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Why do live-bearing fishes usually have fewer and larger young than egg-laying fishes?

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A porcupine fish in the Andaman Sea — fish *Diodon hystrix* is a species of porcupine fish. These fishes are short and broad-bodied, with large eyes and spines covering their skin. Like puffer fishes, they inflate their bodies when threatened.

Live-bearing fishes typically produce fewer and larger young compared to egg-laying species as a result of evolutionary trade-offs driven by natural selection.12

In many egg-laying (oviparous) species, the strategy is to produce a vast number of small eggs that are released into the environment. Because these eggs and the resulting larvae are highly vulnerable to predators and environmental hazards, they face a very high mortality rate, necessitating high fecundity to ensure that at least a few individuals survive to adulthood.32

Conversely, live-bearing species—whether ovoviviparous or viviparous—retain their developing embryos within the female's body. This method provides a significant survival advantage by protecting the young from external threats during their most vulnerable stages. Because the female invests more energy into the development and protection of each individual embryo, she produces fewer offspring. These young are typically born at a more advanced, larger size, which further increases their chances of survival compared to the small, fragile larvae of egg-laying fishes.34