Ascidians, also known as sea squirts, are the evolutionary link between vertebrates and invertebrates, making them valuable subjects of biological studies. For the first time, researchers at Ruhr University Bochum have detected pronounced autofluorescence in Halocynthia papillosa, one of over 3,000 species. Furthermore, the study provides comprehensive insights into the anatomy of this sea squirt. The study demonstrated the potential of modern, multimodal imaging—from light and confocal microscopy to MRT and high-resolution synchrotron tomography, which facilitates three-dimensional imaging of even low-contrast tissues. The study was published in the journal Communications Biology on April 22, 2026.

Detailed Characterization of the Tunic
One key finding is the detailed characterization of the tunic that encases the animal. “For the first time, we were able to detect pronounced autofluorescence in the cuticular spines and reconstruct the complex, spirally organized cellulose architecture of the mantle,” explains Dr. Mareike Huhn from the Ruhr University Bochum Department of General Zoology and Neurobiology. The study publishes the results of the Master’s thesis by Lukas Hessel (Biology, RUB), which was conducted in cooperation with researchers from the Leibniz Institute for Neurobiology in Magdeburg and the European Molecular Biology Laboratory at the German Synchrotron Research Center (DESY) in Hamburg.

Some questions remain
The function of the fluorescent structures remains largely unclear and poses fundamental questions, as similar phenomena have rarely been observed in adult ascidians. “Our data indicate that mechanical states like contraction could influence the optical characteristics of the tunic, with potential ecological functions that have to be examined further,” says Huhn.

Previously unknown, special characteristics of the nervous system
Furthermore, the study presented previously unknown, unique characteristics of the nervous system of Halocynthia papillosa. Its central nervous system is quite different from other species, in particular due to the lack of a nerve thickening at the position where the cerebral ganglion is expected to be localized. This indicates that central neural structures vary more broadly among ascidians than previously believed. “Comparative analyses of other species could reveal new organizational patterns of the cerebral ganglion and provide important insight into their functional significance,” says Huhn.

Basis for future studies
The three-dimensional reconstruction of the tentacles within the oral siphon also reveals species-specific organizational patterns, including distinct sub-tentacle structures as well as the distribution of nerves and blood vessels. “The methods established in this study provide a foundation for systematically comparing such differences between species in the future,” says Huhn. This, she adds, opens up new perspectives for selectively examining correlations between anatomy, filtration function, and environmental factors, such as responses to underwater noise.

Overall, the work illustrates that even common, often disregarded species like H. papillosa have surprising anatomical traits. It also shows that the combination of innovative imaging technology reveals new paths for comprehensively understanding structure-function relationships in marine organisms.