Fish are rarely “pregnant” in the way that term is commonly understood for mammals, which involves a placenta and direct nutrient exchange. The biological term used to describe a female fish carrying developing eggs or young internally is gravid. While most fish species do not involve internal gestation, a few have evolved forms of live birth that are conceptually similar to pregnancy.
The Majority Egg-Laying Fish
The vast majority of fish, approximately 97% of species, reproduce by laying eggs, known as oviparity. Fertilization is typically external, called spawning, where the female releases her eggs (roe) and the male releases sperm (milt) into the water. This method requires little parental energy investment per egg, which is why species like the Ocean Sunfish can release up to 300 million eggs in a single spawning cycle.
The eggs are generally split between pelagic and demersal types. Pelagic eggs are small, buoyant, and float in the water column, often having little yolk and a thin outer layer. Species like tuna and cod produce pelagic eggs, which drift on currents, allowing for wide dispersal but also exposing them to high predation.
Demersal eggs, in contrast, are heavier and non-buoyant, sinking to the bottom where they adhere to rocks, plants, or are buried in sediment. These eggs are typically larger, containing more yolk, and have a thicker, tougher outer envelope for protection. Fish such as salmon and many species of carp lay demersal eggs, and some species exhibit parental care, such as nest guarding, to protect the offspring.
The Closest Comparison Live Birth in Fish
The closest biological comparison to mammalian pregnancy in fish is found in species that give birth to live young, a strategy known as viviparity. This reproductive strategy is divided into two primary types, differentiated by how the developing young receive their nutrition. Both methods require internal fertilization, often achieved using a modified anal fin on the male called a gonopodium.
Ovoviviparity
Ovoviviparity involves the eggs developing internally, but the young receive little or no nourishment directly from the mother after fertilization. The embryos survive primarily by consuming the yolk sac of their own egg, a process known as lecithotrophy. Species like guppies, mollies, and some sharks, such as the angel shark, are considered ovoviviparous, and the young hatch internally before being released live.
Viviparity
Viviparity, or true live birth, involves the young receiving continuous nourishment directly from the mother during gestation, a process called matrotrophy. In these species, structures analogous to a mammalian placenta develop to facilitate the transfer of nutrients, oxygen, and waste. Examples include certain surfperches and some shark species, such as the hammerhead or lemon shark, which develop a yolk-sac placenta that connects the embryonic and maternal blood systems.
When Males Carry the Young
The most unusual form of “pregnancy” occurs in the Syngnathidae family, which includes seahorses and pipefish. In this unique reproductive reversal, the female transfers her eggs into a specialized pouch located on the male’s abdomen. The male then fertilizes the eggs inside this brood pouch and incubates them for a period that can last several weeks.
While incubating, the male is not merely a passive carrier; the pouch functions as a sophisticated internal environment. The pouch tissue develops a rich network of capillaries and secretes a fluid that provides the developing embryos with oxygen, nutrients, and osmoregulation, much like a mammalian placenta. When the young are fully developed, the male undergoes muscular contractions to expel the miniature seahorses, or “fry,” a process that is functionally a form of male live birth.