Kategorie: News
A Master of Adaptation with a Social Life
Of all the large shark species, the bull shark (Carcharhinus leucas) is the only one that can survive in freshwater for extended periods. Females can reach lengths of up to 3.5 meters, while males remain significantly smaller at around 2.1 meters.
Also known as the bull shark or Zambezi shark, this requiem shark lives in warm coastal waters worldwide - but frequently ventures far into rivers and lakes. In the Amazon River system, individuals have been sighted more than 4,000 kilometers inland.
A Body Adapted to Freshwater
This is made possible by a rare trait: bull sharks are euryhaline, meaning they can cope with widely fluctuating salinity levels. While most sharks would perish in freshwater due to osmotic pressure, bull sharks have specially adapted kidneys and a rectal gland that can essentially be shut down in freshwater. This allows the body to retain the necessary salt, filter out less of it, and instead excrete more urea via very watery urine.
Their eyes are also adapted: Their retinas are designed for greenish, murky light, as is typical in rivers and near the coast.
17 Years in a Golf Course Lake
Just how extreme this adaptability can be is illustrated by a particularly remarkable case from Australia: In 1996, six young bull sharks - presumably swept in by a flood - ended up in the freshwater lake at the Carbrook Golf Club near Brisbane - and remained there for at least 17 years.
A study by researcher Peter Gausmann of Ruhr University Bochum analyzed the case: Employees fed the sharks, which became the golf club’s mascots, while the lake’s abundant fish population provided an additional food source. Sharks were last sighted in the lake in 2015 - their exact age and what became of them remain unknown, but they are considered record-holders for the longest known, uninterrupted freshwater phase of a bull shark.
Hunters with a Social Life of Their Own
The bull shark hunts using the so-called “bump and bite” tactic: It first nudges potential prey with its head to assess in a flash whether it is suitable for eating.
This species is viviparous - after a gestation period of over a year, females typically give birth to four to six fully developed pups, at most once every two years.
For a long time, sharks were considered loners, but a study published in 2026 in the journal *Animal Behavior* from the Shark Reef Marine Reserve off Fiji impressively refutes this notion for the bull shark. Over a period of six years, an international research team observed 184 individually tagged sharks and found that: Bull sharks deliberately choose their social partners, distinguishing between closer “friendships” and more superficial “acquaintances.” Both males and females prefer contact with females - which is likely an advantage for the significantly smaller males, as it helps them avoid conflicts with the larger animals. (See also our report on “friendships” among bull sharks)
Status: Endangered!
As a top predator, the bull shark has hardly any natural enemies - and yet it is endangered. Coastal destruction and altered river courses are taking a toll on the species. Added to this is heavy fishing pressure: its meat and fins are highly sought after. Many animals also end up as bycatch, for example in gillnets, trawl nets, and longline fishing. Among sport fishermen, bull sharks are also considered a popular trophy.
Other animals die in shark nets, such as those used on South African beaches. As a result, populations have declined by 30 to 49 percent over the past few decades. The International Union for Conservation of Nature (IUCN) now lists the bull shark as endangered with a declining population trend.
Source and further information:
German Foundation for Marine Conservation (Deutsche Stiftung Meeresschutz)
Friendships among bull sharks: