The Milk shark (Rhizoprionodon acutus) is a species of requiem shark, and part of the family Carcharhinidae. It gets its name from an Indian belief that consumption of its meat encourages lactation. It is a small shark, and is the largest and most widely distributed member of its genus.
Family: Carcharhinidae – Requiem sharks
Genus: Rhizoprionodon
Species: acutus
Phylum– Chordata
Class– Chondrichthyles
Subclass– Elasmobranchii
Common Name– Ground Sharks
Family– Carcharhinidae
Common Name– Requiem Sharks
Genus– Rhizoprionodon
Species– acutus
Status: IUCN Red List LEAST CONCERN
Average Size and Length: Milk sharks typically don’t exceed 3.6 feet in length. Some accounts off West Africa reported 5.8 feet for males and 5.4 feet for females. Although these males are recorded larger, typically the females are the larger.
Average Weight: The West African group has been reported 49 pounds in males and 37 pounds in females. These may be much larger than the norm. Typically, females are the larger of the two sexes.
Current Rare Mythical Sightings: The Milk shark likely once had a contiguous distribution by way of the Tethys Sea, until during the Miocene epoch, when eastern Atlantic sharks were isolated from Indo-Pacific sharks by the collision of Asia and Africa.
The Milk shark gets its name because in Indian culture, they believe that eating the shark’s meat will enhance females lactating.
Teeth and Jaw: There are long furrows at the corners of the mouth on both the upper and lower jaws of the Milk shark. The tooth rows are between 24–25 in both jaws. The upper teeth are finely serrated and strongly oblique; the lower teeth have a similar shape, though the serrations are smaller and the tips curve gently upward. The teeth of juveniles are smooth-edged.
Head: Just behind the corner of the jaw, behind the head, there are between 7 to 15 enlarged pores. The Milk shark has small nostrils with skin flaps. Their snout is long and pointed. Their eyes are large and round and do have nictitating membranes.
Tail: The lower lobe of the caudal fin is well-developed and the upper lobe has a ventral notch near the tip.
Demographic, Distribution, Habitat, Environment and Range: The Milk shark has the widest distribution of any Rhizoprionodon species of shark. In the Indian Ocean, it can be found from South Africa and Madagascar northward to the Arabian Peninsula, and eastward to South and Southeast Asia. In the Pacific Ocean, the Milk shark can be found from China and southern Japan, through the Philippines and Indonesia, to New Guinea and northern Australia. In the eastern Atlantic Ocean, it is found from Mauritania to Angola, as well as around Madeira and in the Gulf of Taranto off southern Italy.
The Milk shark stays close to shore from the surf zone to about a depth of 660 feet. It prefers turbid waters that are just off sandy beaches. On occasion, they will venture into estuaries.
One known nursery area is Shark Bay in Western Australia. Juvenile Milk sharks prefer areas with seagrass. There have been reports that Milk shark have been seen in fresh water in Cambodia and far upstream. However, this still isn’t confirmed, and some scientists believe they avoid low salinities. So currently, both sides are speculation, ad we will have to await further research.
Off KwaZulu-Natal, South Africa, its numbers vary annually with a peak in summer, suggesting some form of seasonal movement.
Diet: The Milk shark feeds primarily on schooling small bony fishes, but will also eat cephalopods and crustaceans. On occasion, they can even eat cuttlefish, octopus, squid, crabs and shrimp. Bony fish species is unique per range.
Since they are a smaller species, Milk sharks do fall prey to larger sharks (specifically Blacktip sharks and Australian Blacktip sharks) and possibly marine mammals.
Aesthetic Identification: Fusiform and slender, the Milk shark is a small and abundant species of shark. It is grey to brown-grey or even purplish-grey on top, and counter-shaded white below. The leading edge of the first dorsal fin and trailing edge of the caudal fin are sometimes darker, and the trailing edges of the pectoral fins are lighter.
The broad, triangular pectoral fins originate below the third or fourth gill slits, and are no longer than the front margin of the first dorsal fin. The anal fin is about twice as long as the second dorsal fin and headed by long ridges. The first dorsal fin originates over the pectoral fin free rear tips, and the smaller second dorsal fin originates over the last third of the anal fin base. There is no interdorsal ridge.
Biology and Reproduction: The Milk shark has copepod parasites.
The Milk shark is viviparous. It has been documented that mating, parturition and reproduction fluctuates according to range. Some females produce young every year and others every other year. Some subpopulations have more complex reproductive cycles than others. Females do not store sperm.
Litter size can be anywhere from 1 to 8 pups, but typically 5 is the norm. In some location’s females do outnumber males. There is a year-long gestation, which is followed by three phases. They are born between 12.8 and 19.7 inches and way less than a pound.
Some known nursery areas with a lot of food and are warmer water temperatures include the Banc d’Arguin National Park off Mauritania, and Cleveland Bay and Herald Bight off Australia. In Herald Bight, large groups of young Milk sharks can be found in shallow tidal pools, as well as in seagrass beds where they are sheltered from predators by the dense, tall vegetation. Once mature, they move. They mature at all different rates depending on location. They reach sexual maturity between 2 and 3 years. The maximum lifespan known is 8 years.
Behavioral Traits, Sensing and Intelligence: Some research suggests that there is possible segregation by sexes.
Milk Shark Future and Conservation: Off KwaZulu-Natal, the decimation of large sharks by the use of gillnets to protect beaches has led to a recent increase in milk shark numbers.
Milk sharks are caught using longlines, gillnets, trawls, and hook-and-line, this shark is marketed fresh or dried and salted for human consumption, and is also used for shark fin soup and fishmeal. They are subject to overfishing, however, since they are resilient and abundant, they are capable or withstanding the heavy fishing.
Milk Shark Recorded Attacks on Humans: The Milk shark is harmless to humans.