The findings, published in the Zoological Journal of the Linnean Society, represent a triumph of what researchers are calling “museomics” — the extraction and analysis of DNA from century-old museum specimens. By combining this historical genetic material with freshly collected specimens and cutting-edge imaging techniques, the team reconstructed the family tree of branching syllid worms and solved a puzzle that has vexed zoologists since the 1870s.

Branching worms belong to a group of annelids — segmented worms related to earthworms and leeches — but their anatomy defies nearly everything we associate with worm body plans. While a typical worm grows by adding length, these creatures grow by splitting. A single head gives rise to a body that repeatedly bifurcates into a sprawling network of posterior branches, creating a living bush whose crown can spread throughout the interior of a sponge host. Scientists believe this bizarre architecture is an adaptation to their symbiotic lifestyle: by branching through the sponge’s internal canals, the worm can occupy its host thoroughly without ever exposing itself to the open water.

Until now, only three species of branching worms had ever been described. The most famous, Ramisyllis multicaudata, was discovered in Australia in 2006 and announced to the world in 2012, decades after the first branching worm, Syllis ramosa, was found inside a sponge collected in the Philippines during the HMS Challenger expedition. That specimen, collected in 1875, launched the mystery: how could such strange animals exist, how many were there, and how on earth did their branching bodies evolve?

The new study answers at least some of these questions with unprecedented detail. The research team, led by Professor Maria Teresa Aguado, Scientific Curator of the Biodiversity Museum at Göttingen University, assembled material from an extraordinary range of sources: worms collected from shallow reefs and deep-sea expeditions across the Indo-Pacific, the Red Sea and New Zealand, alongside fragile historical specimens preserved in museum collections, some of which date back to 1914. Sponge specimens — the hosts in which these worms live hidden — were examined non-destructively using microCT imaging, a technique that uses X-rays to build detailed three-dimensional models of an object’s interior without cutting it open. In several cases, researchers could confirm the presence of branching worms coiled inside sponges that had been collected over a century ago.

“Our findings would have been impossible without access to valuable museum specimens,” said Professor Aguado. “These collections are not just repositories of old samples, but resources that continue to generate new discoveries.”

The genetic and anatomical analyses revealed thirteen distinct forms of branching worms, at least ten of which are likely new to science. Equally significant was what the DNA told the researchers about how these animals relate to one another. Branching worms, it turns out, are not a single loosely related group of oddities — they form a coherent evolutionary lineage that split into two major branches, rather like the bodies of the worms themselves. One lineage retains the genus name Ramisyllis. The other has been assigned to an entirely new genus, Cladosyllis, described in the same study.

The deeper evolutionary story is perhaps the most striking of all. By comparing genomic data across the group, the researchers demonstrated that all branching worms descend from a single common ancestor — meaning that their extraordinary tree-like body architecture evolved only once in the history of life. That single evolutionary event, apparently rare enough that no other animal group has replicated it so dramatically, gave rise to a worldwide radiation of species. Yet despite their shared origin, the two major lineages produce their branches in fundamentally different ways, a discovery that highlights the developmental flexibility evolution can exploit. The same branching outcome, it seems, can be reached through different embryological and growth mechanisms — a finding with implications for how biologists understand the evolution of complex body plans across the animal kingdom.

The distribution of the newly discovered species tells its own story. Branching worms were found across an enormous geographic range: the tropical and subtropical waters of the Indo-Pacific, the Red Sea, and the waters around New Zealand. Some species live in shallow-water sponges accessible to divers, while others were found inside deep-sea glass sponges at depths of up to 1,000 metres — creatures like Crateromorpha meyeri, a delicate lattice of silica whose translucent body can reveal the semi-transparent threads of a resident Cladosyllis worm at its main opening. The researchers believe this close association with specific sponge hosts has been a major engine of diversification. As different worm lineages adapted to different sponge species — and to vastly different environments, from sunlit reefs to the abyssal dark — they diverged into the distinct species now being described.

“For over a century, the rare branching worms were all classified as a single widespread species,” said Dr Guillermo Ponz Segrelles, a co-author of the study. “However, we have shown that – like their bodies – their family tree has many branches, each closely associated with its own sponge species and located in specific areas.”

The revelation that a “single” species actually encompasses a dozen or more distinct lineages is a familiar refrain in modern taxonomy, but rarely has the hidden diversity been so dramatic — or so physically strange. It also underscores the critical role that natural history museums play in twenty-first century science. Specimens collected generations ago, preserved in alcohol and archived in institutional drawers, have become a genetic treasure trove. Techniques that were unthinkable when these worms were first collected — high-throughput DNA sequencing, microCT scanning, computational phylogenetics — can now coax secrets from material that was once considered little more than taxonomic paperwork.

“Some of the sponges hiding these worms came from the Senckenberg collection and date back to 1914,” noted Dr Ekin Tilic of the Senckenberg Research Institute and Natural History Museum Frankfurt, another co-author of the study. “We were able to reveal the worms without damaging the sponges by using microCT imaging – modern techniques are helping us rediscover hidden biodiversity in centuries-old collections.”

The study also raises questions that will keep researchers busy for years. How does a worm with a single head coordinate a body with dozens of posterior ends? Earlier work on Ramisyllis multicaudata suggested its nervous system branches in parallel with its body, and that the worm reproduces in a peculiar fashion — releasing specialized stolons bearing gametes that swim away from the branch tips. Whether the newly described Cladosyllis species share these traits, and how their different branching mechanisms develop, remains to be explored. The researchers suggest that branching worms could become an important model system for studying the evolution of complex body forms, the biology of symbiosis, and the maintenance of biodiversity in marine ecosystems.

There is also a conservation dimension to the discovery. Because branching worms depend on sponge hosts, and because sponges — particularly the slow-growing glass sponges of the deep sea — are vulnerable to trawling, warming oceans and habitat disturbance, the fate of these worms is inseparable from the fate of their hosts. The full extent of their diversity is likely far greater than the current tally suggests. The deep sea remains one of the least explored environments on Earth, and if a single sponge species collected in 1875 could hide a branch of the tree of life for nearly 150 years, the ocean’s unexplored sponges may harbor many more.

What began as a curiosity preserved in the collections of the Challenger expedition has become a window into the creative power of evolution. One head, one ancestor, one evolutionary innovation — and from it, a tree of worms branching through the oceans, some of them hidden for over a century in the quiet halls of the world’s museums, waiting for the right tools to reveal them.