Rhizoprionodon acutus

Geographic Range

Milk sharks ( Rhizoprionodon acutus ) are native to the Indo-West Pacific region. This tropical species is as far north as latitude 23.5 °N and as far south as latitude 23.5 °S. With these coordinates, milk sharks fall within the Tropic of Cancer and the Tropic of Capricorn. These sharks have a range that extends as far west as the southeast portion of the Atlantic Ocean (bordering central Africa), and continues eastward on the east coast of Africa, all of Madagascar. North of Africa, their range includes coastal areas in the Red Sea and throughout the Persian Gulf. Milk sharks' range continues contiguously from the Persian Gulf, eastward along Pakistan and India, to Malaysia and Indonesia. These sharks are common in the southern and southeast waters of Asia including Thailand, Cambodia, and Malaysia. This species has also expanded northward to Japan and near the waters of South Korea. Southwards, their range includes the Gold Coast of Australia. The entirety of waters between Australia and Papua New Guinea are also known places where milk sharks exist.

Although rare, milk sharks have been recently documented in the Mediterranean Sea. According to Amor et al. (2016), this is the first milk shark record from the Tunisian coast and the second Mediterranean record. The specimen was found in southern Tunisia, off the coast of Zarzis.

• Biogeographic Regions
• australian australian
  • native native
• indian ocean
  • native native
• atlantic ocean atlantic ocean
  • native native
• pacific ocean pacific ocean
  • native native
• mediterranean sea
  • native native

• Other Geographic Terms
• cosmopolitan cosmopolitan

Habitat

Milk sharks live in both benthic and pelagic environments. They live on the seafloor, but often swim in the open ocean. While they are primarily found in tropical habitats, they are also known to extend to temperate zones and can be found in brackish waters. Milk sharks often appear near sandy beaches on continental shores (the neritic zone) and have occasionally been observed in estuaries.

This coastal species can be found in shallow waters ranging from 1-200 m below the surface. Although they can withstand water temperatures of 21 °C or more, they are usually found in relatively cool water between 7° to 16 °C. When in tropical waters, these sharks often choose colder waters at greater depths there.

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

• Aquatic Biomes
• pelagic pelagic
• benthic benthic
• coastal coastal
• brackish water brackish water

• Other Habitat Features
• estuarine estuarine

Physical Description

Milk sharks are dusky gray on their dorsum and their ventral half is white. Like most other shark species, milk sharks have large notched eyes and 5 pairs of gills. With 2 pectoral fins and one large vertical dorsal fin, these sharks possess dark edging around the dorsal fin and two anal fins. They also are characterized as having a heterocercal caudal fin where the superior lobe is significantly bigger than the inferior one. Unlike other sharks, milk sharks have long shallow grooves along their mouths. The cusps of their teeth form at an angle greater than 90 degrees and they may or may not be serrated. Milk sharks exhibit sexual dimorphism, as males possess a pair of external reproductive structures called claspers. Males also reach maturity at about 60 cm in total length with the largest recorded at 178 cm. The largest female milk shark was approximately 165 cm. However, the average length for adult milk sharks is closer to 100 cm with sizes ranging between 25 and 39 cm at birth. In addition, milk sharks are homoiothermic, meaning they maintain a constant body temperature.

Milk sharks differ from other members of the genus by size. Milk sharks are usually larger than congenerics. Further, unlike other sharks, the height of their dorsal fin is usually greater than the length of the anterior side of their pectoral fin. All other members of the genus have a shorter ratio of the dorsal height to pectoral fin length.

• Other Physical Features
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger
• sexes shaped differently

Development

A viviparous species, the gestation period for milk sharks is about one year. The young begin as an embryo in the females’ uterus, nourished by a yolk sac. Following the yolk sac's depletion, the placenta is responsible for the milk sharks’ embryonic development. By mid-gestation the developing embryos lack skin pigmentation. Scales and teeth are forming, and external gill filaments are present in this stage. In late gestation, the uteri have fully developed embryos and the umbilical cord is attached between the pectoral fins. Milk sharks are 127-350 g and about 42 cm (range 39-45 cm) at birth. They grow about 10 cm per year in the first 2 years and then growth begins to slow. Males reach sexual maturity at a length of 76.5 cm at an age of 4.8 years. Females average 74cm long and 5.8 years at maturity. These sharks exhibit indeterminate growth, and the oldest, largest male sharks have reached 175 cm.

• Development - Life Cycle
• indeterminate growth indeterminate growth

Reproduction

It’s unknown if milk sharks exhibit polygyny or polyandry, but no sharks are reported to exhibit monogamy. Across most species, female sharks release pheromones - or chemical signals - when ready to mate. Sharks have a common tactic of biting during mating behaviors. The male grasps the female in a way that allows for the correct positioning for inserting a clasper into the female. Sperm is then delivered into the female's cloaca through a groove after a clasper is inserted inside her. Studies have shown that some of these smaller sharks tend to leave females with substantial scarring that may be detrimental to their health.

• Mating System
• polygynandrous (promiscuous) polygynandrous (promiscuous)

Milk sharks breed once yearly, with mating and birth both occurring in the spring or early summer (April to July). Milk sharks are viviparous, with a yolk-sac placenta. Although females may have one to eight pups per litter, the range of two to five pups is more common. Milk shark pups typically weigh between 127-350 grams at birth and are between 25 and 39 cm long. Pups are independent at birth.

Age at maturity varies markedly by geographic location. Along the west African coast, females reach reproductive maturity at about 5.8 years old (as late as age 9), whereas males tend to reach that point at about 4.8 years old. Along the Australian coast, female milk sharks may reach maturity as early a 2 years old, or as late as 8 years old. Differences in male maturity have not been reported.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• sexual sexual
• fertilization fertilization
  • internal internal
• viviparous viviparous

Milk sharks show no signs of parental care after birth; pups are left to fend for themselves. Females provide nourishment and protection of pups before birth, Males exhibit no parental care beyond the act of mating.

• Parental Investment
• no parental involvement
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female

Lifespan/Longevity

Milk sharks live at least 8 years in the wild, but the upper limit to the maximum lifespan is unknown. Milk sharks are kept in captivity, but there are no reports on the longevity of those captive.

A major limit to their longevity is overfishing.

Behavior

Milk sharks are a natatorial, motile species and to move through the water, sharks move their body in a wave-like pattern from side to side. Milk sharks' heterocercal tail is essential for both forward motion and stability in the water.

Despite having specialized means of communication, milk sharks are a solitary species. These sharks primarily rely on their senses of touch and smell to locate prey, mate, and communicate with one another. Due to their increased activity at nighttime, milk sharks typically feed at night. They are mostly active in low light, though they may swim in shallow waters during the day. Milk sharks are thought to be nocturnal.

Milks sharks migrate short distances seasonally, especially in response to reproductive needs.

• Key Behaviors
• natatorial natatorial
• nocturnal nocturnal
• motile motile
• migratory migratory
• solitary solitary

Home Range

Other members of the genus Rhizoprionodon had a reported home range of 1.2 km^2. It is likely milk sharks have a similar range. Further, home range in sharks appears to increase with body size.

Communication and Perception

Milk sharks, like most sharks, use senses such as smell, touch, and sight to communicate. They have eyes to see the environment, catch prey, and recognize conspecifics. Sharks don’t exhibit sounds or any type of vocalizations. Body language and electromagnetism are important aspects for sharks to communicate. These sharks rely heavily on smell and tactile senses as methods of communication among individuals, to mate, and to locate prey. Social interactions are also a key factor with this species, and body language is crucial when trying to communicate, especially when establishing dominance or conveying simple information like which direction they are heading. Vibrations are another way that sharks perceive their environment; this technique allows sharks to detect prey and predators through the electro-sensory system.

Sharks, skates, and rays all have sensory organs called ampullae of Lorenzini that are capable of detecting electrical waves. On the ventral side of the skull, the ampullae present as tiny pores in the skin. Sharks use their ampullae to find predators, prey, and mates. Using the earth’s magnetic field, this might potentially aid sharks in navigating the strong ocean waters and following currents. These ampullae are responsible for certain reflex movements and simple communication.

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

• Other Communication Modes
• pheromones pheromones

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

Food Habits

Milk sharks are carnivores. Although opportunistic generalists, they are primarily piscivores, consuming fish. They also eat molluscs like snails, octopods, squids, and cuttlefish. They consume a variety of non-insect marine arthropods, like crabs and shrimp. Although these sharks' diet may differ based on their sex, stage of development, and geographic location, they primarily consume pelagic aquatic life like annelids. To avoid predation, milk sharks may choose to live in different habitats than larger sharks; according Ba et al. (2013), their dietary intake is modified to help them avoid predators.

Ba et al. (2013) examined 577 milk sharks whose stomachs held food items, and found that 91.6% of these stomachs contained teleosts, or bony fish. These teleosts were comprised of at least 40 species across 20 families. Although nearly 35% of fish remained unidentified, the most common species (in just 7.8% of stomachs) was round sardinalla, Sardinalla aurita . The authors also discovered shrimp or crabs in 6.6% of stomachs, marine gastropods and cephalopods in 5.2% of stomachs, and negligible amounts of marine worms and nematodes.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • eats non-insect arthropods
  • molluscivore molluscivore
  • eats other marine invertebrates

• Animal Foods
• fish
• aquatic or marine worms
• aquatic crustaceans

Predation

Blacktip sharks ( Carcharhinus limbatus ) and Australian blacktip sharks ( Carcharhinus tilstoni ) are known predators of milk sharks. In a study by Dudley et. al (2023), 836 blacktip sharks were captured in shark nets between 1978 and 1991. Elasmobranchs were found in 15.7% of the food-containing stomachs of blacktip sharks. From 1983 to 1991, elasmobranchs were discovered in 16.1% of blacktip shark stomachs; of these shark species, milk sharks were the fourth most common species identified.

Larger marine mammals are believed to prey on milk sharks. Humans ( Homo sapiens ) also catch milk sharks in their fisheries nets. As a means of defense against predators, milk sharks inhabit shallow waters near the surface. Shallow waters and seagrass beds provide cover when milk sharks detect a predator. Milk sharks are known for their speed and agility and can use those tactics to escape predation.

Ecosystem Roles

Milk sharks consume mostly fish, and smaller amounts of molluscs and marine arthropods. They are preyed upon by larger sharks and humans ( Homo sapiens ).

Monogenean parasites were limited to a single species, Loimos secundus . Seven tapeworm parasites of milk sharks are Paraorygmatobothrium floraformis , Nybelinia lingualis , Heteronybelinia peridareus , Nybelinia indica , Phoreobothrium , Poecilancistrum ilishae . Two roundworms parasites of these sharks are larvae of Anisakis and the larvae of Hysterothylacium . A single species of copepod Kroyeria minuta and amphipod Lafystius sturionis also parasitize milk sharks.

Economic Importance for Humans: Positive

Milk sharks are one of the most abundant sharks in the inshore waters where they occur and captured by artisanal and small-scale commercial fisheries and offshore fishing fleets. They are often caught on longlines, hook-and-line, bottom trawls, and pelagic trawls. Milk sharks are utilized fresh, smoked, and possibly dried salted for food and for fishmeal. Milk sharks have hides that are useful for leather and other components that can be used for liver oils. Their fins, though on the small end, were reported to be the most common small-finned species sold in Hong Kon markets in 2019. Just 4 years earlier, thses sharks only constituted 1.4% of all shark fins sold in the same city.

• Positive Impacts
• food food
• body parts are source of valuable material

Economic Importance for Humans: Negative

Milk sharks have no reported negative economic impacts on humans.

Conservation Status

Milk sharks are listed as a “Vulnerable” species on the IUCN Red List. They have no special status on the federal list and the state of Michigan list. Milk sharks are listed under Appendix II on CITES, which means their trade is controlled internationally.

Milk sharks are commonly caught by artisanal or small-scale and commercial fisheries for their meat and fins. Increased legal and illegal fishing efforts in the Mediterranean and the Arabian Sea may put these sharks at greater risk. Although milk sharks may experience declines in populations as a result of overexploitation, their high rates of reproduction and rapid growth make them more resistant to overfishing than other species of sharks. Mortality rates from fisheries efforts for milk sharks, in trawl fisheries, range from 82% in Australian waters and just 9% in the Indian Ocean.

Milk sharks are the most commonly caught shark found at fishing sites along the Senegalese coast. More than half of the 70–85% of females captured between April and June are pregnant, indicating that fishermen target a pupping aggregation in this area. Habitat destruction in Southeast Asia waters also remain a threat.

The impacts of ocean pollution, including the bioaccumulation of pollutants and the consumption of microplastics, can affect milk sharks similarly to many other sharks and marine life. Shark fins have reportedly been consumed as a traditional food in Southeast Asia. Although the consumption of shark fins has been declining in recent years, the trade of shark fins remains a significant issue. Shark fins have a particularly high value and can be worth more than the rest of a shark, which can lead to negative consequences for population sizes.

Conservation efforts in place for milk sharks include research and monitoring, water protection, awareness programs, and species management. Very few species-specific laws exist. On CITES, Appendix II protects them by controlling trade. In South Africa, for small-scale fishing there is a bag limit of one individual milk shark per day for consumption only.

In Iran, a ban on target shark fisheries was introduced ca. 2010, which may benefit milk sharks. Likewise, Australia implemented a design to reduce the unintended capture of non-target species in their marine protected areas. In addition to monitoring population size, capture rates, and taxonomy, more research is required. Humans play a role in indirect conservation measures for milk sharks. Although larger sharks frequently prey on them, the deployment of gillnets to keep huge, dangerous sharks off beaches in Natal, South Africa, has increased the population of milk sharks by reducing the number of larger sharks that prey on them. By indirectly lowering the pressure of predation, humans increase the lifespans of milk sharks.

Additional Links

Encyclopedia of Life

Contributors

Ariel Barber (author), Radford University, Natalie May (editor), Radford University, Alexander McVicker (editor), Radford University, Karen Powers (editor), Radford University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

Amor, M., Y. Diatta, M. Diop, M. Salem, C. Capapé. 2016. Confirmed occurrence in the Mediterranean Sea of milk shark Rhizoprionodon acutus (Chondrichthyes: Carcharhinidae) and first record off the Tunisian coast. Cahiers de Biologie Marine , 57: 145-149.

Ba, A., C. Ba, K. Diouf, P. Ndiaye, J. Panfili. 2013. Reproductive biology of the milk shark Rhizoprionodon acutus (Carcharhinidae) off the coast of Senegal. African Journal of Marine Science , 35/2: 223-232.

Ba, A., M. Diop, Y. Diatta, D. Justine, C. Ba. 2012. Diet of the milk shark, Rhizoprionodon actus (Ruppel, 1837) (Chondrichthyes: Carcharhinidae), from the Senegalese coast. Journal of Applied Ichthyology , 29/13: 789-795.

Ba, A., K. Diouf, F. Guilhaumon, J. Panfili. 2015. Slow growth of the overexploited milk shark Rhizoprionodon acutus affects its sustainability in West Africa. Journal of Fish Biology 87 , 87/4: 912–929.

Barnes, A., D. Sutaria, A. Harry, R. Jabado. 2018. Demographics and length and weight relationships of commercially important sharks along the north‐western coast of India. Aquatic Conservation: Marine and Freshwater Ecosystems , 28/6: 1374–1383.

Caira, J., K. Jensen. 2015. Insights on the identities of sharks of the Rhizoprionodon acutus (Elasmobranchii: Carcharhiniformes) species complex based on three new species of Phoreiobothrium (Cestoda: Onchoproteocephalidea). Europe PMC , 4059/2: 335-350.

Capapé, C., Y. Diatta, M. Diop, O. Guelorget, Y. Vergne, J. Quignard. 2006. Reproduction in the milk shark, Rhizoprionodon acutus (Ruppell, 1837) (Chondrichthyes: Carcharhinidae), from the coast of Senegal (eastern tropical Atlantic). Acta Adriatica , 47/2: 111-126.

Compagno, L. 1984. An annotated and illustrated catalogue of shark species known to date. Part 2 - Carcharhiniformes . San Francisco State University: FAO SPECIES CATALOGUE. Accessed February 10, 2025 at https://www.fao.org/4/ad123e/ad123e00.htm .

Corro-Espinosa, D., J. Márquez-Farías, A. Muhlia-Melo. 2011. Size at maturity of the Pacific sharpnose shark Rhizoprionodon longurio in the Gulf of California, Mexico. Ciecias Marinas , 37/2: 201-214.

Dudley, S., G. Cliff. 1993. Sharks caught in the protective gill nets off Natal, South Africa. 7. The blacktip shark Carcharhinus limbatus (Valenciennes). South African Journal of Marine Science , 13/1: 237-254.

Jabado, R., S. Al Ghais, W. Hamza, A. Henderson, A. Al Mesafri. 2015. Diet of two commercially important shark species in the United Arab Emirates: milk shark, Rhizoprionodon acutus, and slit-eye shark, Loxodon macrorhinus (Rüppell, 1837) & (Müller & Henle, 1839). Journal of Applied Ichthyology , 31: 870-875.

Kalmijn, A. 1972. Bioelectric Fields in Sea Water and the Function of the Ampullae of Lorenzini in Elasmobranch Fishes. UC San Diego: Scripps Institution of Oceanography , 72/83: 3-14. Accessed February 13, 2025 at https://escholarship.org/uc/item/62q699rw .

Kalyan, C., A. Vankara, H. Mannela. 2018. Population dynamics of metazoan parasites of Rhizoprionodon acutus from the Nellore Coast off Bay of Bengal. Annual Research & Review in Biology , 28/6: 1-16.

Ovenden, J., J. Morgan, R. Street, A. Tobin, C. Simpfendorfer, W. Macbeth, D. Welch. 2011. Negligible evidence for regional genetic population structure for two shark species Rhizoprionodon acutus (Rüppell, 1837) and Sphyrna lewini (Griffith & Smith, 1834) with contrasting biology. Marine Biology , 158: 1497–1509.

Peter, L., C. Jeffery, M. John, H. Michael. 2004. Biology of sharks and their relatives . Boca Raton, Florida: CRC press. Accessed March 06, 2025 at https://www.researchgate.net .

Raschi, W. 1984. Anatomical Observations on the Ampullae of Lorenzini from Selected Skates and Galeoid Sharks of the Western North Atlantic (Ph.D. Dissertation) . Williamsburg, Virginia: College of William and Mary-Virginia Institute of Marine Science,.

Rigby, C., A. Harry, N. Pacoureau, K. Herman, L. Hannan, D. Derrick. 2020. "Rhizoprionodon acutus" (On-line). The IUCN Red List of Threatened Species 2020: e.T41850A68642326. Accessed January 29, 2025 at https://dx.doi.org/10.2305/IUCN.UK.2020-3.RLTS.T41850A68642326.en .

Sen, S., S. Chakraborty, E. Vivekanandan, P. Zacharia, A. Jaiswar, G. Dash, S. Bharadiya, J. Gohel. 2018. Feeding habits of milk shark, Rhizoprionodon acutus (Ruppell, 1837) in the Gujarat coastal waters of north-eastern Arabian Sea. Regional Studies in Marine Science , 17: 78-86.

Sen, S., S. Chakraborty, E. Vivekanandan, P. Zacharia, S. Kizhakudan, A. Jaiswar, G. Dash, G. Jayshree. 2017. Population dynamics and stock assessment of milk shark, Rhizoprionodon acutus (Ruppell, 1837) along Gujarat coast of India. Indian Journal of Geo-Marine Sciences , 46/5: 936–946.

Shaaban, A., M. Sabrah, M. Marie, A. Dakrory. 2018. Reproductive biology of the milk shark Rhizoprionodon acutus (Ruppell, 1837) from the Gulf of Suez, Red Sea, Egypt. The Egyptian Journal of Aquatic Research , 44/4: 37-43.

Shen, K., J. Cheow, A. Cheung, . Koh, A. Mun, Y. Lee, Y. Lim, M. Namatame, E. Peng, V. Vintenbakh, E. Lim, B. Wainwright​. 2024. DNA barcoding continues to identify endangered species of shark sold as food in a globally significant shark fin trade hub. PeerJ Life & Environment , 12: e16647. Accessed March 28, 2025 at https://doi.org/10.7717/peerj.16647 .

Speed, C., I. Field, M. Meekan, C. Bradshaw. 2010. Complexities of coastal shark movements and their implications for management. Marine Ecology Progress Series , 408: 275-293.