Biologists Reveal How Sea Worm's Butt Swims Off on Its Own to Find a Mate

There is a type of sea worm that, believe it or not, detaches its own butt from the rest of its body which then swims off in search of a mate. But how this process occurs has always been a mystery, until now.

A research team, led by Professor Toru Miura from the University of Tokyo, set out to discover why sea worms known as green syllid worms, or Megasyllis nipponica, undergo a process known as stolonization—which sees the worm's bottom half detach from the rest of its body.

The detached part of the body is called the stolon, and carries the worm's eggs or sperms. This part then swims around, detached, by itself. It actually develops its own eyes, swimming bristles and antennae while it does this, meaning it becomes completely independent from the original part of its body.

It then continues to swim on its own until it finds a mate.

What has remained a mystery to scientists, however, is how the worm's butt develops these eyes, antennae, and swimming bristles, while still attached to the rest of the body.

But in a new study, published in the journal Scientific Reports, Miura has finally explained why and how.

Miura and colleagues discovered through observations that this process begins as the worms reach sexual maturity. The head then starts to form in the front part of the bottom half of the worm's body. The eyes, antennae, and swimming bristles then form shortly after. Only after the stolon is detached does its brain and nerves begin to develop.

The researchers reached this conclusion through analyzing the gene expression patterns of worms that were reaching sexual maturity.

They discovered that a particular type of gene was present more in the head region of the stolon. Usually, these genes are not present as much in the middle of the body. However when they reach sexual maturity, the head formation genes were found in the middle of the lower end of the worm's original body.

"This shows how normal developmental processes are modified to fit the life history of animals with unique reproductive styles," Miura said in a press release detailing the findings.

The expression of genes across the worm's body tells scientists how and where the body detaches. At first, researchers thought that genes would be expressed differently along the worm's body. But they actually found something different.

"Interestingly, the expressions of Hox genes that determine body-part identity were constant during the process," Miura said. "This indicates that only the head part is induced at the posterior body part to control spawning behavior for reproduction."

This is the first time a study has looked into the development of these stolons. It is also the first piece of research to intricately look at the strange and mysterious reproductive method.

"We would like to clarify the sex determination mechanism and the endocrine regulations underlying the reproductive cycles in syllids," Miura said.