One of the many types of sponges that have been found to act as hosts to branching worms. This one is called Crateromorpha meyeri and it is a deep-sea, glass sponge found in Okinawa, Japan. In this close up of its main opening it is possible to see semi-transparent threads: an example that the newly identified genus of branching worm, Cladosyllis, is living inside. Credit: Naoto Jimi.

An international research team has mined museum collections to solve 147-year-old evolutionary mystery.

Scientists have discovered that one of the most unusual creatures in the animal kingdom is far more diverse and evolutionarily complex than previously known.

An international team led by the University of Göttingen has reconstructed the evolutionary history of branching marine worms – rare annelids that live hidden inside sea sponges and whose bodies branch repeatedly into multiple posterior ends.

Until now, only three species of branching worms have been identified, but by combining DNA sequencing of museum specimens with newly collected specimens and detailed anatomical analyses, the researchers discovered ten new species and solved a long-standing puzzle dating back to the nineteenth century.

Petrosia sponge found in a coral reef of Mayotte, in the Indian Ocean – one of the many types of sponge that can be a host to bizarre branching worms. Credit: Nicole de Voogd.

Strange 

Branching worms are among the strangest animals known to science. Unlike typical worms, they possess a single head that gives rise to a tree-like network of branches.

These species live in symbiosis with and inside sponges, where their unusual body architecture allows them to spread throughout their host.

Using genomic data obtained from both newly collected specimens and decades-old museum material, the researchers found that branching worms form a distinct evolutionary group comprising two major lineages: one group belongs to the genus Ramisyllis, while the second was assigned to a completely new genus, Cladosyllis.

Professor Maria Teresa Aguado, scientific curator of the Biodiversity Museum at Göttingen University, and leader of the study, said: “Our findings would have been impossible without access to valuable museum specimens.

“These collections are not just repositories of old samples, but resources that continue to generate new discoveries.”

Cutting-edge 3D X-ray imaging techniques allowed researchers to create cross-sectional images of the sponge Crateromorpha without damaging this unique museum specimen, which was collected in 1914 from Sagami Bay, Japan. The image shows details of the longitudinal section, with the sponges’ stem internal space occupied by a branching worm from the newly identified Cladosyllis genus. The white arrow points at the worm which lived inside its host sponge. Credit: Ekin Tilic and Maria Teresa Aguado Molina.

Diversity

The researchers also found a remarkable level of hidden diversity, which had remained undetected for more than a century despite these worms being present in a huge part of the world’s oceans.

The team identified thirteen distinct forms of branching worms, including at least ten likely new species distributed across the Indo-Pacific, the Red Sea and New Zealand.

The study suggests that specialisation on different sponge hosts played a key role in driving diversification and evolution: some species inhabit deep-sea glass sponges at depths of up to 1,000 metres, while others live in shallow-water sponges. 

Dr Guillermo Ponz Segrelles, co-author of the study, said: “For over a century, the rare branching worms were all classified as a single widespread species.

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.”

Beyond solving the puzzle of how to classify these creatures, the research provides new insights into the evolution of animal bodies.

The study demonstrates that all branching worms descended from a common ancestor, meaning that their extraordinary body architecture evolved only once.

The researchers also discovered that, despite their overall similarity, the two major lineages produce their branches in different ways, highlighting the developmental flexibility that can arise during evolution.

Modern techniques

Co-author Dr Ekin Tilic at the Senckenberg Research Institute and the Natural History Museum Frankfurt, said: “Some of the sponges hiding these worms came from the Senckenberg collection and date back to 1914.

“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 researchers believe that many more branching worm species remain undiscovered and that these animals could be an important model for studying the evolution of complex body forms, symbiosis and biodiversity in marine ecosystems.

The results were published in the Zoological Journal of the Linnean Society.