The discovery challenges long-standing hypotheses regarding the anatomy and feeding mechanisms of early ambulacrarians. Previously, many researchers believed that the common ancestor of this group resembled a worm that lived in seafloor burrows and filtered food passively through its gills or pharynx. However, the Yujingia fossils indicate a different lifestyle. The animals possessed two distinct sets of appendages. One set consisted of feather-like feeding tentacles used to capture organic particles from the water. The second set was likely used to temporarily anchor the animal to the seabed, although the creature was probably capable of some degree of movement. This active feeding strategy suggests that the ancestor was more similar to modern starfish than to acorn worms.

The fossils were found in the Maotianshan Shales in Yunnan Province, China, a location known for preserving soft-bodied organisms from the Cambrian period. Most Cambrian fossils consist of hard-shelled creatures, but specific geological conditions in places like the Maotianshan Shales allowed for the preservation of delicate soft tissues before they could decay. The Yujingia specimens measure between 8 and 22 millimeters in length. They preserve the body, gut, and appendages, offering rare examples of ambulacrarians without a mineralized skeleton. This preservation is critical for understanding the soft-body anatomy of early deuterostomes, a group that also includes chordates, the phylum containing humans.

Lead author Kaiyue He from Northwest University described the physical appearance of the animal as having an elongated, bottle-like form with tentacles spreading above a rounded lower body. The outline of the fossil closely resembles the ritual vase, or yujingping, in traditional Chinese culture, which inspired the genus name. All specimens clearly preserve a pale, original membrane enclosing the visceral mass. This soft, gelatinous appearance is the species’ most immediately recognizable feature and inspired the specific epithet glutenotunica. The presence of this membrane provides direct evidence of the animal’s soft-bodied nature, contrasting with the hard skeletons seen in later echinoderms.

To determine the evolutionary position of Yujingia, the research team conducted a detailed phylogenetic analysis. Co-author Dr. Giovanni Mussini from Cambridge’s Department of Earth Sciences, along with He, assembled a large dataset comprising 505 morphological characters across 100 living and extinct taxa. They analyzed this data using Bayesian phylogenetic reconstruction. The results place Yujingia closer to the last common ancestor of living ambulacrarians than previously described fossils. This makes Yujingia a key transitional form linking early Cambrian tentacled animals with the distinct lineages of hemichordates and echinoderms. The study suggests that sophisticated feeding structures evolved early in ambulacrarian history, while echinoderm skeletons and other specialized features appeared later in evolutionary time.

The findings have broader implications for understanding the divergence of deuterostome animals. Chordates, echinoderms, and hemichordates represent the principal branches of deuterostome evolution, yet they differ profoundly in body organization and feeding strategies. Reconstructing the ambulacrarian ancestor is central to understanding how these lineages diverged. A longstanding hypothesis proposed that the ancestor was a tentacle-free worm that filtered food through its pharynx, resembling a modern acorn worm. The detailed anatomical study of Yujingia challenges this view with direct fossil evidence. The presence of tentacles indicates that the ancestor actively captured food, a trait that may have influenced the subsequent evolution of its descendant lineages.

Dr. Mussini noted that the fossil suggests the common ancestor of ambulacrarians might have actually fed with tentacles, making it more similar to starfish and their relatives than previously thought. This new perspective helps explain why two closely related lineages evolved such different body plans. The chordates became reliant on movement for finding food, leading to streamlined bodies and developed brains. In contrast, members of the starfish lineage remained relatively anchored to the seafloor and specialized in a particle-feeding lifestyle. The Yujingia fossil illustrates how ecology can drive different lineages onto very different evolutionary paths, resulting in distinct morphological specializations over millions of years.

Co-author Degan Shu, also from Northwest University, emphasized the importance of this discovery in the context of animal evolution. He stated that how the major deuterostome groups diverged during the Cambrian explosion is one of the central questions in the field. Yujingia helps connect primitive, tentacle-bearing animals living on the seafloor with modern echinoderms and hemichordates. The fossil offers a new way to understand the profound transition from bilateral symmetry to the fivefold radial body plan characteristic of echinoderms. By filling a crucial gap in the early animal tree of life, Yujingia provides a tangible link between early soft-bodied ancestors and the diverse forms of life that exist today. The research highlights the significance of soft-bodied fossil sites in reconstructing the history of animal life.

Subject of Research: Earth Science

Article Title: A star is born: half billion-year-old fossil rewrites the origin story of starfish

Article References: A star is born: half billion-year-old fossil rewrites the origin story of starfish. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Paleontology, Evolution, Cambrian Explosion, Starfish, Fossils, Deuterostomes, University of Cambridge, Northwest University, star, born, half, billion-year-old

Coelomocytes are the cellular workhorses of the echinoderm immune system. They circulate in the coelomic fluids that fill the body cavity, performing functions that in vertebrates are distributed among blood cells and immune cells of many kinds. For decades, biologists have classified these cells by looking at their shapes under the microscope, and sea cucumbers, or holothuroids, stand out among echinoderms for displaying the greatest variety of coelomocyte morphotypes of any class in the phylum. Yet morphology alone has proved a poor guide to function. While the overall immune role of these cells is broadly accepted, the specific jobs performed by each cell type have remained murky, and molecular data specific to the different morphotypes have been scarce in the literature.

Single-cell RNA sequencing, or scRNA-seq, has transformed how biologists untangle cellular heterogeneity, allowing researchers to profile the gene expression of thousands of individual cells and group them into transcriptionally distinct populations. The technique, however, has been largely underutilised in studies of non-model organisms, particularly among invertebrates. The new study set out to close that gap for sea cucumbers by applying the method to coelomocytes harvested from the perivisceral fluid of Holothuria forskali, a species collected with the help of the collection service of the Roscoff Biological Station in France. Sequencing was carried out at the Genomic Platform of the GIGA Institute in Liège.

The analysis identified ten distinct clusters of cells, each assumed to correspond to a separate transcriptional coelomocyte population. The clustering revealed a striking architecture: one cluster, designated cluster 0, occupied a central position relative to all the others on the two-dimensional map of gene expression, suggesting that it may represent undifferentiated cells, a reservoir of progenitors from which more specialised populations arise. At the opposite extreme, cluster 6 sat markedly apart from every other group, hinting at a fundamentally different biological identity and function.

To work out what each cluster might actually do, the team performed functional enrichment analyses and hunted for immune marker genes among the transcripts enriched in each population. The results showed that several clusters carry out key immune functions, including the recognition of pathogens, phagocytosis, the engulfing and destruction of foreign particles, the activation of complement-like pathways, and the regulation of redox balance, the delicate chemistry of oxidation and reduction that cells must manage to survive an immune response. These findings provide the first tentative clues about which transcriptional populations correspond to which defensive duties, offering a molecular handle on a system that has until now been described mostly by cell shape.

One of the most intriguing results concerns a recently discovered cell type called the carotenocyte. These cells, rich in carotenoids, the pigments responsible for many of the reds, oranges and yellows in nature, were only recently described in sea cucumbers, and their presence in the perivisceral fluid was confirmed when the researchers examined their processed samples under the microscope. By drawing on transcriptomic data previously generated for this cell type using bulk RNA sequencing, the team was able to confidently assign cluster 6, the most divergent group on the map, to the carotenocyte lineage, and to deepen the picture of what these pigment-laden cells express.

The convergence of two independent datasets proved decisive. Marker genes supported by both the bulk RNA sequencing of carotenocyte-enriched samples and the new single-cell data gave the researchers confidence in the identity of cluster 6, an approach that illustrates how single-cell and bulk methods can be combined to anchor cell identities in non-model organisms where no reference atlas exists. The study’s supplementary tables catalogue the marker genes for each cluster, their functional annotations against multiple databases, and the results of enrichment analyses against KEGG pathways and gene ontology categories, providing a resource for other researchers working on echinoderm immunity.

Rigorous quality control underpinned the analysis. The authors filtered cells on the basis of the number of unique molecular identifiers and detected genes per cell, examined the expression of mitochondrial genes to flag stressed or damaged cells, and used the DoubletFinder tool to detect and remove doublets, artefacts in which two cells are captured and sequenced as one. Sensitivity analyses testing different filtering parameters showed that while individual clusters could merge or split depending on the settings, the overall configuration of the map remained stable, with cluster 0 staying central and cluster 6 remaining the most divergent population, a reassuring sign that the biological signal is robust.

The implications reach well beyond sea cucumbers. Echinoderms are deuterostomes, the same major lineage as chordates, which makes their immune cells valuable comparators for understanding how immune cell lineages evolved on the branch of the tree that eventually produced our own adaptive immune system. A better grasp of holothuroid coelomocyte diversity could also help interpret stress responses in these animals, which are ecologically important grazers and the target of a rapidly expanding aquaculture industry in Asia. Knowing which cells respond to pathogens or environmental stressors, and through which genes, could inform disease management in farmed populations and shed light on how wild sea cucumbers cope with a changing ocean.

The authors are careful to frame the study as a pioneering first step. Linking the remaining transcriptional clusters to the morphotypes described in the historical literature will require further analyses, and the functional assignments drawn from enrichment analyses remain hypotheses to be tested experimentally. Still, the study demonstrates that single-cell transcriptomics can be successfully deployed on a non-model invertebrate with no reference-grade tools, and it delivers a preliminary but detailed map of the functional diversity of holothuroid coelomocytes. For a phylum whose immune system has been studied mostly through a microscope lens, the jump to single-cell genomics marks the beginning of a new era, one in which the hidden division of labour among sea cucumber immune cells can finally be read directly from their genes.

Subject of Research: Single-cell transcriptomic profiling of coelomocyte immune cell diversity in the sea cucumber Holothuria forskali

Article Title: Single-cell transcriptomics reveals transcriptional diversity of sea cucumber perivisceral fluid coelomocytes

Article References: Wambreuse, N., Lavergne, A., Fievez, L., Bureau, F., Zhang, L., Deng, B., Caulier, G., Eeckhaut, I., & Delroisse, J. (2026). Single-cell transcriptomics reveals transcriptional diversity of sea cucumber perivisceral fluid coelomocytes. BMC Genomics. https://doi.org/10.1186/s12864-026-13356-9

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13356-9

Keywords: sea cucumber, coelomocytes, single-cell RNA sequencing, echinoderm immunity, carotenocytes, Holothuria forskali, transcriptomics, immune cells, deuterostomes, phagocytosis, BMC Genomics, non-model organisms