however.
5) Bala shark (Balantiocheilos melanopterus)
The bala shark is also commonly known as the tricolor sharkminnow and silver shark. A member of the carp family (Cyprinidae), this species resembles small sharks because of its large fins and streamlined, silvery body. Its fins have distinct, black margins on their outer edges, giving them a sharper-looking appearance. In optimal conditions, it can grow to a full length of about 14 inches (36 cm).
In the wild, B. melanopterus favors conditions in large-sized and moderately deep rivers and lakes. It grows best in pH-neutral conditions and in temperatures ranging from 22 – 28˚C (72 – 82˚F). As it can quickly grow to a fairly large size, it is not suitable for small tanks. Juveniles can be housed in small aquaria for a short amount of time; eventually, they will need to be given more room.
6) Roseline shark (Sahyadria denisonii)
The roseline shark is a remarkably popular aquarium fish due to its small size, flashy streaks of colors, and peaceful, schooling nature. It is also known as the Denison barb or Miss Kerala. In the wild, this endangered species favors the currents of rapidly moving rivers and streams. Global demand has caused its wild populations to be heavily exploited, with the pet trade as the leading threat to its survival.
Also a member of the carp family, the roseline shark grows to a full length of about 4.3 inches (11 cm). Due to its sociable nature, it is most comfortable in large shoals. Its shoals favor conditions in rocky pools with ample vegetation for shelter, protection from predators, and food. In aquaria, it can subsist on a diet of bloodworms, tiny shrimp, and fish flakes.
7) Violet blushing shark (Labeo boga)
As suggested by its common name, L. boga appears to have cheeks that are perpetually blushing. The vivid tint is actually caused by the transparency of the thin layer of skin overlying the blood vessels found just underneath the gill flap. This distinct feature gives this ornamental species its charm. In juveniles, the skin over the rest of the body is quite transparent as well.
Known for being a placid fish in tanks, the violet blushing shark is actually a member of the carp family. In the wild, it prefers to stay close to the soft bottoms of streams, rivers, and pools. In aquariums, it also keeps close to the substrate and relies on sinking types of food for sustenance. As it grows to a maximum length of about 12 inches (30 cm), it is necessary to house this freshwater shark in an adequately large setup.
8) Harlequin shark (Labeo cyclorhynchus)
The harlequin shark or variegated shark is named for its mottled appearance. This cyprinid fish develops grey to black speckles over the full length of its yellowish body. As it ages, its yellowish coloration disappears until its body becomes a solid black. Those that are sold in pet stores are likely collected from the wild as this species is difficult to breed in captivity. Its adults’ aggressive personality, solitary nature, and lack of sexually dimorphic characteristics make it very challenging to keep in pairs or groups.
In the wild, the harlequin shark keeps to the cool, tropical waters of rainforest streams. As these freshwater bodies tend to be full of vertical vegetation and fallen debris, the tannins from the leaves give them a stained appearance. The mottled-to-dark coloration of the harlequin shark helps keep it camouflaged in vegetated, dark waters.
9) Rainbow shark (Epalzeorhynchos frenatum)
This freshwater, ray-finned fish is typically distinguished by a bright blue to black body and vividly red-orange fins. Due to its popularity in the aquarium industry, it now comes in a handful of colored and albino variants – including a genetically-modified one that seems to fluoresce under aquarium lights (GloFish)! In the wild, this species stays close to the bottom substrates of rivers, where it feeds on plankton and algae found on the surface of substrates.
Though rainbow sharks appear harmless, they are known for being quite aggressive with their own kind and with other species in a tank environment. Larger specimens become territorial and will not hesitate to fight smaller conspecifics that may attempt to encroach into their space. To reduce the chances of injury due to fights, these fish need to be provided with ample space and many hiding places.
10) Black shark (Labeo chrysophekadion)
Also known as the black sharkminnow or black labeo, this shark-like member of the carp family is often a species of interest in the aquarium trade. However, while its juveniles look perfectly content in a well-outfitted freshwater tank, its adults require much larger enclosures as they can grow to a full length of about 3 feet (91 cm)! The black shark is also known for being territorial and aggressive once it matures.
In the wild, this species naturally occurs in streams, rivers, and canals with vegetation and detritus. Its adults feed on plant particles and algae, much like other detritivores and planktivores in its genus. To spawn, fertile adults usually migrate into shallow, upstream areas. They time their spawning period with the onset of the rainy season. This way, their eggs hatch as soon as water levels begin to rise.
