Did you know that now all sharks only live in saltwater!? Many sharks are "stenohaline", meaning they live the salty water of the oceans, but there are several species (not just the bull shark!) which are able to survive in brackish and even freshwater habitats. So which species live in rivers? How can they survive there? And do they share these freshwater habitats with other species?
A Dash of Salt
Different species of fish are evolved to live in either saltwater or freshwater. As their bodies are not capable of maintaining sustaining internal salt levels (known as "osmolarity"), fish will die if they are kept in water of the wrong salinity. Amongst the closely related sharks, skates and rays (collectively known as "Chondrichthyans"), only about 5.8% of species use habitats with low salinity (Dwyer et al, 2020).
Euryhaline = (Greek) eurus = ‘wide’ & halinos = 'salt'
These "euryhaline" sharks, are able to tolerate wide variations in salinity, by changing the levels of salt in their plasma. This means they can enter a very saline area and adjust to maintain "homeostasis", but then move into water with lower salinity and alter their plasma osmolarity accordingly. This is known as "osmoregulation" (Dwyer et al, 2020).
They do this by retaining urea and TMAO in their tissues (substances which counterbalance salt) and their rectal gland and kidneys regulate salts to match the external environment. This process requires energy, so it can be costly for sharks to do over extended periods of time. Therefore, sharks will also often move in order to find more favourable waters, so they can save energy (Dwyer et al, 2020).
In the Bull Pen
The shark which is most famous for being able to live in rivers is the bull shark (Carcharius leucas), but there are actually many other species that live in freshwater habitats too. For a comprehensive list, see Getting Fresh.
For example, river systems are vitally important habitats for the speartooth shark (Glyphis glyphis). These sharks are not wide-ranging, and are only found in Northern Australia and Papua New Guinea (Dwyer et al, 2020).
In the Wenlock and Ducie River systems in Queenslands, bull sharks enter the freshwater to systems give birth to their young, before swimming back out into the open ocean. Comparatively, the speartooth sharks move up the rivers to reproduce, but only live in a small area around the river mouth when they are mature. They never go as far the bulls and their distibutions are much smaller (Dwyer et al, 2020).
Learning to Share
As both sharks occupy the same river, researchers recently conducted a study to understand how the juvenile bull and speartooth sharks share the area in the Queensland river systems. They learned that these two species co-exist by "niche partitioning"; living in slightly different areas and targeting different types of prey (Dwyer et al, 2020).
The speartooths seemed to eat animals lower in the food chain (aka "lower trophic levels"), which they targeted on the "benthos" (meaning they hunted animals that live on the river bottom and in the sandy substrate). Whereas bull sharks consumed higher trophic-level prey from both the river and marine habitats (Dwyer et al, 2020).
The speartooths were often found in areas closer to the river estuary, so they were often living in waters with higher salinity compared to bull sharks of a similar size. Whilst both the species had a wide salinity tolerance, the speartooth sharks were much more restricted in the areas they lived compared to the bull sharks (Dwyer et al, 2020).
Interestingly though, they also found that the sharks' ranges shifted during the different seasons throughout the year. As the monsoonal rains arrived (in January) and more freshwater ran-off into the river, both species migrated down the river and were found closer to the ocean in subsequent months. The bull sharks also popped out into the open ocean during this time (Dwyer et al, 2020).
This work is important because it shows us that these two species do not share a "communal nursery habitat" in these river systems. They mostly try to avoid each other to reduce competition and potential threats from the other sharks (Dwyer et al, 2020).
Do Not Disturb
Tropical river sea estuaries are under serious threat from human disturbance. Many of these systems are under pressure from water extraction, pollution and fisheries, meaning that the habitat becomes significantly degraded. The system in Queensland is especially affected by land clearing, pollution and changes in water flow in response to human activity, which may have a serious affect on the animals living there. Euryhaline sharks come into close contact with human activity, so they are especially vulnerable to human disturbance. (Ward & Larson, 2012; Dwyer et al, 2020).
As speartooth sharks have such a specialised habitat preference in these rivers, they are likely not able to evade these human-induced pressures. Coupled with their small range, this means that this species will be under serious threat from human disturbance in the river systems that are so important for their breeding. As the speartooth is flagged as Endangered by the International Union for the Conservation of Nature (IUCN), and is considered Critically Endangered in Australia, protection to preserve these critical habitats will be vital to ensure their survival (Ward & Larson, 2012).
References
Dwyer RG, Campbell HA, Cramp RL, Burke CL, Micheli-Campbell MA, Pillans RD, Lyon BJ & Franklin CE (2020). Niche partitioning between river shark species is driven by seasonal fluctuations in environmental salinity. Functional Ecology, 1–16. Access online.
IUCN (2020). International Union for the Conservation of Nature Red List of Threatened Species. 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.
Ward S & Larson H (2012). Threatened species of the northern territory: Speartooth shark (Bizant River shark) Glyphis glyphis. Northern Terrirtory Government. Access online.
By Sophie A. Maycock for SharkSpeak
