Latin name

Rhizoprionodon acutus

Other name

Rhizoprionodon acutus

Identification

The Milk Shark is characterized by a slender build, a long pointed snout, large round eyes with nictitating membranes (protective third eyelids), and no breathing apparatus. There are usually seven to 15 enlarged pores on each side of the head behind the angle of the jaw. The nostrils are small, as are the adjacent triangular skin folds. There are long furrows at the corners of the mouth on both the upper and lower jaws. The number of teeth on each jaw is 24-25. The upper teeth are finely serrated and strongly inclined; the lower teeth are similarly shaped, although the serrations are smaller and the tips are slightly curved upward. Juveniles have teeth with smooth edges.

Features of fish fins

The broad triangular pectoral fins begin below the third or fourth gill slit and are no longer than the anterior margin of the first dorsal fin. The anal fin is about twice as long as the second dorsal fin and is preceded by long spines. The first dorsal fin begins above the posterior ends free of the pectoral fins, and the second, much smaller, begins above the last third of the base of the anal fin. The dorsal fins have no ridge between them. The lower lobe of the caudal fin is well developed and the upper lobe has a ventral notch at the tip. 

Fish colouring

This shark has a gray, brownish-gray or purplish-gray coloration on top and white underneath. The leading edge of the first dorsal fin and the trailing edge of the caudal fin may be dark, while the trailing edges of the pectoral fins may be light.

Distribution

Widespread in the eastern Atlantic: from Madeira and Mauritania to Angola. Indo-West Pacific: Persian Gulf, Red Sea and East Africa to Indonesia, north to Japan, south to Australia. An occurrence has also been reported in the Gulf of Taranto in the Mediterranean.

Habitat

Tropical demersal amphidromous species. Habitat depth range 1 - 200 m.

Size

Off the coast of West Africa, the milk shark, the largest member of its genus, reaches 1.78 m (5.8 ft) and 22 kg (49 lb) for males and 1.65 m (5.4 ft) and 17 kg (37 lb) for females, although there is uncertainty about the size of females. However, these figures are considered exceptional, and most individuals do not exceed 1.1 m (3.6 ft) in length. In general, females are heavier and reach a larger maximum size than males.

Behavior

Males and females of this shark live separately. It is found on continental shelves, near the surface in shallow waters, often on sandy beaches and rarely in estuaries. It has been reported to enter freshwater and has been recorded several times in Cambodia as far upstream as the Great Lake. 

Food and feeding habits

One of the most abundant coastal sharks in its range, it feeds primarily on small demersal and gregarious bony fish. Squid, octopus, cuttlefish, crabs, shrimp and snails are also occasionally caught. In Shark Bay, the main prey are silver sharks, herring, smelt and guban; it is also the only native shark species that preys on the sea bass Psammoperca waigiensis, which lives in thickets of algae that other sharks avoid. In the Gulf of Carpentaria, it feeds mainly on halfbeak, herring and mullet, and is a major predator of foam shrimp. Smaller sharks eat proportionally more cephalopod molluscs and crustaceans, switching to fish as they mature. The milk shark eats many predators, including larger sharks such as Carcharhinus limbatus and Carcharhinus tilstoni, and possibly marine mammals. 

Reproduction

Milk sharks are viviparous. Females usually have one functional ovary (left) and two functional uteri divided into separate compartments for each embryo. Litter size ranges from one to eight individuals, usually two to five, and increases with the size of the female. The gestation of the embryos lasts about one year and occurs in three phases. During the first stage, which lasts two months and reaches an embryo length of 63-65 mm (2.5-2.6 in), the embryo is fed on egg yolk and gas exchange occurs through its surface membranes and possibly also through the yolk sac. During the second stage, which also lasts two months and reaches an embryo length of 81-104 mm (3.2-4.1 in), the external gill filaments develop and the yolk sac begins to resorb, with the embryo absorbing histotrophs (a nutrient secreted by the mother). In the third stage, which lasts six to eight months, the emptied yolk sac becomes the placental junction through which the fetus is nourished until birth. Young sharks are typically born 32.5-50.0 cm (12.8-19.7 in) long and weigh 127-350 g (0.280-0.772 lb). Pregnant females use coastal nursery areas to give birth, taking advantage of warmer water and abundant food. Sharks leave these coastal nursery grounds when they become adults. The age of maturity is thought to be 2-3 years, with a maximum life span of at least 8 years.

Fishing

It is caught with longlines, gillnets, trawls and rods. Its abundance makes it an important component of artisanal and commercial fisheries throughout its range. It is also one of the most commercially important sharks harvested off the coasts of Senegal, Mauritania, Oman and India. 

Relationship with a person

This shark is harmless to humans due to its small size and teeth. It is sold fresh or dried and salted for human consumption and is also used to make shark fin soup and fish meal.