This sea worm’s ‘butt’ swims away to reproduce. Now, researchers know why

To study that, researchers investigated the developmental gene expression patterns of sexually maturing worms. Typically, “head formation genes” are not expressed as much in the middle of the body. But when the worms’ reproductive parts develop, those genes are highly expressed in the middle of the rear end of the original body.

“Hox genes” determine the body segmentation along the worms’ bodies. The researchers initially thought that these genes would be expressed differently along the “anterior-posterior” axis.”

“Interestingly, the expressions of Hox genes that determine body-part identity were constant during the process,” said Miura, co-author of a paper published in the journal Scientific Reports, in a press statement. As a result, the stolons don’t have a differentiated digestive tract and have repeated uniform body segments (except for the head and tail). “This indicates that only the head part is induced at the posterior body part to control spawning behavior for reproduction.”