Bull Shark

Carcharhinus leucas

By: Samantha Fisher

Introduction

A bull shark was blamed for a series of infamous shark attacks in 1916, which resulted in four human fatalities over a span of twelve days. It was this series attacks that inspired Peter Benchley to write his book Jaws, which was eventually turned into the well-known Steven Spielberg movie Jaws in 1975 (Curtis et al., 2018)!

Carcharhinus leucas, commonly known as the bull shark, is large in size, ranging from 7-11.5 feet in length and weighing between 200-500 pounds (National Geographic, 2018). The largest bull shark in recorded history was caught by a University of Miami researcher in southern Florida in 2012, weighing between 800-1,000 pounds (Morgan, 2012). Distinguishing characteristics of the bull shark include a large and triangular first dorsal fin that “originates over or slightly behind [the] pectoral insertion” and is followed by a much smaller second dorsal fin, a blunt and rounded snout that is shorter than the width of the mouth, and small eyes. Bull sharks are light to dark gray with a cream-colored underside. They can live for over 25 years and can be found in a variety of environments ranging from freshwater to saltwater (Curtis et al., 2018).

Taxonomy

Carcharhinus leucas is commonly known as the bull shark. Bull sharks are a member of the Class Chondrichthyes, which include all species of sharks, skates, and rays. Members of this class do not have any true bones and their skeleton is completely made of cartilage. Their teeth and vertebrate are the only calcified portions of their bodies (University of California Museum of Paleontology, n.d.). Members of the Order Carcharhiniformes are commonly known as the ground sharks. They are the most diverse group of sharks, with over 200 described species. These sharks “feature an anal fin, two dorsal fins, an elongated snout, and a long mouth that reaches behind the eyes, with teeth ranging from small and cuspidate to large and bladelike” (PBS, 2002). In addition to these features, they also have a third eyelid. Species in the Family Carcharhinidae are known as the requiem sharks. This family is one of the largest family of sharks. Approximately 50% of all reported shark attacks are from requiem sharks (Beller, 2000)! The bull shark is joined by 35 other species in the Genus Carcharhinus, including local species such as the blacktip shark, dusky shark, and sandbar shark (FishBase, 2013).

The taxonomic classification below is provided by the Integrated Taxonomic Information System (n.d.).

Distribution of bull sharks around the world. Image Source: https://kids.nationalgeographic.com/animals/bull-shark/#bull-shark-swimming-ocean-floor1.jpg

Bull shark that was caught in the Brisbane River in Australia. Image Source: https://www.abc.net.au/news/2019-01-23/how-shark-infested-is-brisbane-river/10731866

Niche and Habitat

Bull sharks are apex predators of coastal waters and connected water bodies worldwide. The bite of the bull shark actually “exerts more force than a similarly-sized white shark”, which allows them to feed on a variety of prey items. They often feed on other large marine species, including other bull sharks or shark species, marine mammals, sea turtles, and large fishes. When found in low-salinity environments, they may feed on anadromous species such as red drum, American eel, and summer flounder. Because they are an apex predator, their presence in low-salinity environments may negatively impact the movement, habitat use, and population sizes of the prey items that can be found in these locations (Ogburn & Hines, 2016).

The bull shark can be found in subtropical to tropical coastal waters around the world. Not only are bull sharks found in coastal waters, but they can also be found in low-salinity systems such as estuaries, rivers, harbors, bays, and lagoons. Additionally, bull sharks can also be found in high-salinity systems, tolerating salinities of up to 53 parts per thousand. Bull sharks can be found at a range of depths when in coastal waters, often between 3-450 feet deep, but prefer waters less than 100 feet deep (Curtis et al., 2018). The movement of bull sharks is not well studied; however, they have been tracked making regular movements along shorelines, into low-salinity systems like estuaries and rivers, and between reefs (Ogburn & Hines, 2016).

Evolutionary History

The bull shark has one major evolutionary advantage over almost all other shark species: its ability to traverse from the ocean into freshwater environments. The only other shark species in the world that has this ability are the river sharks (Bryant, 2008). In order to thrive in environments with varying salinities, bull sharks must be excellent osmoregulators.

Osmoregulation is the “ability of an organism to maintain a constant concentration of water in its body even when its outside environment would normally cause it to lose or gain water”. The mechanism used by sharks to osmoregulate include having high concentrations of urea in their blood, which allows sharks to absorb water from its surroundings, and the removal of salts in their bloodstream though urine, which allows sharks to dispose of the salt they absorb. This process of osmoregulation is controlled by the kidneys (Shark Savers, n.d.).

Bull sharks can readily adapt their ability to osmoregulate. Their kidneys can be gradually adjusted based on the water’s salinity, which, in turn, adjusts them to their new environment. For example, when a bull shark moves from the ocean into a lower-salinity environment, such as an estuary, the bull shark’s kidneys will remove less salt and more urea from the bloodstream and dispose of the salt and urea in its urine. This allows the bull shark to survive and thrive in low-salinity environments (Shark Savers, n.d.).

Osmoregulation in bull shark that is located in a high-salinity environment. Image Source: https://twitter.com/elasmobranchjkm/status/744278919524802560

Osmoregulation in bull shark that is located in a low-salinity environment. Image Source: https://twitter.com/elasmobranchjkm/status/744278919524802560

Bull shark feeding. Image Source: https://mpo.photoshelter.com/image/I00000YD4vQk3b_c

Environmental Impacts

As previously mentioned, bull sharks are apex predators, consuming various prey items ranging from large prey, such as other sharks and marine mammals, to smaller prey, such as small fish species. Apex predators, like the bull shark, have a top-down effect on the food web (Ogburn & Hines, 2016). By consuming other predatory species, they directly impact that species’ preferred prey population and so on. Additionally, because bull sharks have a variety of prey items, they can switch to a more sustainable prey when the populations of their preferred prey begin to decrease.

In addition to regulating population dynamics, apex predators, such as the bull shark, can also influence the spatial distribution of other species through intimidation. The fear of predation by sharks “causes some species to alter their habitat use and activity level, leading to shifts in abundance in lower tropic levels”, which can impact community structure. The presence of bull sharks can also lead to greater species diversity within ecosystems, as they can help to prevent one species from dominating a limited resource (Griffin et al., 2008).

Conservation

The bull shark is listed as “near threatened” globally on the IUCN Red List; however, the last assessment was performed in 2005 and it was not published until 2009. In order to determine the current conservation status of the bull shark, the population would need to be reassessed (Simpfendorfer & Burgess, 2009). Studies have shown that the average length of the bull shark has “declined significantly over the past few decades” (National Geographic, 2018), which may indicate that individuals are not living as long as previously documented.

In the past, bull sharks have been targeted by commercial fisheries for their meat, skins, and oils (Oceana, n.d.; National Geographic, 2018). This species is also targeted by recreational fisheries. Currently, a major threat to bull sharks is getting caught as accidental bycatch in other fisheries and most of these incidents occur when they are upriver. Occasionally, bull sharks are the target of shark culling efforts, which are meant to promote the safety of beachgoers by killing the sharks before they have the opportunity to attack. Culling has not been proven to effectively protect beachgoers from sharks and is looked down upon by scientists, conservationists, and members of the general public alike (Oceana, n.d.).

Bull shark caught by recreational fishermen off of Port Canaveral, Florida. Image Source: Jeff Fisher

Bull shark versus crocodile in Australia. Image Source: https://www.ntnews.com.au/news/northern-territory/monster-croc-snacks-on-surprised-shark/news-story/6648566da6f616bb79cd4789ec9fc3f5

Friends or Foes?

Bull sharks have a limited number of natural predators. Juvenile bull sharks may fall prey to other species of sharks, such as tiger sharks, sandbar sharks, and other bull sharks. Adult bull sharks may fall prey to larger-sized sharks. A report was previously released about a crocodile consuming a bull shark in South Africa (Curtis et al., 2018). Similarly, saltwater crocodiles hunting for bull sharks in the surf zone have been reported in Australia (Bittel, 2017). Additional predators include Pandarus sinuatus and Perissopus dentatus, both of which are species of copepods that are parasitic and attach to the bull shark’s pectoral and pelvic fins (Curtis et al., 2018). Bull sharks may compete with other apex predators in their environment, such as other species of sharks, for prey items.

Videos

The video below showcases the bull shark on an episode of Animal Planet's "River Monsters".

The video below showcases the bull shark in its natural habitat while divers observe and film.

The video below showcases five facts about the bull shark, its characteristics, and its life cycle.

Additional Resources to Explore

Visit the Florida Museum for an overview of the bull shark. Link: https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/carcharhinus-leucas/

Visit NOAA's Greater Atlantic Fisheries to identify the bull shark and other shark species using a key based on their characteristics. Link: https://www.greateratlantic.fisheries.noaa.gov/sustainable/recfishing/helpful/rec_shark_id_placard.pdf

Visit Shark Savers to learn more about osmoregulation in bull sharks. Link: http://www.sharksavers.org/en/education/biology/how-bull-sharks-survive-in-fresh-water/

Visit Animal Diversity Web for a detailed description of the bull shark. Link: https://animaldiversity.org/accounts/Carcharhinus_leucas/

Visit Australia's Parks and Wildlife Service to view a fact sheet about the bull shark. Link: https://www.dpaw.wa.gov.au/images/documents/conservation-management/riverpark/fact-sheets/Fact%20sheet%20-%20bull%20shark.pdf

Resources Utilized

Citations

Beller, P. (2000). Carcharhinidae (Requiem Sharks). Retrieved from https://www.sdnhm.org/oceanoasis/fieldguide/carcharhinus.html

Bittel, J. (2017). Alligators Attack and Eat Sharks, Study Confirms. Retrieved from https://news.nationalgeographic.com/2017/09/american-alligator-sharks-predators-stingrays/

Bryant, C. W. (2008). How Bull Sharks Work. Retrieved from https://animals.howstuffworks.com/fish/sharks/bull-shark2.htm

Curtis, T., French, L., & Naylor, G. (2018). Carcharhinus leucas. Retrieved from https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/carcharhinus-leucas/

FishBase. (2013). Genus: Carcharhinus. Retrieved from https://www.fishbase.se/identification/SpeciesList.php?genus=Carcharhinus

Griffin, E., Miller, K. L., Freitas, B., & Hirshfield, M. (2008). Predators as Prey: Why Healthy Oceans Need Sharks [PDF File]. Retrieved from https://oceana.org/sites/default/files/reports/Predators_as_Prey_FINAL_FINAL1.pdf

Integrated Taxonomic Information System. (n.d.). Carcharhinus leucas. Retrieved from https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=160275#null

Morgan, C. (2012). University of Miami Researchers Bag a Big -- Really Big -- Bull Shark. Retrieved from https://www.miamiherald.com/latest-news/article1940452.html

National Geographic. (2018). Bull Shark. Retrieved from https://www.nationalgeographic.com/animals/fish/b/bull-shark/

Oceana. (n.d.). Bull Shark. Retrieved from https://oceana.org/marine-life/sharks-rays/bull-shark

Ogburn, M. & Hines, A. (2016). Bull Shark Species Profile. Retrieved from https://serc.si.edu/node/38812

PBS. (2002). Island of the Sharks. Retrieved from https://www.pbs.org/wgbh/nova/sharks/world/carcharhiniformes.html

Shark Savers. (n.d.). How Bull Sharks Can Live in Both Ocean and Fresh Water. Retrieved from http://www.sharksavers.org/en/education/biology/how-bull-sharks-survive-in-fresh-water/

Simpfendorfer, C., & Burgess, G. H. (2009). Bull Shark. Retrieved from https://www.iucnredlist.org/species/39372/10187195#assessment-information

University of California Museum of Paleontology. (n.d.). Introduction to the Chondrichthyes. Retrieved from https://ucmp.berkeley.edu/vertebrates/basalfish/chondrintro.html

Image Citations

Header: https://www.nwf.org/Educational-Resources/Wildlife-Guide/Fish/Bull-Shark

Image Carousel (in order): https://otlibrary.com/bull-shark/, http://www.animalplanet.com/tv-shows/river-monsters/fish-guide/bull-shark-adventure-fishing/, https://www.sunshinecoastdaily.com.au/news/bull-sharks-lurking-in-suburban-coast-lakes/3299741/, https://www.abc.net.au/news/science/2016-02-04/great-barrier-reef-bull-sharks-secret-lives-revealed/7136950, and https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/carcharhinus-leucas/.

Lesson Plan

Osmoregulation Lesson Plan.docx

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