This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 had been identified. By combining DNA sequencing of museum specimens with newly collected specimens and detailed anatomical analyses, the researchers discovered 10 new species and solved a long-standing puzzle dating to the 19th century. The results were published in the Zoological Journal of the Linnean Society.
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. "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," said Professor Maria Teresa Aguado, scientific curator of the Biodiversity Museum at the University of Göttingen and leader of the study.
The researchers also found a remarkable level of hidden diversity that 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 13 distinct forms of branching worms, including at least 10 likely new species distributed across the Indo-Pacific, the Red Sea and New Zealand. The study suggests that specialization 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 meters (3,280 feet), while others live in shallow-water sponges.
"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," said Dr. Guillermo Ponz Segrelles, co-author of the study.
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