Worm’s butt grows eyes and then swims away to make babies

Life finds a way. But that way is just plain weird at times. Some segmented worms have butts which have a mind of their own and then swim away to reproduce.

The Japanese greed syllid (Megasyllis nipponica) detaches its entire posterior, gonads included, in a process called stolonisation. The stolon, which is full of gametes (eggs or sperm), then swims until it meets the opposite sex and spawns.

Basically, the whole worm exists to jettison its reproductive organs.

This allows the original body to remain protected from environmental dangers, while also helping the gametes to disperse over larger distances.

It gets even weirder though! To swim around autonomously, the stolon develops its own eyes, antennae, and swimming bristles while still attached to the original body. It even develops nerves and a ‘brain’ to sense and behave independently.  

A swimming Megasyllis nipponica with a stolon in its posterior end. Credit: Nakamura et al 2023

Researchers from the University of Tokyo, Japan, have now figured out the mechanisms that make it possible.

They studied the expression patterns of developmental genes in sexually maturing worms and found  a well-known group of genes involved in head formation are highly expressed in the head region of the stolon. Typically, these genes are not expressed much in this part of the worm’s body, but become so during gonad development.

“This shows how normal developmental processes are modified to fit the life history of animals with unique reproductive styles,” explains Professor Toru Miura, who led the research published in Scientific Reports

Another set of genes, known as Hox genes, determine the body segmentation along the head to tail axis of the worms. Basically, they determine that the head forms at the front, followed by the body segments, and end with the posterior at the rear.

“Interestingly, the expressions of Hox genes that determine body-part identity were constant during the process,” says Miura.

As a result, the stolons do not have a digestive tract and instead have repeated, uniform body segments (except for the head and tail). No need for unnecessary digestive machinery when your job description is simply to reproduce!

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