Meet the worm with a thousand tails

A worm with one head but hundreds of tails, each of which can sprout tiny reproductive bodies that swim off on their own. This is no fantasy, it's the daily reality of Ramisyllis kingghidorahi, a rare marine worm discovered living inside sponges off the coast of Japan.

Recently, a team of researchers from Germany, Japan, Australia, and Spain published the first genetic study of this strange species in BMC Genomics. Their work dives deep into the biology of the worm, exploring how its many “tails” coordinate reproduction, and what that can teach us about the flexibility of life itself.

Most worms have a simple body plan: head at one end, tail at the other. R. kingghidorahi, first described in 2022, breaks all the rules. It lives embedded in sponges, sending its body branching through the sponge like roots of a tree. At the tips of those branches lie hundreds, sometimes thousands, of posterior ends.

From these tips, the worm produces stolons, which are little detachable reproductive units complete with eyes, tiny brains, and swimming bristles. Once mature, the stolons break off and swim away to mate, leaving the parent worm safe inside the sponge to repeat the process.

This remarkable reproductive trick is called stolonization, and while it occurs in many syllid worms, no other animal does it on such a grand scale.

Scientists have long wondered how does one small head coordinate reproduction across hundreds of tails? In simpler syllid worms, hormones and signals from the front end of the body regulate when stolons form. In a branching species like R. kingghidorahi, this control system must somehow be amplified across its sprawling body.

That's where genetics comes in. By comparing patterns of gene activity (which genes are turned on or off) in different body parts of male, female, and non-reproductive worms, the researchers hoped to uncover the hidden choreography of stolonization.

The team collected specimens by scuba diving off Sado Island. Since these worms only live inside certain sponges, the task was anything but easy. Back in the lab, they extracted RNA, a molecule that reflects which genes are active in a cell, from three regions, which were the head end, the mid-body, and the stolons themselves. By sequencing and comparing this RNA, they created the first transcriptomes (a kind of genetic activity map) for a branched annelid worm.

They made several discoveries. First, location matters more than sex. A worm's body region influenced gene activity more strongly than whether the worm was male or female. The stolons, in particular, showed the most unique gene expression patterns, reflecting their intense role in producing eggs or sperm and developing into free-swimming units. The second discovery was that stolons steal the spotlight, so to speak. In fact, male and female stolons displayed hundreds of differences in gene activity, linked to sperm production, egg development, and physical dimorphism (male stolons are slimmer, female ones are egg-filled and bulkier). The scientists also discovered that the head is quieter than expected. Earlier studies suggested that the head plays a central role in triggering stolon formation through hormones. Yet in this species, far fewer head-specific genes were differentially active than expected. That raises new questions: does the worm rely on non-genetic mechanisms, or are the genetic signals simply harder to detect? Finally, classic developmental genes like Wnt and Hox, which are famous for controlling body segmentation and growth, barely showed up in the data. This aligns with a growing idea that stolons, despite resembling little worms, aren't true body segments but rather unique reproductive offshoots.

When we think of worms, we often imagine simple, dull creatures. Hidden in coral reefs and sponge forests are organisms that rewrite the rules of biology. A worm with one head and a thousand tails, sending out fleets of tiny, free-swimming offspring, it's a story as marvelous as any myth, but written in genes and seawater.

If you want to learn more, read the original article titled "Sex-specific differential gene expression during stolonization in the branching syllid Ramisyllis kingghidorahi (Annelida, Syllidae)." on BMC Genomics at http://dx.doi.org/10.1186/s12864-025-11587-w.