Freshwater Tolerance in Bull Sharks, by Christian Park

The bull shark, (Carcharhinus leucas) is one of the few cartilaginous fish species that are known to be diadromous and euryhaline. It is also the only recorded shark species that can thrive in fresh and saltwater while remaining a dominantly saltwater species. The genus Glyphis contains three species of true freshwater sharks, which does not include the bull shark, but is often confused to be associated with that genus. The bull shark is in the genus Carcharhinus, which is composed of several species of saltwater sharks that are not capable of freshwater survival.

Bull Shark.  Photo by Brook Ward.  CC BY-NC 2.0.  Flickr.com

The distribution of the bull shark is one of the widest ranges within the genus Carcharhinus and can be found along both coasts of North America, South America, Africa, Australia, Southeastern Asian Islands, West Coast of India, and other scattered regions (Karl et al. 2010). 

Distribution of Bull Shark.  CC BY-SA 3.0  IUCN.org

The bull shark’s diet consists of fish, rays, sharks, birds, turtles, crustaceans, dolphins, and even terrestrial mammals. They are known to actively and aggressively hunt in turbid waters and tend to be solitary hunters but have been observed in congregations with other bull sharks when it provides as a feeding advantage (Snelson et al. 1984).

The bull shark is the most well-known species of shark that has been reported travelling and surviving in freshwater systems. Other elasmobranch species that have been known to travel into rivers include stingrays and sawfish, but cannot thrive in freshwater systems for long periods of time like the bull shark (Pang et al. 2015). Most elasmobranchs are unable to thrive in freshwater due to their blood saline concentration typically matching their surrounding environment through the accumulation of urea (Werry et al. 2011). Bull sharks, however, tend to show a significantly lower concentration of urea within their blood. Even so, the osmolarity of the bull shark’s blood remains much higher than a freshwater external environment. To help regulate the osmolarity even further, the bull shark utilizes four organs: the rectal gland, liver, gills, and kidneys. When in freshwater, the bull shark decreases its excretory activity, which in turn conserves saline osmolarity within its body. The gills are responsible for the uptake and extraction of sodium chloride from freshwater, although in minute concentrations in comparison to saltwater. The liver produces the urea needed to maintain osmolarity within the shark and changes its production levels based on the surrounding environment’s salinity. The kidneys produce large amount of dilute urine and actively reabsorb solutes into the blood of the bull shark. These organs working in unison allow the shark to maintain osmolarity in freshwater environments, perpetually forever, unlike other elasmobranchs (Pillans et al.  2004). The ability for the bull shark to do this has been assumed through evolutionary disconnect; one assumption being that the bull shark encountered a population bottleneck during the last ice age and resulted in separation from the rest of the Elasmobranchii subclass and favored genes for an osmoregulatory system.

Osmoregulation in salt water and freshwater in Bull Sharks.   (c) Joshua K. Moyer

The ability to survive and thrive in fresh water has allowed the bull shark to gain an advantage in its reproductive cycle. Elasmobranchs typically mate and spawn in saltwater systems, and sometimes even brackish waters. Elasmobranchs that are born in saline systems are still subject to being prey items, disregarding their proportionally larger size than freshwater species. Bull sharks are viviparous, meaning they give live birth from anywhere between 1 and 13 pups. The pups average about two feet in length at birth, which is much larger than most any species of freshwater predators can consume. With this advantage, bull shark pups are able to grow and develop without predators in their current ecosystem. This reproductive advantage is the only known advantage that freshwater tolerance provides the bull shark (Jenson, 1976).
 

Though the reasoning behind the bull shark’s ability to survive and thrive in freshwater remains somewhat of an anomaly, it can be seen that the bull shark has utilized its ability in order to provide an advantage over its other cartilagenous relatives. It is unsure as to whether the bull shark has always had this ability, but through evolution and adaptation, it has become one of the most ecologically versatile fish on the planet. Bull shark pup survival and dominance in freshwater is a key adaptation that has allowed it to sustain healthy numbers in world with loose and unregulated shark protection laws, while many other species suffer high mortality rates due to anthropogenic activities. However, the bull shark has gained a hostile reputation among many people due to their aggressive behavior, and being able to thrive in freshwater is taken as a “scary” and “dangerous” behavior that many people fear the bull shark for. In turn, this has made the bull shark a target for capture and kill when discovered in freshwater systems for human safety. This may be the only slight disadvantage that the evolution of freshwater tolerance has brought to the bull shark, but their adaption has nonetheless allowed their populations to thrive while other species struggle to recover.

References

Jenson, N. H. (1976). Reproduction of the Bull Shark, Carcharhinus leucas, in the Lake Nicaragua-Rio San Juan System. Pages 539-559 T.B. Thorson, editor, Investigations of the Ichthyofauna of Nicaraguan Lakes.  University of Nebraska, Lincoln.

Karl, S. A., Castro, A. L., Lopez, J. A., Charvet, P., & Burgess, G. H. (2010). Phylogeography and conservation of the bull shark (Carcharhinus leucas) inferred from mitochondrial and microsatellite DNA. Conservation Genetics,12(2), 371-382.

Pillans, R. D. (2006). The physiological ecology of the bull shark Carcharhinus leucas in the Brisbane River. PhD Thesis, University of Queensland, Australia. 

Pillans, R. D., Good, J. P., Anderson, W. G., Hazon, N., & Franklin, C. E. (2004). Freshwater to seawater acclimation of juvenile bull sharks (Carcharhinus leucas): Plasma osmolytes and Na /K -ATPase activity in gill, rectal gland, kidney and intestine. Journal of Comparative Physiology B,175(1), 37-44.

Reilly, B. D., Cramp, R. L., Wilson, J. M., Campbell, H. A., & Franklin, C. E. (2011) Branchial osmoregulation in the euryhaline bull shark, Carcharhinus leucas: A molecular analysis of ion transporters. Journal of Experimental Biology 214(Pt 17):2883-2895. doi: 10.1242/jeb.058156.

Snelson, F., F., Mulligan, J., T., Williams, & E., S. (1984, January 01). Food habits, occurrence, and population structure of the Bull Shark, Carcharhinus leucas, in Florida coastal lagoons.  Bulletin of Marine Science 34:71-80.

Pang, P.K.T., R.W. Griffith, & J.W. Atz. (2015) Osmoregulation in Elasmobranchs. American Zoologist 17:365-377.

Werry, J. M., Lee, S. Y., Otway, N. M., Hu, Y., & Sumpton, W. (2012).   A multi-faceted approach for quantifying the estuarine–nearshore transition in the life cycle of the bull shark, Carcharhinus leucas.  Marine and Freshwater Research 62:1421-1431.