Worm’s Butt Grows Eyes and Then Swims Away to Make Babies

Swimming Megasyllis nipponica

To study a life form that could be considered peculiar, possibly originating from a foreign planet, one only needs to take a glance into the world’s oceans. The marine worm Megasyllis nipponica, belonging to the Syllidae family, appears extraterrestrial, or at least reminiscent of a horror film. Since its discovery in the 1960s, it has astounded researchers with a peculiar trait. Throughout its life, it separates from its posterior, which not only swims away for reproduction but also develops its own eyes.

Second Head in the Body

The phenomenon of certain annelids transforming parts of their own bodies, or the entire body, for reproduction is well documented. The formation of a structure called a stolon capable of swimming independently, as observed in Megasyllis nipponica, is not unique. In the case of this Japanese marine worm, the stolon carries sperm or eggs. After detachment from the original body, it autonomously swims to find other stolons, ensuring reproduction.

However, what has perplexed researchers is the mechanism by which eyes, antennae, and swimming bristles can develop while the stolon is still an integral part of the worm. It serves as the worm’s posterior until separation, including the site for waste elimination. Just before detachment, it develops nerve cells to perceive its surroundings with a “mini-brain” and act independently of the original body. The original excretory canal gradually regresses.

Curious Peculiarity in the Animal Kingdom

A team led by Toru Miura from the University of Tokyo discovered that gene expressions responsible for head growth, typically occurring in the upper body region of animals, suddenly appear in the middle segment of these enigmatic worms. These expressions are detectable at the point of sexual maturity and the formation of their reproductive organs, known as gonads.

Miura emphasized that the modification of developmental steps in this marine worm, allowing it to grow a new head in the middle of its body, is an impressive example of animals with unique reproductive mechanisms.

In addition to the identified gene expressions, researchers were astonished that the stolon practically inherits all features, from the digestive tract to individual ring segments, from the original body, and these features remain unchanged during the detachment process. The so-called Hox genes are responsible for separating the stolons from the rest of the body. According to the research team, which published its findings in the journal Scientific Reports, only the head in the middle of the animal seems to be added as a new element for the purpose of reproduction.

Swimming Megasyllis nipponica. The rear part of the worm develops its own head and then swims away.

Other Unresolved Issues

Despite decoding some of the fundamental developmental steps in this marine worm, researchers are still unclear about the hormonal and other mechanisms responsible for determining whether the posterior becomes female or male reproductive organs. Further research aims to uncover the conditions under which stolons with sperm or eggs are formed.