3D Imaging Unveils Sea Squirt Anatomy

Red sea squirt (Halocynthia papillosa)
Photo Credit: Diego Delso
(CC BY-SA 4.0)

Scientific Frontline: Extended "At a Glance" Summary: Unique Anatomical Structures of Ascidian Species

The Core Concept: Researchers have utilized multimodal imaging to comprehensively map the anatomy of the sea squirt Halocynthia papillosa, uncovering previously unknown biological features such as tunic autofluorescence and an atypical central nervous system.

Key Distinction/Mechanism: Unlike traditional marine dissections, this research employs a combination of advanced modern imaging techniques—including MRI, confocal microscopy, and high-resolution synchrotron tomography—to successfully map three-dimensional models of delicate, low-contrast tissues..

Major Frameworks/Components: 

• Multimodal 3D Imaging: Integration of light microscopy, MRI, and synchrotron tomography for deep tissue visualization.
• Tunic Analysis: Identification of pronounced autofluorescence in cuticular spines and the mapping of a complex, spirally organized cellulose mantle.
• Neuromorphology: Discovery of a central nervous system that fundamentally differs from expected models, notably lacking a conventional cerebral ganglion thickening.
• Tentacle Reconstruction: High-resolution mapping of the species-specific distribution of nerves and blood vessels within the oral siphon.

Branch of Science: Marine Biology, Neurobiology, Evolutionary Biology, and Zoology.

Future Application: The established baseline data and imaging methods provide a foundation for examining correlations between species anatomy, filtration functions, and environmental stressors, such as ecological responses to underwater noise.

Why It Matters: As the closest invertebrate relatives to vertebrates, ascidians serve as crucial evolutionary model organisms. Revealing their diverse and hidden anatomical traits provides vital insights into chordate evolutionary biology and physiological ecology.

Stefan Herlitze, Til Böttner, Mareike Huhn, and Ida Siveke (from left) participated in the study. 
Photo Credit: © RUB, Marquard

As the closest relatives of vertebrates, ascidians are important model organisms. Thanks to modern imaging, researchers now have new insights into their anatomy.

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

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 of the Ruhr University Bochum Department of General Zoology and Neurobiology. The publication presents the results of a master’s thesis by Lukas Hessel (biology, RUB), 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, such as contraction, could influence the optical characteristics of the tunic, with potential ecological functions that must be examined further,” says Huhn.

Previously Unknown 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 that of other species, particularly due to the lack of a nerve thickening at the position where the cerebral ganglion is expected to be localized. This absence 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 insights 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 subtentacle 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 among anatomy, filtration function, and environmental factors, such as responses to underwater noise.

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

Published in journal: Communications Biology

Title: Insight into Unique Anatomical Structures of Ascidian Species

Authors: Lukas Hessel, Jonas Albers, Annika Michalek, Til Böttner, Elizabeth Duke, Ida Siveke, Stefan Herlitze, Jürgen Goldschmidt, and Mareike Huhn

Source/Credit: Ruhr-Universität Bochum | Diego Delso

Edited by: Scientific Frontline

Reference Number: mb052726_01

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