Featured Species Friday: Bull Shark (Carcharhinus leucas)

This week’s featured species is named for its large, blunt snout. The bull shark (Carcharhinus leucas) is a powerhouse of a shark that is known to aggressively defend its territory with great bursts of speed (Parker, 2008).

bull_shark1
Kok, A. (Photographer). (n.d.). Bull Shark (Carcharhinus leucas) [Digital Image]. Retrieved from https://a-z-animals.com/

The bull shark hunts a wide variety of prey including bony fishes, invertebrates, sea birds, turtles, other sharks, even carrion and garbage (Skomal, 2016). Just like their prey items, their feeding habits resemble those of their cousin, the tiger shark (Galeocerdo cuvier) They both are ambush predators, utilizing warm, shallow, coastal waters which are often murky, and are most actively hunting during twilight hours (Stevens, 1997; Skomal, 2016). Unlike the tiger shark, bull sharks utilize a combination of teeth to handle a diverse diet. Their upper jaws consist of serrated triangular teeth that act as their steak knife, cutting through tough meaty flesh and removing it in large sections. Their bottom jaw has sharp, dagger-like teeth that can pierce and hold slippery prey. (Stevens, 1997).

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Left: Top jaw of a bull shark. Center: Jaw of a bull shark. Right: Bottom jaw of a bull shark. Flannery, A. (Photographer). (2018) Original Content.

 

The bull shark is found in warm, temperate and tropical waters across the globe. Unlike other members of the Carcharhinidae family, the bull shark is known to swim up river into fresh water. They have been found 2,000 miles (3218 km) up the Amazon River and 1,800 miles (2897 km) up the Mississippi River- that’s as far as Illinois (Skomal, 2016)! In Africa, the bull shark is known to inhabit the Zambezi river, the fourth largest river in Africa. Locals know the bull shark as the “Zambezi Shark” or “Zambi.” A population of bull sharks has been observed and studied in Lake Nicaragua. This population leaves the ocean, moves through the Rio San Juan water system to the Lake where they may live for up to a year. Pregnant females have been observed moving into these freshwater areas to give birth to their young, which shelter the pups until they are large enough to move into open water (Simpfendorfer, & Burgess, 2009).

Bull-shark-distribution-300x139
Khalid, U. (Author). (n.d.). Bull shark world distribution [Digital Image]. Retrieved from https://animalsanswers.com

Many shark species are able to tolerate a range of salinity. The speartooth shark (Gylphis gylphis), for example, lives its life in low salinity environments at the mouths of rivers. However, unlike most species of shark, the bull shark is able to move freely from marine environments with high salinity to brackish -or water that is mixed with fresh and salt water- and onward into fresh water for great periods of time. They accomplish this because of an adaptation in their kidneys. Typically, in a salt water environment, a shark ingests various salts through their mouths as they swim. As a result, their kidneys and rectal glands need to remove the excess salt they ingest from their bodies in the form of urine, but with a small amount of water. When a shark moves into a fresh water system, they do not ingest salts, but their bodies still try to remove this excess salt from the bodies. As a result their cells become dehydrated and they die. A bull shark has the ability to turn off this need to remove excess salts by shutting down their rectal gland which secretes excess salt. This means their cells do not become dehydrated and they are able to survive in freshwater systems (Skomal, 2016; Klimley, 2013; Shark MOOC, 2016).

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Top: Illustration highlighting osmoregulation of the Bull Shark, Carcharhinus leucas in marine habitats Bottom: Illustration highlighting osmoregulation of the Bull Shark, Carcharhinus leucas in fresh water habitats (Shark MOOC, 2016).

 

Because the bull shark is often found in fresh and brackish waters that are dark and muddy, it often encounters humans where we swim and fish. Dr. Greg Skomal recounts his time tagging bull sharks in the bayous of Lake Pontchartrain in Louisianan. “In all the areas where we caught bull sharks, we found coffee-colored muddy water that was virtually impossible to see through… this species is also one of the top three species implicated in unprovoked attacks on humans. Needless to say, this is one area where I felt it best to work from a boat and not from the water” (2016). And the bull shark is likely responsible for more attacks than it is currently credited for. In remote areas of the Amazon and the Zambezi, likely bull shark attacks go unreported (Stevens, 1997). One infamous series of attacks in New Jersey 1916 may likely have involved a bull shark and a great white. In New Jersey 1916, over a period of 12 days, five people were brutally attacked, four were fatally wounded, along the New Jersey coast. The first attack occurred at Beach Haven, New Jersey on July 1st. Charles Vansant did not survive. The second attack came a few days later on July 6th in Spring Lake New Jersey. Charles Bruder did not survive. The last three attacks happened on July 12th in Matawan Creek, New Jersey. Lester Stilwell and Stanley Fisher did not survive. Joseph Dunn lost his leg (Capuzzo, & Cariou, 2001). For years the great white shark was the only shark species thought capable of being a these attacks. The series of attacks went on to inspire Peter Benchley’s Jaws. But after a century of exploration into shark biology and ecology, experts believe it was likely two species responsible for these attacks. The great white along the coast, and the bull shark in Matawan Creek.

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Library of Congress (Archive). (1916 Jul 17). Front page of the Evening Public Ledger in Philadelphia, PA.

 

While many around the world view the bull shark as one of the “Top Three Man Eaters” along with the great white shark and the tiger shark, the bull shark is an apex predator with a critical role in the ecosystem. Apex predators sit a top a carefully balanced trophic pyramid. They keep other mid-level predators in check, who in turn keep other predators and omnivores below them in check, who in turn keep the herbivores in check, who keep the primary producers in check. When the ecosystem becomes unbalanced because one level of the pyramid is removed, it can cause trophic cascades, throwing all the other levels of the pyramid into complete disarray. Bull sharks, and other apex level sharks, have their place in the ecosystem (Baum & Worm, 2009; Estes et al., 2011; Frisch et al., 2016). While I personally have mixed feeling about the practice of hand feeding sharks, in some areas of the world, bull shark ecotourism is a thriving enterprise that is aiding in their conservation and helping to educate people about sharks and their roles in the ecosystem.

 

BBC Earth (Videographer). (2012 December 26). Feeding Bull Sharks | South Pacific| BBC Earth [Video Clip]. Retrieved from https://www.youtube.com/

 

Shark Stats

Authority: Müller & Henle,1839

Family: Carcharhinidae, 59+ species

Length: Maximum 11 feet (3.4 m) females; 7 feet (2.1 m) males

Weight: Females 700 lbs (318 kg); Males 200 lbs (91 kg)

Habitat: Shallow coastal waters including estuaries, bays, and rivers

Depth: Typically no deeper than 100 ft (31 m) but occasionally found as deep as 500 ft (152 m)

Reproduction: Placental Viviparous

Gestation: 10 – 11 months

Litter Range: 1-13 pups

Home Range: Worldwide in tropical and temperate waters

Diet: Broad range: fishes, invertebrates, other sharks, carrion, sea bird, turtles, and garbage

IUCN Status: Near Threatened

(Parker, 2008; Simpfendorfer, & Burgess, 2009; Skomal, 2016)

Thanks so much for checking out the bull shark! Remember to swim over to last week’s featured species: the Bonnethead Shark. If there is a species of shark you’d love to know more about, leave me a comment or send me a message! I would love to do a feature on your favorite species.

The new Ocean For Sharks Shop is open! There’s handmade ocean inspired plush animals, canvas paintings, and of course my children’s book, Winifred the Wondrous Whale Shark, available in print and PDF. Be sure to stop by. Remember proceeds benefit shark research and conservation with a donation to Project AWARE!

As always, remember you can make a difference for marine ecosystems by contacting your Congress man or woman and telling them that you care about the quality of our waters! Sharks, rays, and other marine organisms cannot speak or be represented in Congress. They need your voice. Get involved and stand up for sharks. Until next time finatics!

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Featured Image Source

Ocean Treasures (n.d.). Feeding Bull Shark [Digital Image]. Retrieved from http://otlibrary.com/

Literature Cited

Baum, J. K., & Worm, B. (2009). Cascading top-down effects of changing oceanic predator abundances. Journal of Animal Ecology, 78(4), 699–714.

Capuzzo, M., & Cariou, L. (2001). Close to Shore. Random House.

Estes, J. a, Terborgh, J., Brashares, J. S., Power, M. E., Berger, J., Bond, W. J., … Wardle, D. a. (2011). Trophic downgrading of planet Earth. Science (New York, N.Y.), 333, 301–306.

Frisch, A. J., Ireland, M., Rizzari, J. R., Lönnstedt, O. M., Magnenat, K. A., Mirbach, C. E., & Hobbs, J. P. A. (2016). Reassessing the trophic role of reef sharks as apex predators on coral reefs. Coral Reefs, 35(2).

Klimley, A. P. (2013). The biology of sharks and rays. University of Chicago Press.

Parker, S. (2008). The encyclopedia of sharks (2nd ed.). Buffalo, NY: Firefly Books Ltd.

Shark MOOC. (2016, June). Pit Stop: Kidneys. Lecture presented at Sharks! Global Biodiversity, Biology, and Conservation.

Simpfendorfer, C. & Burgess, G.H. (2009). Carcharhinus leucas. The IUCN Red List of Threatened Species 2009: e.T39372A10187195. http://dx.doi.org/10.2305/IUCN.UK.2009-2.RLTS.T39372A10187195.en

Skomal, G. (2016). The Shark Handbook: The Essential Guide for Understanding the Sharks of the World. (2nd ed.). Kennebunkport, ME: Cider Mill Press.

Stevens, J. D. (Ed.). (1997). Sharks (6th ed.). New York: Facts on File.

Tricas, T. C., Deacon, K., Last, P., McCosker, J. E., Walker, T. I., & Taylor, L. (1997). The Nature Company Guides: Sharks and Rays. (L. Taylor, Ed.). Hong Kong: The Nature Company, Time Life Books.

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Featured Species Friday: Bonnethead Shark (Sphyrna tiburo)

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