While most of the world's thousands of fish species lay eggs, more than 500 species of fish reproduce by giving live birth. This includes fish from wildly different classes and families. Vivpary, or giving live birth, has some advantages over egg-laying, along with a few drawbacks.
Vivipary evolved multiple times in fish. This is evidenced by the fact that it shows up in unrelated species. Some groups of fish that reproduce by live birth include such varied species as the coelacanth, some sharks and the familiar guppy. Unlike mammals, most female fish that give live birth provide little nutrition for their developing young. However, like most rules in biology, a few exceptions exist. Some fish related to the guppy provide nutrition to their developing young like mammals do.
Advantages of Livebearing
Vivipary offers a few advantages over egg-laying. The mother's body provides the developing young protection. The arrangement also allows the young to start life on the outside more developed than egg-layers. Live-bearing fish usually start life as miniature versions of the adults, better able to care for themselves. Additionally, vivipary allows more mobility during development. While eggs are usually stuck wherever they landed, a pregnant fish can move to new areas if conditions change. Tying in with this, a single pregnant female can spread the species to a new area.
Disadvantages of Livebearing
Vivipary does have a few minor drawbacks. Female livebearers usually have smaller broods than their egg-laying counterparts. However, since their young are more developed, they typically have higher survival rates. The biggest reason that vivipary is not more common in fish is that it requires a series of modifications to the "standard" fish reproductive system, making it less like likely to evolve.
Some egg-laying fish make up for the disadvantages of egg laying through behavioral changes, particularity parental care. This takes a number of different forms across different species of fish. For example, sicklebacks and cichlids are both known for creating nests for their young. Additionally, some fish, like cardinalfish and jawfish, carry their developing eggs, and even young, around in their mouths. Other fish, like bettas and certain catfish, go as far as creating nests out of bubbles and plant matter.
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