Did you know that in India eating shark meat was once considered beneficial for lactating woman? It was said to promote breast milk production – this is the reason for the name of the milk shark (Rhizoprionodon acutus). I hope that this belief has changed since then – like every other predatory fish, the milk shark (even if not a top predator) contains much methyl mercury, which is highly dangerous for babies. Since methyl mercury mimics an essential amino acid, it is transported freely throughout the body and consequently goes to the fetus and the mammary glands, too.

Milk shark, image by Randall, John E.
Milk shark, image by Randall, John E., licensed under a Creative Commons Attribution-Noncommercial 3.0 Unported License.

The milk shark or white-eyed shark (look at these eyes) is a rather small (3.6 ft – 1.1 m- long), common wide spread species of requiem sharks and can be found in coastal tropical waters throughout the eastern Atlantic and the Indo-Pacific regions. It likes sandy beaches, estuaries and rivers.

The milk shark is viviparous. Each year, females give birth to up to 8 living young (after the two uteruses have been divided into separate compartments for each embryo). The embryo relies at first on yolk for sustenance, later it ingests histotroph (a nutritious substance secreted by the mother). At last, the depleted yolk sac is converted into a placental connection through which the fetus receives nourishment until birth.

The milk shark reaches maturity after two or three years and probably lives to a maximum of at least eight years. Different from other requiem sharks, it shows a extreme sex imbalance: females outnumber males in a litter by more than 2:1 (sometimes all young are females). The reason is unknown.

Milk sharks are heavily fished for its meat, fins and fishmeal. In northern Australia it is one of the most commonly taken shark species in fish and prawn trawls, and it represents 2% of the catch in gillnets and 6% of the catch on longlines. But its reproductive characteristics seem to help withstanding this high level of exploitation, at least for now.

Sources: here and here

Advertisements