Freshwater Sharks - Do They Exist?
- bioworld090
- Apr 24, 2023
- 5 min read
While sharks are usually seen as the apex predators of the marine world such as the oceans and seas, there are a few specific sharks that live in freshwaters such as the rivers. These sharks are commonly known as river sharks. River sharks are sharks that belong to the genus Glyphis in the family Carcharhinidae. The genus contains a total of 5 species, three extant and two extinct.
The three extant river shark species are the Ganges shark (Glyphis gangeticus), Speartooth shark (Glyphis glyphis) and Northern River shark (Glyphis garricki). The river shark group also includes two extinct species, Glyphis hastalis and Glyphis pagoda. Another species of shark that lives in both fresh and marine water is the bull shark, but it does not belong to the genus Glyphis.
The name of the group, river sharks, implies that they are freshwater fish and, in a way, it’s true. However, only the Ganges shark is exclusively a freshwater species.
GANGES SHARK
The Ganges shark (Glyphis gangeticus) is found in the Hoogly, Mahanadi, Ganga and Brahmaputra Rivers. It is the world’s only shark that lives exclusively in freshwater. In the river Ganges, it shares its habitat with the bull shark and is often confused with them. This led it to have a reputation for being a man-eater.
The Ganges shark is 56-61cm long at birth and reaches a length of around 178 cm at maturity. The maximum size can be 204 cm. The color of the body is mostly uniform gray or brown in color with no distinct patterns and markings. The teeth are long and sharp which possibly aid in catching and impaling fish. As a kind of requiem shark, the Ganges shark possesses five gills.
It possesses very small eyes, and it is suggestive of its possible adaptation to the poor visibility of the turbid waters it lives in. The small eyes and turbid surroundings also suggest the possibility of a not-so-good or limited sense of sight. It can indicate that this animal can have a heightened sense of hearing, smell and electroreception to compensate for the possible limited sense of sight.
This freshwater shark is already rare as very less is known about the Ganges shark, and it has already been declared critically endangered on the IUCN Red List due to some major threats it faces in the form of rapid habitat changes, habitat degradation, pollution, dams and barrages.
SPEARTOOTH SHARK
The speartooth shark is another member of the genus Glyphis. Its scientific name is Glyphis glyphis. It is found in Australia and New Guinea where it inhabits the areas of tropical rivers and coastal marine waters. It, unlike the Ganges shark, is not exclusively a freshwater animal.
The speartooth shark is around 50-65 cm in length at birth but when it becomes an adult, it can get as long as 2.6 meters. The body is gray-colored with a white underside. Two tiny eyes with a white ring surrounding them lie behind the short and broad snout. The eyes are equipped with nictitating membranes that allow the speartooth shark to protect its eyes from the muddy water without even closing them. The mouth contains two types of teeth – 26-29, large, triangular and serrated teeth in the upper jaw and 27-29, narrow, spear-like in the lower jaw. Respiration takes place through the five gills that it possesses.
The speartooth shark lives in turbid waters. Young to subadult sharks only live in waters with muddy bottoms and strong currents. The salinity level of the water can range from 0.8 ppt (nearly fresh) to 28 ppt (nearly marine). Adult speartooth sharks are generally found more towards the downstream as compared to the younger ones. Since the speartooth shark lives in turbid waters and is equipped with tiny eyes, its sense of sight is seemingly not strong. The tiny eyes and abundant ampullae of Lorenzini (gel-filled sensory organs that detect electric fields produced by other fishes) suggest that this creature uses electroreception to detect and hunt its prey.
The diet of speartooth shark is generally composed of small bony fishes and crustaceans. The dietary habit of adult speartooth shark is almost unknown. However, the skeletal structures found from the jaw of a female speartooth indicates that different types of bony fishes and stingray are a part of the adult speartooth shark’s diet. The dietary habits of the juveniles, on the other hand, are pretty well known. They are known to feed on a variety of fishes and crustaceans such as the Macrobrachium prawn, burrowing gobies, Ariid catfish, king threadfish, Nibea squamosa, bony bream and smalleye gudgeon.
Just like the other river sharks, the speartooth shark is also rare with a total population of around 2500 globally. It is declared vulnerable in the IUCN Red List. It poses no threat to humans but is threatened by human activities. Sometimes, commercial fisheries can catch it as a bycatch. It is also a victim of recreational fishing and bow fishing. The third threat it faces is habit degradation.
NORTHERN RIVER SHARK
The northern river shark, also known as the New Guinea river shark, is the third member of the river sharks and is found in the tidal rivers and coastal waters of northern Australia and Papua New Guinea. Just like the other two members of the river sharks, the northern river shark also lives in waters with poor visibility and muddy bottoms.
At birth, the northern river shark is around 65 cm long. It becomes mature at the length of 1.2-1.4 meters in case it’s a male and at a length of 1.4-1.7 meters, in case it’s a female. The maximum length it can attain is 2.5 meters. The color of the body is gray in color which transitions into a white color near the sides and on the belly. On both sides of the wide and flattened head of the animal, lie two tiny eyes that are equipped with nictitating membranes. The mouth contains two types of teeth – 31-34 rows of upright, triangular teeth with serrated edges in the upper jaw and 30-35 rows of narrow, straight to slightly curved teeth in the lower jaw.
The habitat of the northern river shark includes tidal rivers, estuaries and coastal bays that are highly turbid and have muddy bottoms. The young and juvenile sharks live in waters with a wide range of salinity (2-36 ppt). They are found in fresh, brackish and salt water. The adults, unlike the young ones, are restricted only to marine waters.
The northern river shark is extremely rare with only 250 individuals living in the wild, according to the current data. Like the other river sharks, it is also victim to commercial fishing, recreational fishing and habit degradation. It is a vulnerable species on the IUCN list.
BULL SHARKS
Even though the bull shark is not a member of the rivers shark group, the genus Glyphis, it is known for its ability to thrive in both fresh and marine water. It is found in oceans, rivers and even lakes. Some of the locations where they are found other than the oceans include the Zambezi River, Lake Nicaragua and Mississippi River and Ganges River.
The bull shark is a large animal with females averaging a length of 2.4 m and males averaging a length of 2.25 m. The newborn bull shark is generally about 70 cm long but can measure up to 81 cm in length.
The bull shark can easily switch between fresh and marine water. It reproduces in freshwater which seems to be a strategy to improve the survival rate of the juveniles. The juvenile bull sharks, after birth, stay in freshwater for some time before moving to the seas and oceans.
REFERENCES:
Comments