CM – Branched worm with dividing internal organs that grow in the sea sponge


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May 3, 2021

from the University of Göttingen

The sea worm Ramisyllis multicaudata, which lives in the inner canals of a sponge, is one of only two such species that have a branched body with a head and several posterior ends. An international research team led by the Universities of Göttingen and Madrid is the first to describe the internal anatomy of this fascinating animal. The researchers discovered that the complex body of this worm spreads widely in the ducts of its host sponges. In addition, they describe the anatomical details and nervous system of its unusual reproductive units, the stolons, which form their own brain when severed for fertilization, allowing them to navigate their surroundings. The results were published in the Journal of Morphology.

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The research team found the host sponges and their guest worms in a remote area in Darwin, Australia where these animals live. They collected samples, some of which are now in the collections of the Biodiversity Museum at the University of Göttingen. For their analysis, they combined techniques such as histology, electronic optical microscopy, immunohistochemistry, confocal laser microscopy and X-ray computed microtomography. This made it possible to obtain three-dimensional images of both the various internal organs of the worms and the interior of the sponges in which they live. The scientists show that when the bodies of these animals divide, so do all of their internal organs, which has never been observed before.

In addition, the three-dimensional models developed during this research made it possible to to find a new anatomical structure, reserved exclusively for these animals, formed by muscle bridges that cross between the various organs when their body has to form a new branch. These muscle bridges are important because they confirm that the bifurcation process does not take place in the early stages of life, but rather when the worms become adults and then throughout their lives. In addition, the researchers propose that this unique « fingerprint » of muscle bridges theoretically makes it possible to distinguish the original branch from the new at each bifurcation of the complex body network.

In addition, this new study examines the anatomy of the reproductive units (stolons) which develop at the rear ends of the body during reproduction of these animals and which are characteristic of the family to which they belong (Syllidae). The results show that these stolons form a new brain and have their own eyes. This enables them to navigate their environment as they break away from the body for fertilization. This brain is connected to the rest of the nervous system by a ring of nerves that surrounds the intestines.

« Our research solves some of the puzzles these curious animals have posed since the first branching ringlet was discovered in the late 19th century » , explains lead author Dr. Maite Aguado from the University of Göttingen. “However, there is still a long way to go to fully understand how these fascinating animals live in the wild. For example, this study found that the intestines of these animals may be functional, but no trace of food has ever been seen inside them and so it is still a mystery how they can feed their huge branched bodies. Other questions raised in this study are how blood flow and nerve impulses are affected by the branches of the body. « This research lays the foundation for understanding how these creatures live and how their incredible branchy body evolved.

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