I Need Some Space
Updated: Apr 27, 2021
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. Many sharks are "stenohaline", meaning they live the salty water of the oceans. However, it may surprise you to learn that not all sharks live in saltwater... In fact there are several species, such as the speartooth shark (Glyphis glyphis) and the bull shark (Carcharius leucas), which have a very wide salinity tolerance. They are able to maintain their internal equilibrium (known as "homeostasis") though "osmoregulation" which allows them to move between oceans and rivers.
Amongst the closely related sharks, skates and rays (collectively known as "Chondrichthyans"), only about 5.8% of species use habitats with low salinity. These "euryhaline" sharks, are able to tolerate wide variations in salinity, by changing the levels of salt in their plasma. Their rectal gland and kidneys regulate salts to match the external environment, by the movement and excretion of "ions", and the production of "urea" (a substance which counterbalances salt). 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. Yet, this process requires energy, so it can be costly for sharks to do over extended periods. Therefore, sharks will also often move in order to find more favourable waters, so they can save energy (Dwyer et al, 2020).
Euryhaline = (Greek) eurus = ‘wide’ & halinos = 'salt'
Commonly, euryhaline sharks use river habitats when they are younger and move into marine areas as they grow to maturity. For example, bull sharks move into river systems in Australia to give birth to their young, before swimming back out into the open ocean. In the Wenlock and Ducie River systems in Queensland, the juvenile bull sharks therefore share space with juvenile speartooth sharks. Compared to the bulls, these sharks are not wide-ranging, and are only found in Northern Australia and Papua New Guinea. The adults also move into the rivers to reproduce, but when mature, only live in a small area around the river mouth. Therefore, it is thought that these low salinity river environments might be used as "nursery habitats" for these species (Dwyer et al, 2020).
As both sharks occupy the same river, researchers recently conducted a study to understand how the juveniles share the area in the Queensland river systems. They caught sharks to equip them with a tracking tag, which allowed them to understand how they moved within the river, and where they were found versus where they were absent. They also took tissue samples for analysis, in order to compare their tissue osmolarity and analyse what food they had eaten. This allowed them to determine whether each species occupied a different "niche" in the system (Dwyer et al, 2020).
As these tropical river systems are extremely dynamic environments (with daily and seasonal fluctuations in salinity, temperature, and water flow and turbidity), they sampled throughout the year to assess how the different species shifted their ranges in response to changing envrionmental conditions (Dwyer et al, 2020).
Their chemical analysis of the sharks' tissues suggested that these two species were targeting different kinds of prey. 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).
Their catch and tagging work revealed that juvenile speartooth sharks and bull sharks generally were not found in the same areas of the river. The speartooths were found in waters with higher salinity compared to bull sharks of a similar size. This meant that the speartooth sharks were often found in regions closer to the river estuary than the bull sharks. They called this a "partitioned habitat along a salinity gradient" (Dwyer et al, 2020).
Yet, they found that the sharks' ranges shifted during the different seasons. Habitat segragation was clear during the dry season (especially between September to December). However, 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 two species were still segregated to a degree, but they had much more overlap in their ranges between February and April. At this point they both experienced similar water salinities. However, the bull sharks also made short runs into considerably more saline areas, in the river mouth, where the speartooth sharks did not (Dwyer et al, 2020).
The researchers concluded that both of these sharks conducted these seasonal migrations in response to freshwater inflow. As the monsoon rainwater reduced the salinity of the water, the sharks would have to shift their internal osmoregulation. Therefore, in order to minimise energy expenditure, they sought out more saline waters, closer to the ocean. During this time, the bull sharks made short trips into more saline waters, when hunting for prey. As the seasons changes, the rains stopped and upstream salinities became more favourable, both sharks swam back upriver to their original areas (Dwyer et al, 2020).
They hypothesised that, during the dry season, the juvenile sharks were segregated in the river, as they were targeting different prey and avoiding each other in order to reduce "competition". Therefore, it seems that these two species do not have a "communal nursery area" that they share (Dwyer et al, 2020).
What is especially important about this work, is that it taught us that, whilst both these sharks have a wide salinity tolerance, juvenile bull sharks moved into extreme high and low salinity areas, where the speartooths were much more restricted. These sharks were only found in a narrow salinity range (a "brackish zone"). This means that these sharks have a very specialised habitat preference (Dwyer et al, 2020).
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 (Dwyer et al, 2020).
As euryhaline sharks come into close contact with human activity, they are especially vulnerable to human disturbance. Whatsmore, their restricted ranges can mean they are not able to evade human-induced pressures. This study showed that the speartooth shark has an especially specialised habitat preference in these rivers (Dwyer et al, 2020). Coupled with their limited range, this species will be under serious threat from human disturbance in the river systems that they use for reproduction (known as "natal grounds") in Australia (Ward & Larson, 2012). If the sharks were no longer able to use this habitat to breed, their populations could seriously decline. As the speartooth is currently classified as 'endangered' by the International Union for the Conservation of Nature (IUCN), and is considered 'critically endangered' in Australia, so protection to preserve these critical habitats will be vital to ensure their survival.
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.
IUCN (2020). Internation Union for the Conservation of Nature Red list. 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.