Carcharhinus obscurus

Geographic Range

Dusky sharks, Carcharhinus obscurus , have a wide geographic range. They are a coastal species that covers the southern tip of California going southward along the western coast of Mexico at Guadalajara. They also exist in waters from the Texas-Mexico border of the Gulf of Mexico, eastward through the western coast of Florida and the northern coast of Cuba. They have been known to inhabit the eastern coast of North and South America, with some populations being located in areas on the western coast of Chile. They are located across the western coast of Portugal, the eastern coast of Spain, the northern coast of Algeria in the Mediterranean Sea, as well as covering the entire west coast of Africa. The coast of all of Madagascar, locations off the coast of Mozambique, the eastern southern tip of Africa, and all throughout the Red Sea are all areas where they are commonly found. There are large populations of dusky sharks known to congregate in areas off the coasts of of Australia. Some other locations where they convene are along the coast of Bangkok, Thailand and Hong Kong, China, as well as off the eastern coast of Japan.

• Biogeographic Regions
• oceanic islands oceanic islands
  • native native
• indian ocean
  • native native
• atlantic ocean atlantic ocean
  • native native
• pacific ocean pacific ocean
  • native native
• mediterranean sea
  • native native

Habitat

Dusky sharks are commonly found in tropical coastal waters, but have been spotted in temperate environments when migrating. They descend to 400 meters at times. The depths that they inhabit depend on the environment they are in at the time. When swimming near the coast, they tend to stay at depths ranging from 50 to 100 meters. However, when inhabiting southern gulf waters they tend to stay at depths of 20 to 50 meters.

• Habitat Regions
• temperate temperate
• tropical tropical
• saltwater or marine saltwater or marine

• Aquatic Biomes
• pelagic pelagic
• coastal coastal

Physical Description

Adult females typically range from 76 to 287 cm in length, whereas males are slightly smaller, having a range from 74 to 276 cm long. Length at sexual maturity is reported to be 235 cm (range 220-300 cm). The largest male dusky shark was 420 cm long. Adults weigh an average of 180 kilograms, but pregnant females have been known to weigh as much as 450 kilograms. The smallest adults weigh 160 kg. At birth, sharks are 48 to 100 centimeters in length.

As their name suggests, dusky sharks have dusky tips on their fins that are not very prominent. Their undersides are white and their snouts are rounded. Their pectoral fin is in the shape of a sickle. They are known to have the strongest bite of any shark species, being able to produce 60 kilograms of force when their jaws are fully closed. Adding to this bite force, their teeth are sharp, in the shape of a triangle, and serrated on both exposed sides.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger

Development

The embryos at full term for a dusky shark can range from 48-100 cm (more typically 70-100 cm). This is a viviparous species with a yolk sac intially providing nourishment directly to the 3-14 young as they develop. When the yolk sac runs out of nutrients, the placenta then feeds the young. The female gestates the embryos for 16-22 months. Pups are born with working jaws and teeth and are able to hunt and kill from the time of birth. Like all other sharks, dusky sharks have indeterminate growth, meaning they continuously grow throughout their lifetime. Although more development data could and should have been collected during the era in which they were overfished, they were not. Currently, with smaller populations, very little research on development and other reproductive metrics is being conducted.

• Development - Life Cycle
• indeterminate growth indeterminate growth

Reproduction

Male sharks have been known to mate with multiple females. Other than this there is no known information about the mating behavior of the dusky shark.

• Mating System
• polygynous polygynous

Female dusky sharks are viviparous, giving birth to live young. They have a gestation period that typically lasts from 16 to 22 months. Potentially taking up to 2 years for their gestation period, they have to wait a year before being able to bear pups again, making their overall reproduction cycle 3 years long. They have a litter size ranging from 3-14 pups in one litter, with each pup ranging from 70 to 100 centimeters in length (or as small as 48 cm). They give nourishment to the pups by utilizing a yolk-sac placenta. Late term females lack a large yolky ova; this is one reason why they have to wait a year to mate again (Romine, 2009). Males tend to reach sexual maturity at the age of 19 years old, with a range from 17 to 24 years old, whereas females tend to reach sexual maturity at the age of 21 years old, with a range of 17 to 24 years old as well. There is no information about the mass of the pups when they are born.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• viviparous viviparous

Hussey (2010) reported that the pups are born with an oversized liver that accounts for 20 percent of the pups mass at birth. The liver contains stored energy for the pups to use within the first months of their lives. After the stored energy is used up, they are able to hunt and live freely. Ebert (2010) stated that immediately after birth, the mother moves away from the pups to avoid cannibalism. Ebert (2010) also suggested that the pups remain in a group shortly after birth. Hussey (2010) suggests that although they do not stay with the mother, the pups are not independent from their mothers' resource allocation after birth due to the excess nutrients within their liver that they are given in the womb.

Paternal investment is non-existent for the dusky shark. After impregnation, the father has nothing to do with the pups.

• Parental Investment
• female parental care female parental care
• pre-hatching/birth
  • provisioning
    • female
• pre-weaning/fledging
• pre-independence
• post-independence association with parents

Lifespan/Longevity

Carpenter and Valdestamon (2017) report the oldest dusky shark was 40, but it's suspected the maximum age they can reach is mid-40s to 50s. The information does not specify whether this is for captive sharks or wild sharks. Compagno (1984) reports in the FAO Species Catalogue that younger dusky sharks fare well in captivity, but maximum lifespan has not been reported in captivity.

Behavior

Dusky sharks migrate from location to location depending on the temperature and season at the time, trying to stay in the more tropical areas during all times of the year. The females move closer to the shore when they are about to give birth, then move away from shore afterwards. The pups typically form schools that are sometimes divided up between males and females and hunt together. As they get older, they move away from the shore to more open waters but still moderately close to the shore. They have been known to follow ships that are leaving offshore areas at times. They have also been known to be extremely aggressive. There have been multiple reports of them attacking humans (Ebert, 2013).

• Key Behaviors
• natatorial natatorial
• motile motile
• migratory migratory

Home Range

Dusky sharks don't have a reported home range, nor do they defend a territory.

Communication and Perception

All sharks have sense organs that help them communicate. These organs monitor both the outside and inside environments at all times. Some of these organs are spread around the entirety of the sharks' bodies and transmit nerve impulses. These nerve impulses are sent to two different areas in the sharks' bodies, their brain and their spinal cord. When there are signals sent to the brain, they comprehend them as just sensations. When signals are sent to the spinal cord, it results in a reflex reaction. They also have another system called the neuromast system. This systems organs consist of lateral lines, cephalic canals, ampullae of Lorenzini, and pit organs. Neuromasts are all over the body as well as on the surface of the skin and in sunken canals that are beneath the skin. These organs help detect a wide variety of sensations and assist the sharks with the sensations of their surroundings. They can detect water movement and use it to orient their movement, detect sound waves, detects potential threats, and locate food. Some of the neuromast system uses electroreceptors that, when electric pulses are fired off, an electric field is transmitted all around the shark. When the electric pulses cease, the field fades away.

• Communication Channels
• visual visual
• tactile tactile
• chemical chemical
• electric electric

• Other Communication Modes
• vibrations vibrations

• Perception Channels
• visual visual
• tactile tactile
• vibrations vibrations
• chemical chemical
• electric electric

Food Habits

Dusky sharks are generalist carnivores, more typically feeding close to the marine substrate. When they are young, they tend to prey on smaller organisms such as sardines ( Sardina pilchardus ) and small squid ( Teuthida ). As they grow and become adults, they start to hunt for larger prey such as groupers ( Epinephelus striatus , Epinephelus polyphekadion , Epinephelus flavocaeruleus ). Dusky sharks have been known to eat skates, rays, sea turtles, other sharks as well. Invertebrate materials include gastropods, crustaceans, and sea stars. They can sometimes mistakenly consume human trash. In few stomachs, bottlenose dolphins Tursiops have been found. However, it's thought that the marine mammals are consumed already dead, as carrion. Dicken et al. (2015) do report one case of dusky sharks attacking a calf humpback whale ( Megaptera novaeangliae ).

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • molluscivore molluscivore
  • eats other marine invertebrates

• Animal Foods
• mammals
• reptiles
• fish
• blood
• body fluids
• carrion carrion
• mollusks
• aquatic crustaceans
• echinoderms

Predation

Adult dusky sharks have very few predators that hunt them. When they are born, they have a risk of being prey to larger sharks in the area, but as they grow this threat disappears. The only predator of adults is humans, Homo sapeins , who catch them in fishing efforts and seek them out for their fins.

Ecosystem Roles

The dusky shark is a generalist, preying on a wide variety of animals. They are an apex predator in their system. Some of the parasites that inhabit these sharks are tapeworms Anthobothrium laciniatum , and the parasitic monogenean (a flatworm), Dermophthirius carcharhini .

Economic Importance for Humans: Positive

Dusky sharks are very beneficial to humans in the food industry. Their meat is valued at a high price and their fins are valued at an even higher price. The fins are used in a soup called shark fin soup as well as the skin being used for leather. According Musick et al. (2009), the oil from the livers can be used to make an assortment of vitamins. They are also the subject to ecotourism in many areas because of their tendencies to live near coastal waters.

• Positive Impacts
• food food
• body parts are source of valuable material
• ecotourism ecotourism
• source of medicine or drug source of medicine or drug

Economic Importance for Humans: Negative

Dusky sharks have been known to be aggressive towards humans.

• Negative Impacts
• injures humans
  • bites or stings

Conservation Status

According to the IUCN Red List, dusky sharks are listed as "Vulnerable." Ebert et al. (2013) list them as endangered in the northwest and central Atlantic ocean. The US Federal List, CITES, and the State of Michigan List all state there is no special status involved with this species.

This is a severely overfished shark, caught as by-catch in tuna nets. Their fins, livers, and hides are highly sought-after by humans. Musick et al. (2009) reported that 144,000 and 767,000 dusky shark fins are traded each year. With fishermen seeking out juveniles in many portions of their range, and fishing tournaments (e.g., in Florida) purposely catching and keeping them, the populations have declined precipitously. Given that it takes these sharks nearly 20 years to reach breeding status, these sharks are imperiled.

Dusky sharks are now illegal to fish or kill in the United States. The ban on fishing of these sharks in the U.S. Atlantic is already seeing populations improve incrementally. In other areas, the ban hasn't worked; in 2003, despite the ban, over 2000 dusky sharks were harvested. Other countries, as well as international conservation efforts, have mentioned monitoring of these and other sharks - but implementation is slow. The long-term prognosis for this species is unclear.

Additional Links

Encyclopedia of Life

Contributors

Justin Alouf (author), Radford University, Alex Atwood (editor), Radford University, Karen Powers (editor), Radford University, Joshua Turner (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

Bullard, S., S. Dippenaar, E. Hoffmayer, G. Benz. 2004. New locality records for Dermophthirius carcharhini (Monogenea: Microbothriidae) and Dermophthirius maccallumi and a list of hosts and localities for species of Dermophthirius. Comparative Parasitology , 71/1: 78-80.

Carpenter, K., R. Valdestamon. 2017. "Carcharhinus obscurus" (On-line). FishBase. Accessed November 21, 2017 at http://www.fishbase.org/summary/Carcharhinus-obscurus.html .

Castro, J., D. Peebles. 2010. The Sharks of North America . Oxford: Oxford University Press.

Dicken, M., A. Kock, M. Hardenberg. 2015. First observations of dusky sharks (Carcharhinus obscurus) attacking a humpback whale (Megaptera novaeangliae) calf. Marine & Freshwater Research , 66/ 12: 1211-1215.

Ebert, D., S. Fowler, L. Compagno, M. Dando. 2013. Sharks of the World A Fully Illustrated Guide . St. Maurice, Plymouth, U.K.: Wild Nature Press.

Geraghty, P., W. Macbeth, A. Harry, J. Bell, M. Yerman, J. Williamson. 2014. Age and growth parameters for three heavily exploited shark species off temperate Eastern Australia. ICES Journal of Marine Science , 71/ 3: 559-573.

Hall, B. 2005. Bones and Cartilage: Developmental and Evolutionary Skeletal Biology . Cambridge, Massachusetts: Academic Press.

Hereward, J., C. Riginos, D. Blower, J. Hereward, C. Riginos, J. Ovenden, S. Corley. 2015. Characterisation and cross-amplification of 21 novel microsatellite loci for the dusky shark, Carcharhinus obscurus. Conservation Genetics Resources , 7/ 4: 909-912.

Hoffmayer, E., J. Franks, W. Driggers, J. McKinney, J. Hendon, J. Quattro. 2014. Habitat, movements and environmental preferences of dusky sharks, Carcharhinus obscurus, in the Northern Gulf of Mexico. Marine Biology , 161/ 4: 911-924.

Hussey, N., S. Wintner, S. Dudley, G. Cliff, D. Cocks, A. MacNeil. 2010. Maternal investment and size-specific reproductive output in Carcharhinid sharks. Journal of Animal Ecology , 79/ 1: 184-193.

Linzey, D. 2012. Vertebrate Biology . Baltimore, Maryland: The Johns Hopkins University Press.

Marshall, H., G. Skomal, P. Ross, D. Bernal. 2015. At-vessel and post-release mortality of the dusky (Carcharhinus obscurus) and sandbar (C. plumbeus) sharks after longline capture. Fisheries Research , 172: 373-384.

Musick, J., R. Grubbs, J. Baum, E. Cortés. 2009. "The IUCN Red List of threatened species 2009" (On-line). Carcharhinus obscurus. Accessed September 12, 2017 at http://dx.doi.org/10.2305/IUCN.UK.2009-2.RLTS.T3852A10127245.en. .

Natanson, L., B. Gervelis, M. Winton, L. Hamady, S. Gulak, J. Carlson. 2014. Validated age and growth estimates for Carcharhinus obscurus in the northwestern Atlantic Ocean, with pre- and post management growth comparisons. Environmental Biology of Fishes , 97/ 8: 881-896.

Rogers, P., C. Huveneers, S. Goldsworthy, J. Mitchell, L. Seuront. 2013. Broad-scale movements and pelagic habitat of the dusky shark Carcharhinus obscurus off southern Australia determined using pop-up satellite archival tags. Fisheries Oceanography , 22/2: 102-112.

Romine, J., J. Musick, G. Burgess. 2009. Demographic analyses of the dusky shark, Carcharhinus obscurus, in the northwest Atlantic incorporating hooking mortality estimates and revised reproductive parameters. Environmental Biology of Fishes , 84/ 3: 277-289.

Ruhnke, T., J. Caira. 2009. Two new species of Anthobothrium van Beneden, 1850 (Tetraphyllidea: Phyllobothriidae) from carcharhinid sharks, with a redescription of Anthobothrium laciniatum Linton, 1890. Systematic Parasitology , 72/3: 217-227.

Shoou-Jeng, J., C. Jun-Hsu, C. Chien-Pang, L. Kwang-Ming. 2015. Age and growth of the dusky shark, Carcharhinus obscurus, in the western North Pacific Ocean. Terrestrial, Atmospheric & Oceanic Sciences , 26/ 2: 153-160.

Simpfendorfer, C., R. McAuley, N. Hall. 2007. A method for evaluating the impacts of fishing mortality and stochastic influences on the demography of two long-lived shark stocks. ICES Journal OF Marine Science , 64/ 9: 1710-1722.

Simpfendorfer, C. 2001. Diet of three commercially important shark species from western Australian waters. Marine and Freshwater Research , 52/ 7: 975-985.

Smoothey, A., C. Gray, S. Kennelly, O. Masens, V. Peddemors, V. Robinson. 2016. Patterns of occurrence of sharks in Sydney harbour, a large urbanised estuary. PLoS ONE , 11/ 1: 1-16.

2001. "Appendix 1 Frequently asked questions about sharks" (On-line). Sharks and sustainability activites booklet. Accessed September 21, 2017 at http://www.mesa.edu.au/seaweek2005/pdf/appendix1.pdf .

Food and Agriculture Organization of the United Nations. FAO Species Catalogue, Vol.4. Sharks of the world An annotated and illustrated catalogue of shark species known to date Part 2. Carcharhiniformes. ISBN 92-5-101383-7. Rome, Italy: Food and Agriculture Organization of the United Nations. 1984. Accessed November 21, 2017 at http://www.fao.org/docrep/009/ad123e/ad123e00.HTM .