In what distant deeps: The terrifying tigerfish

William Blake may not have had the tigerfish in mind when he composed the “The Tyger” but the primary question posed by the poem still applies. How, in fact, did the fearful symmetry of this African tigerfish —named for an Asian predator— arise?

The short answer is: somebody had to do it. Most healthy ecosystems have one or two predators that dominate the rest. The 5 to 6 species of African tigerfish, in the genus Hydrocynus, are among the most fearsome aquatic hunters on Earth, let alone in Africa. In the freshwater realm, they have few equals. They are usually apex predators in the rivers and lakes that they frequent; only crocodiles best them in size and appetite.

Per Leander J. McCormick, scion of the McCormick dynasty and game fish expert, it “is the fiercest fish that swims.” Tigerfish resemble the saltwater barracuda, though they are less gracile and boast even more intimidating dentition. It’s perhaps unsurprising that they are related to the South American piranhas, also known for their carnivorous tendencies. In the same family are such fish as the tetras most home aquarists will be familiar with. Take a look at a close up image of a tetra and one of a tigerfish. The similarities in their teeth and jaws are striking, though those of the tigerfish are much exaggerated in comparison.

Tigerfish range from northern Africa down into central Africa and parts of southern Africa. They prefer large lakes and rivers, where they are the sharks of their ecosystems. They are known in the Congo River basin and Lake Tanganyika, as well as in the Nile, Gambia, Zambezi, and Limpopo rivers.

Their Latin name means “water dog” and like canines, they often live in groups, at least until they reach full adulthood. In areas where they are found, the other large predatory fish of the region, the African pike (Hepsetus odoe) is often absent or restricted to sub-prime real estate due to competition and predation. (Young fish of both species appear to share the same habitat without much issue, however.)

The largest species, the goliath tigerfish (Hydrocynus goliath), has been recorded at 154 pounds and may reach lengths of nearly 5 feet. The smallest, H. tanzaniae, is a modest 2 feet at the largest. Females attain larger sizes than males. The large size and muscular body of the tigerfish make it among the most sought-after African sportfishes. While they can be sustainably harvested and have been introduced in some places, in their native range, it is advised that they should be thrown back after landing, especially during breeding season. They are slow breeders and depleted populations may take decades to rebound. Their flesh, while edible, is oily and bony and not considered particularly delicious.

These predators patrol the depths in search of fish, their primary food source. They have been recorded taking fish nearly 60% their size, though they’ll even take prey as small as an aquatic insect, especially as juveniles. They appear to prefer cichlids, but will also take gobies, barbs, and whatever else might be available.

One 2013 study noted that H. vittatus will even take barn swallows (Hirundo rustica) in flight. Like most characins, they possess a set of bones that link their swim bladders to their inner ears. This is thought to improve hearing and is likely a useful adaptation for a predatory species. Tigerfish love the turbulent areas where receding floodwaters meet the boundaries of the rivers and lakes they inhabit. The cloudy water likely disorients some fish and allows the tigerfish to take its pick. Tigerfish, especially large ones, are somewhat territorial. And they have been known to cannibalize smaller specimens.

Because of their distinctive, conical teeth, tigerfish have served as useful index fossils. No other fossil fish in Africa has similar teeth and so they are unmistakable. While of course the fossils are helpful in understanding the evolution of the fish itself, they have also served to elucidate the hydrogeographic histories of the areas where they are found. That is: these fossils, along with knowledge of their contemporary ranges, can be used to fill in gaps in the knowledge of the historic extent of various rivers and floodplains.

Notably they are absent from Lake Malawi and Lake Victoria, which is helpful in piecing together the historical connections (or lack thereof) of these bodies of water to others. A 2016 study used tigerfish fossils to propose a hydrological link, no longer extant, between the eastern and western parts of northern Africa. Because of similarities between the fossils found in these areas, the authors proposed that there must have been at least seasonal connections between this area that allowed fish to disperse from one part of the continent to another.

Similarly, genetic analysis has been used to investigate how and when these fish radiated southward from the Congo Basin and into southern Africa. Because of their apex predator status, these radiations have major implications for other organisms and their adaptations.

Imagine the shock of previously isolated populations of fish when floodwaters first dumped a tigerfish into their homes. It must have been like Jaws showing up in your swimming pool. Luckily for us, the species only rarely attacks humans, and even then, only when a careless angler doesn’t handle it with appropriate care.

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By Richard Pallardy, Earth.com Contributing Writer