These Baby Fish Are Born Knowing How to Kill

By Elizabeth Preston | August 4, 2016 9:15 am


Some babies are born totally useless (I’m looking at you, Homo sapiens). Others can wobble upright shortly after birth and start teetering around. And still other animals are almost frighteningly precocious.

For example, the metallic livebearer, a little golden fish native to Cuba, hatches from an egg while still inside its mother. That means the mom gives birth to live young. The more traditional fish-y way is to lay eggs. But some other fish also bear live young, including guppies and most kinds of sharks.

Metallic livebearers (Girardinus metallicus) are hunters very with precocious babies. As soon as a newborn G. metallicus swims into the world, it’s ready to murder.

Physiologist Martin Lankheet and his colleagues at Wageningen University in the Netherlands wanted to know how much of a baby livebearer’s skill is inborn. Which deadly skills develop while it’s still in the womb, and which does it learn in its first days of life?

They studied 28 G. metallicus to learn more. The fish were born at night, and early the next morning, researchers placed each one in its own Petri dish. They set up a high-speed camera to observe the fish hunting. Then, one at a time, they dropped tiny prey animals into the dishes and waited.

The prey were brine shrimp larvae—babies that, unfortunately for them, aren’t born with any special skill for escaping. Researchers tested the little livebearers on their first, second, and third days of life. Each day, they fed the fish until they were full. The video camera captured more than 2,000 prey captures in all, and from these videos the researchers analyzed the hunters’ every movement.

Metallic livebearers hunt by swimming close to a prey animal and slurping it up like a vacuum cleaner. Hours-old fish were already proficient hunters, succeeding in about 80 percent of their attempts to catch prey. And they got even better as time went on. On their second day of life, they succeeded about 90 percent of the time. By day three they were close to perfect. (The fish in the video above is two days old. Its target is the oblong speck near 10:00 in the Petri dish.)

Video analysis showed that the babies had honed some skills while they were still in the womb. The way they tracked prey with their eyes, for example, didn’t change much as they got older. Neither did their pattern of speeding up just before striking. They knew how to do these things despite not having room to exercise their muscles before birth, or light to see by, or prey to practice capturing.

Other skills improved with time. “The most surprising finding for me was that the newborn fish cannot use their tail for directed swimming during prey capture,” Lankheet says. They can flick their tails to escape a threat, but rely on their pectoral (side) fins to aim themselves at prey. Once the fish get a couple of days older, their mastery of their tails improves. This lets them whip their bodies toward prey more quickly.

Adult livebearers are adept and eager hunters. In fact, they’ll even eat their own babies if it’s convenient—apparently that time inside the mother’s body doesn’t create much affection.

Image: by Michael Göllnitz (via Wikimedia Commons)

Lankheet, M., Stoffers, T., van Leeuwen, J., & Pollux, B. (2016). Acquired versus innate prey capturing skills in super-precocial live-bearing fish Proceedings of the Royal Society B: Biological Sciences, 283 (1834) DOI: 10.1098/rspb.2016.0972



Like the wily and many-armed cephalopod, Inkfish reaches into the far corners of science news and brings you back surprises (and the occasional sea creature). The ink is virtual but the research is real.

About Elizabeth Preston

Elizabeth Preston is a science writer whose articles have appeared in publications including Slate, Nautilus, and National Geographic. She's also the former editor of the children's science magazine Muse, where she still writes in the voice of a know-it-all bovine. She lives in Massachusetts. Read more and see her other writing here.


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