Jelly babies

Some animals never grow up. Scientists have discovered that a population of tiny marine creatures near northern Europe contains only larvae. These are the immature form of the animal. That’s like finding an entire town of kids — and no adults.

The animal is a comb jelly called Mertensia ovum. This jellyfish-like sea creature has a round body and tentacles. Adults can grow as big as 10 centimeters (4 inches) long. But among this newly studied population, researchers couldn’t find any individuals bigger than 1.6 millimeters (less than a tenth of an inch). That’s the size of a marker pen point.

The team found these jellies during 13 trips to the Baltic Sea. This body of water lies between Sweden, Finland and Poland. The researchers sampled the critters from four parts of the sea.

In the lab, comb jelly larvae bigger than 0.75 millimeter (about one-thirtieth of an inch) could lay eggs. That means even the young of this species might reproduce.

The researchers think that the larvae are reproducing enough to keep the population going — even though none ever become adults. The study is “actual proof from nature that there is an entire population maintained by larval reproduction,” team member Cornelia Jaspers told Science News. Jaspers is a graduate student at the Centre for Ocean Life at the Technical University of Denmark in Charlottenlund.

The scientists think that the jellies might be reproducing early because a lot of predators are nearby. If many comb jellies get eaten before they reach adulthood, the population could dwindle. So by developing the ability to lay eggs while they’re still young, these creatures can make sure their population doesn’t die out.

This shift toward early reproduction is an example of evolution. Evolution occurs when a species changes over time — usually many generations — improving its chances of survival. For example, an animal might evolve sharper claws or thicker fur.

Many people think that “evolution sort of goes forward. Everything gets bigger and more complicated and smarter,” Mark Martindale told Science News. Martindale is the director of the Kewalo Marine Laboratory at the University of Hawaii at Manoa in Honolulu. But this new research suggests that evolution can sometimes make a species become simpler or smaller.

Power Words

comb jelly A jellyfish-like animal with a round body and tentacles that lives in the ocean.

population A group of individuals from the same species that lives in the same area.

reproduce To create offspring.

evolution A process by which a species changes gradually over generations.