Lots of animals go to extreme lengths to protect their young. For some, like polar bears, that means taking care of their offspring for years before they venture into the world. For others, like dolphins, adults form protective groups around the young to shield them from predators.
But there’s one group that takes parenthood to a whole new level. And that, my friends, is the mouthbrooding fish.
Mouthbrooding is a strategy where parents keep eggs (and sometimes newly hatched fry) in—you guessed it—their mouths. There are three types: paternal, maternal and biparental mouthbrooders, depending on whether it’s the father, mother or both who store offspring in their mouths. There are a number of families of fish in both salt and freshwater who practice this unusual technique.
Take, for example, the Banggai cardinalfish. This small and flashy saltwater fish is found off of Indonesia and is a popular aquarium fish. They are paternal mouthbrooders, so after the spawning pair mates, the female deposits about 40 eggs into the male’s mouth. The eggs are just a few millimeters in diameter and remain warm and safe inside the male until they hatch. Then, the juvenile cardinalfish will remain in the male’s mouth for a little longer before he releases them into the world to fend for themselves. About half the eggs will survive to become a juvenile.
There are some pretty clear pros and cons of this strategy. An advantage of mouthbrooding is the eggs are protected from predation, so eggs are more likely to survive to hatch than if they were loose in the ocean. But the tolls of mouthbrooding can be significant, as it limits the parents’ ability to eat. Also, it’s the definition of “putting all your eggs in one basket”—if the parent is eaten, its young are eaten along with it.
There is a particularly crafty example of mouthbrooding found in freshwater systems. The cuckoo catfish, which is found in a Lake Tanganyika in Africa, is a brood parasite. That means they rely on another fish to raise their offspring (brood parasites are seen all over the animal kingdom, including in birds that use egg mimicry to trick other birds into caring for their eggs). The cuckoo catfish will sneak in and quickly eat the laid eggs of a cichlid, another mouthbrooding species, before the cichlid parent can take the eggs in their mouth. Then when the cichlid scoops up their eggs, there are catfish eggs mixed in. Then the cichlid will incubate the eggs, allowing the cuckoo catfish to enjoy the benefits of mouthbrooding without doing any of the work! The cuckoo catfish eggs hatch first, and those freshly hatched fry end up eating the cichlid eggs.
Mouthbrooding is just one of many creative ways animals have evolved to care for their young and ensure the greatest chance of survival. Thankfully, humans have nothing of the sort—instead of keeping our kids in our mouths for a month or two, we get to care for them for 18 years!