It may come as a surprise to you to learn that sharks are not only found in the oceans. In fact, the brackish waters of estuaries, tidal regions of rivers and even freshwater lakes are home to a few different types of sharks and rays. You may never have even heard of some of these creatures, but today these elusive freshwater species are some of the most threatened sharks and rays on the planet today! So where can you find freshwater sharks and rays? How do they survive in freshwater habitats? And why are they so at risk?
Sharks & Rays in Rivers & Lakes
As many as 171 different species of sharks and rays (known collectively as "elasmobranchs") can be found in freshwater environments. This is 5% of all the species known to science! These animals fall into two different groups: those that live permanently in freshwater ecosystems (known as "obligate freshwater species") and those which can move between freshwater and marine habitats (known as "euryhaline species") (Martin, 2005; Lucifora et al, 2015; IUCN SSG, 2022).
Most elasmobranchs which live permanently in freshwater are myliobatoids - a type of ray. The vast majority are neotropical stingrays (Family Potamotrygonidae), which make up 38 of the world's 45 true freshwater elasmobranch species. There are also several species of stingrays (Family Dasyatidae) which live permanently in freshwater (Martin, 2005; Lucifora et al, 2015; IUCN SSG, 2022; Constance et al, 2023).
These freshwater species are restricted to only a few river drainage systems around the world. The potamotygonid rays are only found in the river drainage systems of South America, including the Amazon. Other freshwater rays are restricted to Indo-Pacific and African river systems, including (but not limited to) the Mekong, and Sumatran and peninsular Malaysian rivers, and the Congo and Niger drainages in Africa (Martin, 2005; Lucifora et al, 2015; IUCN SSG, 2022).
The euryhaline elasmobranchs are more diverse. At least four different species of sharks use freshwater and "brackish" (slightly salty) habitats. The most famous of these is the bull shark (Carcharius leucas), which can be even be found in the hypersaline St Lucia Lake system of South Africa and as far as 4200 km upstream in the Amazon River! There are also several other sharks, including the speartooth (Glyphis glyphis) and Ganges shark (Glyphis gangeticus), several stingrays (Family Dasyatidae), a skate (Family Rajidae) and the magnificent sawfishes (Family Pristidae) which can all move between salt- and fresh-water areas (Martin, 2005; Lucifora et al, 2015; IUCN SSG, 2022).
Euryhaline elasmobranchs are much more widely distributed than their freshwater relatives. They can be found in most major tropical and subtropical river basins (Martin, 2005; Lucifora et al, 2015; IUCN SSG, 2022).
There are also a few examples of euryhaline species which have taken to living in freshwater habitats permananelty. Bennett's stingray (Hemitrygon bennetti) have made a home in the Pearl River, China and similarly the Atlantic stingray (Hypanus sabinus) can be always be found in Lake Jesup in Florida, USA. The magnificent largetooth sawfish (Pritis pristis) also lives year-round in Lake Nicaragua (IUCN SSG, 2022).
How Do Sharks & Rays Survive in Freshwater?
You could certainly be forgiven for not knowing that any elasmobranchs could be found in rivers and lakes because the vast majority of sharks and rays would not be able to survive in freshwater. Most elasmobranchs have evolved to live in a marine environment. Their bodies have adapted to be able to regulate their internal osmolarity (salt levels) when they are submerged in salt water. This is possible thanks to a chemical called urea in the shark's blood and a specialised organ called the "rectal gland" (Lucifora et al, 2015). To learn more you can check out I Need Some Space.
Elasmobranchs which have made their homes in freshwater have evolved to lose these abilities, as they are no longer necessary. Rectal glands are often atrophied and urea levels are not as high in the blood of obligate freshwater species (Lucifora et al, 2015).
Freshwater Sharks are In Hot Water
The adaptation to live in a freshwater environment might, in the end, be the downfall of some of these wonderful species of sharks and rays. As obligate freshwater species have lost some or all of their ability to regulate their homeostasis in salt water, they are restricted to a limited freshwater region. This means that they are not able to escape and find a new place to live if their habitat becomes degraded... And freshwater habitats are becoming increasingly damaged all over the world! (Martin, 2005; Lucifora et al, 2015; IUCN SSG, 2022; Constance et al, 2023).
"Globally, freshwater areas are probably being degraded and eliminated faster than tropical forests, mak[ing] them the fastest disappearing major biome in the world."
- Martin, 2005
Because a large proportion of the human population is clustered around water resources, freshwater habitats have been been exposed to extreme human disturbance and degradation (Martin, 2005; IUCN SSG, 2022; Constance et al, 2023).
Sewage release is a huge threat to freshwater ecosystems, as this can lead to uncontrolled growth of plant life (known as "eutrophication") and reduction of oxygen levels in the water ("hypoxia"). Engineering projects, like dams, land reclamation and the removal of water for human use can all alter the natural flow of water through a particular region and reduce the amount of habitat available to the animals that live there. Increased recreational use of waterways causes disturbance to the animals that live in rivers and lakes. Urbanisation, industrialisation and land repurposing along the river banks lead to sedimentation and pollution. In short, animals that rely on freshwater habitats are constantly competing against humans for aquatic resources (Martin, 2005; IUCN SSG, 2022; Constance et al, 2023).
Freshwater sharks and rays are additionally threatened by intensive overfishing. In some places they are considered to be dangerous vermin, so are killed to protect human beings. Other communities rely on elasmobranchs as a food source. Disturbingly, as some of these species are so beautifully patterned, they are also extremely valuable on the global market either dead or alive - they can fetch a pretty penny in the ornamental or aquarium trade respectively (Martin, 2005; Lucifora et al, 2015; IUCN SSG, 2022).
"Non-marine elasmobranchs are threatened with extinction at twice the rate of their marine counterparts ... 76% of species are listed as threatened with extinction."
- IUCN SSG, 2022
Today, there is serious concern for freshwater elasmobranchs on a global scale! It is thought that their populations are declining more severely than terrestrial animals or even their marine counterparts (IUCN SSG, 2022).
Over one third of all obligate freshwater sharks and rays are threatened with extinction. In fact, there are only two species which scientists believe are secure. Many of these animals are also classified as 'data deficient' (DD), meaning scientists have yet to assess their populations, so these numbers could be much higher. Many of the eurylahline species are also threatened. The river sharks (Glyphis species) and sawfish (Pristidae species) have an especially high risk of extinction in the wild (Lucifora et al, 2015; IUCN SSG, 2022).
A Fresh Hope?
There is serious concern for the futures of all freshwater sharks and rays and action must be taken immediately to protect these animals and their habitats! So how do we save them? This is certainly not something that is easy to solve, as protecting these animals requires the consideration of different issues:
sociological issues - many communities rely on their fisheries for food and income,
political upheaval - the regions some of these animals inhabit are struggling through civil unrest and war,
cultural issues - it can be challenging to educate people not to fear sharks, when it is ingrained in their communities that these animals are dangerous (Martin, 2005; Lucifora et al, 2015; IUCN SSG, 2022).
What is certain though, is that we need to fill the gaps in our knowledge about these species if we have any hope of saving them. We know very little about the biology and ecology of many freshwater species. These means we have no idea where their critical habitats are, what their natural population sizes are or how quickly they may recover after declines. Only through learning more about these wonderful, elusive animals, will we stand any hope of being able to save them from extinction (Martin, 2005 Lucifora et al, 2015; IUCN SSG, 2022).
References
Constance JM, Garcia EA, Pillans RD, Udyawer V & Kyne PM (2023). A review of the life history and ecology of euryhaline and estuarine sharks and rays. Reviews in Fish Biology and Fisheries. Access online.
IUCN SGG (2022). A Step Towards Contextualizing the Conservation of Non-Marine Elasmobranchs Within the Global Freshwater Biodiversity Crisis. In: (Eds.) Scholl M, Jabado R, Scott M & Stein C. IUCN SSG Shark News Magazine, 4 Access online.
Lucifora LO, de Carvalho MR, Kyne PM & White WT (2015). Freshwater sharks and rays: A quick guide. Current Biology, 25, 965–979. Access online.
Martin MR (2005). Conservation of freshwater and euryhaline elasmobranchs: A review. Journal of the Marine Biological Association UK, 85, 1049-1073. Access online.
By Sophie A. Maycock for SharkSpeak
