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Description

A selection of sharks and rays that live in freshwater, to scale.

Compared to the teleost (bony) fish, very few elasmobranchs (only 5% of all sharks and rays) spend time in freshwater environments. Although the reasons are largely unknown, marine elasmobranchs maintain a high requirement for urea in their bodies. Species which freely travel between marine and freshwater environments are unable to eliminate urea from their bodies, and as a result energy and nitrogen losses limit the time they can spend in freshwater. Effectively euryhaline (saline tolerant) sharks and rays get dehydrated in freshwater. The few obligate freshwater groups, specifically potamotrygonid and a few dasyatid rays, are completely saline intolerant and have actually lost the capacity to concentrate urea when exposed to higher levels of salinity. Adaptations to freshwater life also include giving birth to live young and a modified electrosensory system to work in low conductivity environments. All elasmobranchs control buoyancy with an oil-rich liver. In freshwater environments, elasmobranchs suffer from a lack of buoyancy and get around this by fattening up their livers. However this results in larger bodies and thus less efficient swimming, with studies showing that bull sharks spend 50% more energy on lift when swimming in freshwater compared to seawater.

* Not to be confused with a number of popular cyprinid aquarium fish, there are two groups of sharks which frequent freshwater environments. Best known of all the freshwater elasmobranchs is the common bull shark, found ubiquitously in warm marine and freshwater environments around the world. While capable of living entirely in freshwater, a lack of suitable prey and a higher tolerance to saltwater results in older bull sharks moving back to coastal waters fulltime. Similarly the poorly known and rarely seen river sharks Glyphis of Southern Asia and Northern Australia also spend time between rivers and coastal areas. To offset the energy of swimming, speartooth sharks will often follow the tidal movements up and down rivers.

* Potamotrygonids or river stingrays are by far the most speciose group known with 4 genera (a fifth is totally marine) and at least named 35 species found throughout South America. They are the most highly adapted of the elasmobranchs to freshwater being found across a wide range of aquatic habitats such as lakes and fast-flowing rivers, living highly sedimented through to clear water. Species that live over loose substrates such as sand, will readily bury themselves, while others happily live on rocky river bottoms.

* Only a small number of whiptail stingray species spend time in freshwater and brackish environments. Based on sheer size alone, the most familiar is the Giant freshwater whipray. Previously considered a member of Himantura, this large genus was split up and now this ray is part of the Urogymnus rays. This is one of the largest freshwater fish alive, with a disk width of nearly 2 metres across. It can be almost impossible to lift a hooked giant ray, as they will bury themselves in the sediment and boats have been capsized in the past.

* Probably the oddest of the freshwater elasmobrachs, and definitely the largest, are the sawfish. Although they are actually a type of ray, sawfish have a long body form which is exaggerated by the rostrum. This elongated extension of the skull has an array of 'teeth' along the edge (actually denticles) used to detect and stun fish. While all species are regularly found in estuarine water, of the five species, only largetooth sawfish are frequently found in freshwater environments.

Description

A selection of sharks and rays that live in freshwater, to scale.

Compared to the teleost (bony) fish, very few elasmobranchs (only 5% of all sharks and rays) spend time in freshwater environments. Although the reasons are largely unknown, marine elasmobranchs maintain a high requirement for urea in their bodies. Species which freely travel between marine and freshwater environments are unable to eliminate urea from their bodies, and as a result energy and nitrogen losses limit the time they can spend in freshwater. Effectively euryhaline (saline tolerant) sharks and rays get dehydrated in freshwater. The few obligate freshwater groups, specifically potamotrygonid and a few dasyatid rays, are completely saline intolerant and have actually lost the capacity to concentrate urea when exposed to higher levels of salinity. Adaptations to freshwater life also include giving birth to live young and a modified electrosensory system to work in low conductivity environments. All elasmobranchs control buoyancy with an oil-rich liver. In freshwater environments, elasmobranchs suffer from a lack of buoyancy and get around this by fattening up their livers. However this results in larger bodies and thus less efficient swimming, with studies showing that bull sharks spend 50% more energy on lift when swimming in freshwater compared to seawater.

* Not to be confused with a number of popular cyprinid aquarium fish, there are two groups of sharks which frequent freshwater environments. Best known of all the freshwater elasmobranchs is the common bull shark, found ubiquitously in warm marine and freshwater environments around the world. While capable of living entirely in freshwater, a lack of suitable prey and a higher tolerance to saltwater results in older bull sharks moving back to coastal waters fulltime. Similarly the poorly known and rarely seen river sharks Glyphis of Southern Asia and Northern Australia also spend time between rivers and coastal areas. To offset the energy of swimming, speartooth sharks will often follow the tidal movements up and down rivers.

* Potamotrygonids or river stingrays are by far the most speciose group known with 4 genera (a fifth is totally marine) and at least named 35 species found throughout South America. They are the most highly adapted of the elasmobranchs to freshwater being found across a wide range of aquatic habitats such as lakes and fast-flowing rivers, living highly sedimented through to clear water. Species that live over loose substrates such as sand, will readily bury themselves, while others happily live on rocky river bottoms.

* Only a small number of whiptail stingray species spend time in freshwater and brackish environments. Based on sheer size alone, the most familiar is the Giant freshwater whipray. Previously considered a member of Himantura, this large genus was split up and now this ray is part of the Urogymnus rays. This is one of the largest freshwater fish alive, with a disk width of nearly 2 metres across. It can be almost impossible to lift a hooked giant ray, as they will bury themselves in the sediment and boats have been capsized in the past.

* Probably the oddest of the freshwater elasmobrachs, and definitely the largest, are the sawfish. Although they are actually a type of ray, sawfish have a long body form which is exaggerated by the rostrum. This elongated extension of the skull has an array of 'teeth' along the edge (actually denticles) used to detect and stun fish. While all species are regularly found in estuarine water, of the five species, only largetooth sawfish are frequently found in freshwater environments.