Why do sharks eat other fish

Pisces: 10 Truths About Sharks and Rays

The great white shark is considered to be the horror of the seas. Far fewer people die from him than from selfies. Basking mouth sharks are harmless giants, but Greenland sharks prey on polar bears. Bull sharks and giant stingrays also live in fresh water ... We present these and other facts about rays and sharks below.

Bathers and surfers in the ocean fear little more than sharks - although the number of deaths is negligible when you put it in relation to the number of people who venture into the sea. However, if you still think that you are only safe in fresh water, you should avoid certain waters in the tropics. In fact, a dozen species of shark regularly visit brackish water, rivers or even lakes - at least temporarily. The best-known species among them is probably the bull or bull shark, which lives around the world in the course or estuary of larger rivers in warmer regions. It has been found in the Amazon as well as in the lower Mississippi, it swims in the Brisbane River as well as in the Ganges. Via the Río San Juan it has also reached Lake Nicaragua, where a larger population is native. It is potentially dangerous to humans; several deaths are attributed to him in the "International Shark Attack File". In contrast to the bull shark, which also lives in the sea, the river sharks are of the genus Glyphis almost exclusively freshwater inhabitants. They are under pressure from fishing and are very polluted, making them one of the most endangered species of shark on earth.

Greenland sharks (Somniosus microcephalus)are considered to be the longest-lived vertebrates on earth: the oldest specimens found to date are estimated to be 400 years, and the average life expectancy has so far been determined to be 272 years. In their home in the far north, they are among the top predators of the sea - which may even kill swimming polar bears. This is at least indicated by bone finds in the stomach of Greenland sharks, in which, among other things, a polar bear's jaw has been detected. However, scientists are not entirely sure whether the polar bear was actively shot or whether the sharks simply ate a drowned bear. One of the arguments in favor of the thesis of a captured victim is that no remains of carrion-eating marine invertebrates have been found. According to studies, the sharks also hunt in shallower water and prey on seals.

The brown-banded bamboo shark (Chiloscyllium punctatum) With a maximum length of 105 centimeters, it belongs to the rather small representatives of the large shark family. Instead, it is characterized by something different: Females can store male sperm in their bodies for at least four years and use it to fertilize eggs. This was proven by a baby shark that hatched in the Steinhart Aquarium in San Francisco in 2015 - 48 months after its mother last had contact with a male conspecific. In contrast to many bony fish, which release sperm and eggs into the water, the cartilaginous fish fertilization takes place in the womb: Many sharks and rays then lay relatively large sacs of ice in which the young animals develop. Other species, on the other hand, give birth to few live baby sharks. The sperm are stored in special glands and the female can fall back on them when eggs are ripening in her body or when conditions are particularly favorable.

Unfortunately, we humans only get new teeth twice in our lives - if we lose our adult teeth, artificial replacements have to be found. Sharks are better off in this case: if they lose part of the set of teeth, biting tools from the second row move up because sharks have several rows of teeth in their mouths at the same time. And that happens continuously like on an assembly line. This is also necessary because, for example, a great white shark loses an average of one tooth per week, for example because it gets stuck in its prey. Scientists have already found fossil shark teeth in the fossilized skeletons of pterosaurs, among other things. Basic sharks, which include hammerheads, develop up to 30,000 teeth in the course of their life. The biologist Gareth Fraser from the University of Sheffield is researching how this property, which is beneficial for sharks, could perhaps be used to allow us humans to regrow real third teeth.

In the deep sea, time - and life - passes more slowly: goblin sharks (Mitsukurina owstoni) are a good example of this. Fossils of the species date back to the Eocene 50 million years ago. However, it is still unclear what the purpose of its long snout, which protrudes over its mouth, is used for. It may offer additional space for the Lorenzini ampoules, a typical sensory organ of cartilaginous fish, with which sharks, rays and sea cats can detect electromagnetic fields and the finest temperature differences and thus recognize prey. The goblin sharks can catapult their mouths forwards by up to 20 centimeters when they snap shut, so that their snouts are not in the way. Their peculiar appearance ultimately also ensured their naming: Japanese fishermen pulled goblin sharks more often from the sea and called the animals »tengu-zame« - »zame« means shark, and »tengu« is the name of a long-nosed, red-faced demon in English and German usage became "Kobold".