Britannica Sources used in this answer

How do fish eggs develop in open water and freshwater habitats?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Illustration showing examples of vertebrates: a turtle for reptiles, a songbird for birds, a bluegill fish, a salamander for amphibians, and two mammals-a whale and a groundhog. — Vertebrate The major groups of vertebrates include fishes, amphibians, reptiles, birds, and mammals.

Fish egg development varies significantly based on the environment and the species' reproductive strategy. Most bony fishes are oviparous, meaning they lay eggs that are fertilized externally in the water.12

Open Water (Marine) Habitats

In marine environments, many species produce pelagic eggs that are neutrally buoyant, allowing them to drift with ocean currents in the surface layers. Because these eggs are exposed to the open water, they are often produced in large numbers to compensate for high mortality rates from predation and environmental factors.231

Freshwater Habitats

In freshwater, egg development is often tied to the substrate. Many freshwater species lay demersal eggs, which are heavier than water and settle on or within the bottom of streams or lakes, often among gravel, sand, or vegetation. Some species produce adhesive eggs that stick to these surfaces to prevent them from being swept away by currents.23

Because freshwater environments can be perilous, some species have evolved to produce fewer, larger eggs with more yolk to support direct development, allowing the young to hatch in a more advanced, miniature-adult stage. Other protective strategies include nest-building and parental guarding to increase offspring survival.21