External Fertilization of Eggs and Embryonic Care in Fish

In fish species that engage in external fertilization, males ordinarily take over the bulk of embryonic care once females have left their eggs behind.

external fertilization of fish eggs
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Females may simply take off and leave their eggs to a male to fertilize and then care for.

External Fertilization Followed By Care From Dad

In some bony fish species like salmon and trout, mothers will dig out small nests or redds. They use their tails to excavate depressions in the substrate. They then deposit their eggs. Some females will remain with their eggs once they are fertilized. However, most will leave all egg guarding and maintenance to the father.

Since males are often defending a territory, females can choose where to lay their eggs based on qualities of the territory or on characteristics of the male himself. In many species females are more likely to deposit their eggs into the nest of a male that already has some. This indicates that other females have chosen him, so he’s likely a good provider.

External Fertilization With A Twist

There are always exceptions to any generalization in the animal kingdom. Male fish don’t always take over embryonic care after external fertilization has taken place.

mouth brooding cichlid external fertilization
Photo credit: Shutterstock

The female mouth-brooding cichlids in Lake Tanganyika do not leave their eggs to the care of a specific male. Instead, once they have extruded their eggs, they scoop them back up into their own mouths. Females then solicit sperm donations from various potential fathers. Since it’s best for a female to maximize the genetic diversity of her offspring, she may seek the sperm of several males for fertilization. Females keep the fertilized embryos in their mouths for the duration of their gestation. New hatchlings will continue to seek protection in mom’s mouth for a short time after they hatch.

Although it’s a strange sight to imagine – a female accepting sperm into her mouth from several males in sequence – a mom’s gotta do what a mom’s gotta do!


This post is an excerpt from my book “Wild Moms: Motherhood in the Animal Kingdom.”

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