Scientists have uncovered 10 new species of branching marine worms, revealing that these peculiar creatures are far more diverse and evolutionarily complex than previously thought. An international team led by the University of Göttingen has traced the evolutionary history of these rare annelids, which live hidden inside sea sponges and have bodies that branch repeatedly into multiple posterior ends. Until now, only three species of these worms had been identified. By combining DNA sequencing from museum specimens with newly collected samples and detailed anatomical studies, the researchers discovered 10 new species and solved a long-standing mystery that dates back to the 19th century. Their findings were published in the Zoological Journal of the Linnean Society. Branching worms are among the most unusual animals on Earth. Unlike typical worms, they have a single head that gives rise to a tree-like network of branches. These worms live in a symbiotic relationship with sea sponges, using their unique body structure to spread throughout their host. By analyzing both newly collected samples and material from museum collections, the researchers found that branching worms form a distinct evolutionary group with two major lineages: one belonging to the genus Ramisyllis, and the other to a newly identified genus, Cladosyllis. The study also uncovered a surprising level of hidden diversity that had gone unnoticed for over a century, despite these worms being widespread across the world’s oceans. The team identified 13 distinct forms, including at least 10 new species found in regions such as the Indo-Pacific, the Red Sea, and New Zealand. The research suggests that the worms' adaptation to different sponge hosts has played a significant role in their evolution. Some species live in deep-sea glass sponges at depths of up to 1,000 meters, while others thrive in shallow-water sponges. For over a century, these rare worms were all classified as a single, widespread species. However, the study shows that their evolutionary history is as complex as their bodies, with multiple branches in their family tree, each closely linked to specific sponge species and geographic regions. The research also provides new insights into how animal bodies evolve. It shows that all branching worms descended from a common ancestor, meaning their unusual body structure evolved only once. Despite their overall similarity, the two main lineages produce branches in different ways, demonstrating the flexibility of evolutionary development. Some of the sponges containing these worms were collected in 1914 and are part of the Senckenberg collection. Using modern techniques like microCT imaging, researchers were able to examine the worms without damaging the sponges, revealing hidden biodiversity in old collections. The scientists believe that many more branching worm species remain undiscovered and that these animals could serve as important models for studying the evolution of complex body structures, symbiotic relationships, and marine biodiversity.