Sharks do not produce milk. Milk production, known as lactation, is a biological characteristic exclusive to mammals, which possess specialized mammary glands. These glands are not present in any fish species, including sharks.
The Definitive Answer
Sharks are classified as cartilaginous fish, meaning their skeletons are made of cartilage rather than bone. This fundamental biological difference distinguishes them from mammals. Lactation is a complex physiological process unique to mammals, who possess mammary glands for this purpose. Sharks, lacking these glands, are unable to synthesize or secrete milk. Their methods of nourishing offspring therefore differ significantly from those of mammals.
Diverse Shark Reproductive Methods
Sharks employ a range of reproductive strategies, none involving postnatal milk production. These methods ensure the survival and development of their young through various forms of internal or external nourishment. The three primary categories are oviparity, ovoviviparity, and viviparity.
Oviparity
Oviparous sharks are egg-laying species. The female deposits fertilized eggs, often encased in tough, leathery egg cases known as “mermaid’s purses,” into the marine environment. The developing embryo inside relies on a yolk sac within the egg case for all its nutritional needs. Examples include horn sharks, catsharks, zebra sharks, and the Port Jackson shark. Some egg cases feature tendrils or corkscrew shapes to anchor them securely to the seabed or vegetation.
Ovoviviparity
Ovoviviparous sharks retain their eggs inside the mother’s body, where they hatch internally, leading to live birth. Embryos primarily receive nourishment from their yolk sac. Once the yolk is depleted, some species exhibit oophagy, consuming unfertilized eggs produced by the mother. This strategy results in larger, more developed offspring at birth, increasing their chances of survival. Great white sharks and tiger sharks are also ovoviviparous.
Viviparity
Viviparous sharks give birth to live young that develop with a more direct maternal connection, often resembling a placental link. In these species, embryos receive nourishment directly from the mother through a uterine connection or via nutrient-rich secretions. This fluid, sometimes referred to as “uterine milk,” provides essential lipids and proteins for the growing pups. Bull sharks, hammerhead sharks, and milk sharks are examples of viviparous species, allowing their young to reach a substantial size before birth.
Evolutionary Paths to Nourishment
The distinct reproductive strategies in sharks and mammals result from their vastly different evolutionary histories. Fish, including sharks, and mammals diverged hundreds of millions of years ago, leading to separate adaptive pathways for offspring nourishment.
For sharks, the evolution of diverse reproductive methods like egg-laying, internal egg hatching, and various forms of internal maternal feeding provided adaptive advantages within their aquatic environment. Internal development offers protection from external predators and allows offspring to be born at a larger, more independent stage.
In contrast, the evolution of lactation in mammals provided a significant advantage, particularly in terrestrial or varied environments. Lactation allows mothers to provision their young with a portable and readily available food source, independent of immediate foraging success. This maternal provisioning supports the rapid growth and development of offspring after birth. These divergent evolutionary paths highlight how different environmental pressures led to unique, yet effective, strategies for nurturing the next generation.