Fish do not drink milk from their mothers because lactation is unique to the class Mammalia. This biological process requires specialized organs and hormonal pathways that are entirely absent in all species of fish. The methods fish use to sustain their offspring are varied and efficient, but they do not involve the transfer of a liquid nutrient produced through mammary glands.
The Definition of Milk
Milk is biologically defined as a complex liquid food produced by the mammary glands of female mammals. This secretion is rich in specific components designed to nourish and protect the young during their earliest stage of life. Milk provides a balanced supply of lipids, proteins, and the sugar lactose, all tailored to the offspring’s rapid growth requirements.
The production of milk, known as lactation, is regulated by a complex interplay of hormones, including prolactin and oxytocin. Milk also contains immune factors, such as immunoglobulins, which provide passive immunity to the newborn. Mammals are the only animals to possess mammary glands and undergo this process.
How Fish Reproduce and Provide Parental Care
The reproductive strategy of most fish species eliminates the need for post-birth lactation. The vast majority of fish are oviparous, reproducing through spawning where the female releases eggs and the male releases sperm for external fertilization. This method, often called broadcast spawning, typically involves the parents leaving the eggs to develop on their own with no subsequent parental care.
Some fish species exhibit various forms of parental care, such as building nests, fanning the eggs to increase oxygen flow, or guarding the developing clutch against predators. Even in species that give live birth (viviparous or ovoviviparous fish), the young receive nutrition inside the mother through structures analogous to a placenta or nutrient-rich fluids, not through lactation.
Nutritional Strategies for Young Fish
Young fish obtain their initial sustenance through mechanisms entirely separate from milk, primarily relying on a maternally provisioned energy source. The most common source of nutrition for newly hatched fish, or fry, is the yolk sac. This yolk sac is a remnant of the egg and contains a concentrated supply of lipids and proteins deposited by the mother before fertilization.
The developing fish absorbs this yolk internally, which fuels its growth until it is large enough to begin feeding independently. Once the yolk sac is depleted, the fry must transition to exogenous feeding by consuming external food sources like plankton or small crustaceans.
In rare, specialized cases, some fish have evolved a form of parental feeding often compared to lactation, such as the Discus fish (Symphysodon spp.). Both male and female Discus parents secrete a thick, nutrient-rich mucus from their skin that the young feed on by nibbling at the parents’ sides. This mucus contains proteins, lipids, and immunoglobulins, similar to mammalian milk. However, this secretion is produced by the skin’s epidermal cells, not by mammary glands, and is not true milk.