This Male Cannibal Fish Eats Its Eggs When It Wants A Better Batch

The male blenny fish will occasionally eat its babies when it feels the first group of eggs wasn’t good enough or won't be worth it to guard.

Researchers from Nagasaki University in Japan found out recently why these fish have adopted this odd behavior, called filial cannibalism. The paper, which was published on Thursday in Current Biology, found that the barred-chin blenny eats its eggs so it can breed again and create more eggs as soon as possible.

The barred-chin blenny is found near reefs in parts of Asia, such as southern Japan, Taiwan, Malaysia and the Philippines. After the female blenny lays its eggs, the male blenny must stay with them to protect them until they hatch.

But if the female has less than 1,000 eggs, the male blenny will sometimes eat them instead. Previously, scientists thought the male fish would eat their offspring because the nutritional value of the eggs was more than what they would gain from protecting them. 

This new research revealed that the blenny more likely eats the eggs so that their presence can’t influence him any longer and he can instead go find a different female fish to breed with. The fish wants the new set to be larger and healthier than the last, 

The reason the fish must eat the eggs, instead of just swimming away from them, is because a male blenny’s testosterone level drops when the eggs are laid into their nest. During that time, their testosterone level is too low to mate. After the eggs hatch and the babies leave the nest, the blenny’s testosterone level will go back up. 

Fish with Divers Antwerp, Belgium Mayor Bart De Wever (L) and Antwerp director of the Zoo Dries Herpoelaert (R) feed fishes in the Antwerp zoo during the opening of one of the biggest reef aquariums of Europe. The barred-chin blenny lives near reefs in Asia. LUC CLAESSEN/AFP/GETTY IMAGES

The fish will often eat the eggs as quickly as possible, or even spit them out of the nest, in hopes that the empty nest will trigger their body to develop testosterone again. “It strongly implies that egg removal is urgent and that males eat the eggs primarily to remove them, not merely for nutrition,” Yukio Matsumoto, the leader of the study, told New Scientist. The scientists found that after eating their eggs, the fish mated the very next day.

The barred-chin blenny isn’t the only fish to eat its eggs—the sand goby will selectively eat eggs when they aren’t developing as quickly as the others. The males in a species of sail-fin silversides will eat their eggs when they suspect they may not actually be the father.