The Most Extreme “Marine” Habitat: Freshwater
For a salt water adapted shark, few environments could be as extreme as the freshwater environment. Only 5% of elasmobranchs (sharks and rays) frequently enter freshwater and only 3-4% spend their entire lives there, compared to 41% of teleost (ray-finned) fish.
Marine sharks are constantly surrounded by highly salty seawater, which can cause the shark to lose precious water through a process called osmosis, where water molecules move across a membrane from an area of high water concentration (low dissolved solute concentration) to an area of low water concentration (high dissolved solute concentration). In other words, if the amount of dissolved solutes in the shark’s body doesn’t match the amount in the surrounding seawater, they will lose water from their cells and body fluids and become dehydrated, eventually resulting in death.
Obviously, this doesn’t actually happen as sharks are well adapted to living in the sea, and they deal with this osmotic problem by maintaining high levels of urea and tri-methylamine oxide (TMAO) in their cells. These organic compounds are osmotic regulators that work by increasing the concentration of dissolved solutes in the cell, which draws water in through osmosis. Marine sharks also use their kidneys and rectal glands to actively remove the excess sodium and chloride ions (the ‘salt’ from the sea) that they are constantly absorbing.
These adaptations, however, are fixed and uncontrollable, and if marine sharks entered freshwater, they would not be able to regulate the amount of water and solutes in their bodies. Freshwater also poses difficulties for their sensory systems, as it is a poor conductor of electricity compared to seawater, and so the sharks electroreceptive sense (used for hunting, avoiding predators and detecting mates) is severely compromised.
A new study also found that most sharks actually sink in freshwater. Sharks remain almost neutrally buoyant by having huge livers (taking up to a third of their body cavity space) filled with a low-density oil called squalene. Freshwater is less dense than seawater as it has less dissolved ions in it, which means it gives less buoying force, so for sharks to remain neutrally buoyant they would need livers 8 times the size they currently are to be able to stay afloat.
Exceptions to the Rule
There are 43 species of elasmobranch known to spend at least some of their lifecycle in freshwater, but probably the most famous of these is the bull shark (Carcharinus leucas). Bull sharks frequently spend long periods in rivers, lakes and estuarine environments. They use rivers as nursery grounds, with the smallest, youngest bull sharks usually found upstream, and the larger, sexually mature sharks found in estuaries and the surrounding marine environment. This size segregation protects the young sharks from being predated upon by the cannibalistic adults.
Bull sharks are adapted to survive in marine habitats by reversing the usual osmoregulatory processes in marine sharks. Rather than maintaining high urea levels and low sodium and chloride levels, they have smaller rectal glands to stop them from removing as much sodium and chloride from the body when in a freshwater environment, and they also appear able to decrease the amount of urea they produce and increase the amount they excrete depending on the salinity of their environment. These famous sharks are well known and their adaptations to the freshwater environment are fairly well studied. However, there is a genus of sharks that nearly no one seems to have heard of and about very little is known…
The Glyphis Genus
The Glyphis sharks are among some of the least known and least well understood of all the elasmobranch species. It is thought that there are 6 species of Glyphis shark, half of which are completely undescribed. The Glyphis genus is special as it is believed to contain the only obligate freshwater shark species, currently named “Glyphis A” and “Glyphis C”. Unfortunately, too little is known about any of these species to know if any are truly obligate freshwater species, but the currently described sharks are freshwater tolerant and appear to have similar life histories to the bull shark.
Just two species of Glyphis shark are comparatively well described and studied: the speartooth shark Glyphis glyphis and the northern river shark Glyphis garricki. Both are found in turbid tidal rivers, estuaries and marine habitats. Glyphis glyphis appear to use rivers as nursery areas like bull sharks, as no sexually mature individuals have been found in rivers, and they show similar size segregation in rivers to bull sharks. However, G. garricki are found at all sizes in freshwater, estuarine and marine environments, implying they are less reliant on rivers as nursery habitats. So far, it is unclear what benefits they gain from being able to travel between fresh and saltwater environments.
The first ever Glyphis shark caught on film. These incredible sharks are so rare that under the IUCN’s protection, they can only be handled for 60 seconds before having to be released. Credit: River Monsters, Animal Planet.
These sharks are well adapted for hunting in the dark, murky waters of rivers and estuaries, with small eyes but many ampullae of Lorenzini (electrosensory organs) to detect prey when visibility is poor. Unfortunately, that is pretty much the extent of what we know about about these incredibly rare sharks, and opportunities to learn more about them are limited.
All Glyphis species are listed as endangered or critically endangered on the IUCN red list, which means they can’t be handled for long periods of time if caught, they can’t be targeted by fishermen and they are so rare that they are almost never caught as by-catch. It is unsure if they osmoregulate in the same way as bull sharks do, or if they have some novel coping mechanism to survive in freshwater. Other questions left unanswered about these sharks include things such as their litter sizes and gestation period, and regrettably, it is possible that these questions may never be answered.