Sharks do not produce milk. This is because milk production is a defining characteristic of mammals, a biological class to which sharks do not belong. As fish, sharks employ diverse reproductive strategies that do not involve lactation or nursing their young.
Sharks: Fish, Not Mammals
Sharks are cartilaginous fish, classified under the class Chondrichthyes. Their skeletons are primarily made of cartilage rather than bone, distinguishing them from bony fish. Mammals, belonging to the class Mammalia, have fundamental biological differences from fish. Mammals are characterized by mammary glands, which produce milk to nourish their young. They are also typically warm-blooded, maintaining a constant internal body temperature regardless of their environment.
In contrast, most fish, including sharks, are cold-blooded, with their body temperature largely influenced by the surrounding water. Fish respire using gills to extract oxygen from water, while mammals breathe air using lungs. These physiological distinctions, particularly the absence of mammary glands, confirm sharks’ classification as fish, despite some exhibiting traits like live birth or internal temperature regulation.
How Sharks Reproduce and Nourish Their Young
Sharks employ various reproductive strategies to ensure the survival of their offspring. These methods provide nourishment to the developing embryos through different internal or external means.
Oviparity
Some sharks lay eggs externally, a method called oviparity. These eggs are encased in tough, leathery pouches, often called “mermaid’s purses,” which can anchor to underwater structures. The embryo develops inside the egg, sustained by a yolk sac, as seen in species like horn sharks and catsharks.
Ovoviviparity
A more common reproductive strategy among sharks is ovoviviparity. Eggs hatch inside the mother’s oviduct, and the embryos continue to develop internally. The primary source of nourishment for the young comes from the remnants of their yolk sacs, supplemented by fluids secreted by glands in the oviduct walls. Great white sharks and tiger sharks are examples of ovoviviparous species, with pups born alive and fully functional.
Specialized Ovoviviparous Nourishment
Within ovoviviparous species, some exhibit unique nourishment methods. Oophagy involves embryos feeding on a continuous supply of unfertilized eggs produced by the mother. This practice can lead to larger, more robust embryos. Adelphophagy, a more extreme form, occurs when the largest and strongest embryo consumes its developing siblings within the uterus, as seen in some sand tiger sharks.
Viviparity
The most advanced form of shark reproduction is viviparity, or placental viviparity, where there is a direct connection between the mother and the developing young, similar to a mammalian placenta. In this method, a yolk-sac placenta forms, allowing for direct nutrient and oxygen transfer from the mother. Hammerhead sharks and certain requiem sharks are viviparous, giving birth to live young that are nourished through this placental-like connection. Some species, like the white shark, also produce a lipid-rich secretion, often referred to as “uterine milk” or histotroph, which embryos absorb for nutrition.