Spawn consists of the reproductive cells (gametes) of aquatic animals, some of which will become fertilized and produce offspring. The process of spawning typically involves females releasing ova (unfertilized eggs) into the water, often in large quantities, while males simultaneously or sequentially release spermatozoa (milt) to fertilize the eggs.
Most fish reproduce by spawning, and so do most other aquatic animals, including crustaceans such as crabs and shrimps, molluscs such as oysters and squid, echinoderms such as sea urchins and sea cucumbers, amphibious animals such as frogs and turtles, aquatic insects such as mayflies and mosquitoes, and corals (which are small aquatic animals and not plants). Fungi, such as mushrooms, are also said to "spawn" a white fibrous matter that forms the matrix from which they grow.
There are many variations in the way spawning occurs, depending on sexual differences in anatomy, on how the sexes relate to each other, on where and how the spawn is released, and on whether or how the spawn is subsequently guarded.