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Syllid worms have a butt that grows its own organs and swims away to mate

Japanese green syllid worms have a posterior body part that develops its own set of organs and transforms into a mobile reproductive unit. 

In a peculiar reproductive process called stolonization, the posterior part detaches from the worm and swims away for spawning. Scientists at the University of Tokyo have now unraveled the mystery of this bizarre reproductive strategy. 

Autonomous movement

During stolonization, the posterior segment of the syllid worm, which contains the gonads, detaches from the main body. This independent unit, called the stolon, is filled with gametes – either eggs or sperms. 

The stolon is not just a passive carrier of reproductive cells. It swims autonomously, a feature that not only shields the original body from environmental hazards but also aids in spreading the gametes across broader areas.

Puzzling anatomy

The stolon is a marvel of biological engineering. Before detaching, it develops its own sensory and navigational tools. It grows eyes, antennae, and swimming bristles while still attached to the main body. 

These features allow the stolon to move and behave independently. But a question that has puzzled scientists is – how does the stolon head form in the middle of the original body? 

Developmental mechanism revealed

The research, led by Professor Toru Miura, has shed light on the developmental mechanism of the stolon for the first time. 

The formation of the stolon begins with the maturation of gonads at the worm’s posterior end. Subsequently, a head-like structure forms at the anterior part of the evolving stolon. The development of sensory organs – eyes and antennae – along with swimming bristles soon follows. 

Crucially, before detachment, the stolon also develops nerves and a rudimentary “brain,” equipping it to sense its environment and act independently.

Unexpected findings

“Furthermore, expression profiles of genes involved in the anterior–posterior identity (Hox genes), head determination, germ-line, and hormone regulation were compared between anterior and posterior body parts during the stolonization process,” wrote the study authors. 

“The results reveal that, in the posterior body part, genes for gonadal development were up-regulated, followed by hormone-related genes and head-determination genes.” 

“Unexpectedly, Hox genes known to identify body parts along the anterior–posterior axis showed no significant temporal expression changes. These findings suggest that during stolonization, gonad development induces the head formation of a stolon, without up-regulation of anterior Hox genes.”

Study implications 

The study not only reveals the developmental mechanism of stolons but also opens new avenues for understanding the complex reproductive behaviors of annelids. 

According to Professor Miura, the next steps involve clarifying the sex determination mechanism and the endocrine regulations that govern the reproductive cycles in syllid worms. 

This research enhances our understanding of the evolutionary transitions in developmental systems of animal life cycles. It also adds a fascinating chapter to the study of life’s reproductive strategies.

The study is published in the journal Scientific Reports.

Image Credit: Nakamura et al 2023

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