When it comes to reproduction, fish are among the weirder types of animals out there. The distinction doesn't end with live-bearers vs. egg-layers, since each set has different types. There's external fertilization, internal fertilization and a question where eggs will be left to develop. In some cases, the fish in question may be hermaphroditic, lending more variety to the act of reproduction.
Oviparity is the most common form of reproduction among fish. Oviparous fishes are those that lay unfertilized eggs in the water. The female lays eggs into a cloud of sperm released by the male for fertilization en masse. After the eggs are laid and covered by the cloud of sperm, the eggs will either float or sink prior to hatching -- the difference between being either pelagic or demersal. Pelagic eggs are lighter than water and float, often near the surface; most marine fish spawn pelagic eggs. Demersal eggs, on the other hand, are heavier than the water and often have adhesive properties, allowing them to sink and stick onto structures and develop in these locations. Smelt, greenlings and herring produce demersal eggs.
Internally Fertilized Eggs
Other egg-layers may copulate and have internal fertilization but still lay eggs. Like other egg layers, the immature fishes are completely nourished by the yolk of the egg; the mother doesn't need to provide any nutrients or other nourishment for her offspring to develop. This method of egg-laying and fertilization isn't nearly as common as oviparity.
Clouds, Nests & Mouths
Some egg-laying fishes let their eggs float away in cloud-like masses. This allows for a widespread distribution of the hundreds or thousands of offspring. Since mortality is high among fish eggs, this method spreads out the population to improve the odds that some will survive. Other fish species will create nests in which the male or female places the eggs and guards them until they hatch. Still other species, such as some cichlids and catfish, will hold the fertilized eggs in their mouths until the offspring hatch.
Live-bearing fishes are those that, like humans, give birth to live offspring. Mating is necessary for live-bearing fish; the eggs must be fertilized prior to development. Therefore, copulation must take place between a male and a female. Once the young are fully developed, the female gives birth to offspring that are free to swim through the water. In some cases, the female provides direct nourishment to her young inside her womb, much like most mammals. In other cases, however, the offspring get their prenatal nourishment via the egg yolk. The eggs simply hatch within the mother and are then released at birth as developed fish.
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