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

The new study, published in the open-access journal Microplastics and Nanoplastics, focused on Eisenia fetida, the composting earthworm that has become the standard model organism for terrestrial ecotoxicology. Rather than examining gross measures of animal health such as weight change, reproduction or survival, the researchers zoomed in on the cellular level, specifically on coelomocytes, the immune cells that circulate in the coelomic fluid of earthworms and perform functions analogous to those of white blood cells in humans. These cells come in two principal flavors: amoebocytes, which patrol tissues and engulf foreign material, and eleocytes, which are derived from chloragocytes and contribute to immunity and nutrient transport.

To test how plastic ingestion reshapes this immune cell population, the team exposed earthworms outside of soil for six days to food pellets under three conditions. One group received pellets with no microplastic particles at all, serving as the mock-treated control. A second group received food containing 10 percent by weight of pure polystyrene microplastic particles ranging from 25 to 75 micrometers in size. A third group received the same polystyrene diet but supplemented with 0.5 percent by weight of Irgafos 168, a phosphite antioxidant widely used in plastic manufacturing to prevent polymer degradation during processing. This additive-design decision is what gives the study its sharpest edge, because most laboratory toxicity tests rely on pristine, additive-free plastic spheres that bear little resemblance to the weathered, chemically loaded particles found in real environments.

One of the study’s most notable methodological achievements was the use of a non-invasive technique to harvest the earthworms’ immune cells. Instead of sacrificing the animals, the researchers induced them to expel coelomic fluid containing coelomocytes, allowing repeated sampling from the same individuals and reducing experimental variability. The recovered cells were then analyzed by flow cytometry, a laser-based technique that can distinguish and count thousands of individual cells per second based on their size, internal complexity and fluorescent labeling. This allowed the team to quantify not just the total number of immune cells, but their viability and the relative proportions of amoebocytes and eleocytes within each sample.

The first key finding was deceptively reassuring: the total number of cells isolated from the worms was not significantly affected by microplastic ingestion. In other words, earthworms eating polystyrene did not simply produce fewer coelomocytes overall. But when the researchers looked at cell viability, a different picture emerged. Earthworms that had ingested pure polystyrene particles showed significantly reduced coelomocyte viability compared with the mock-treated controls. The cells were still there, but a larger fraction of them were dead or dying, a sign that something in the plastic-exposed animals was harming the immune cells themselves.

The damage grew worse when the plastic carried its industrial additive. In worms fed the Irgafos 168-containing polystyrene, the number of living cells per milligram of body fresh weight dropped even further than in the pure-polystyrene group, indicating that the additive amplified the toxicity of the particles. The researchers backed up this in-vivo result with ex vivo assays, exposing isolated coelomocytes directly to the two particle types in the laboratory. Those experiments independently confirmed that particles supplemented with Irgafos 168 were more toxic to the cells than the pure polystyrene particles, strengthening the argument that the chemical additive, not merely the plastic polymer, drives part of the harm.

Why would an antioxidant designed to protect plastics from degradation be harmful to living cells? Irgafos 168 belongs to a class of phosphite compounds that can oxidize over time into phosphate derivatives, and laboratory studies have suggested that both the parent compound and its degradation products can interact with cell membranes and intracellular signaling. Because additives are not covalently bound to the polymer matrix, they can leach out of plastic particles once the material enters the environment and encounters warmth, digestive fluids or microbial action. When an earthworm swallows a contaminated particle, its gut becomes a reaction vessel in which these compounds can be released at close range to the very tissues responsible for defense and nutrient absorption.

Beyond the viability data, the study documented that ingestion of microplastic particles and their additives caused measurable shifts in the distribution of immune cell subpopulations compared with mock-treated worms. Changes in the balance between amoebocytes and eleocytes are more than a bookkeeping detail; they point to a reprogramming of the immune system itself. Amoebocytes are the earthworm’s first line of cellular defense against pathogens, phagocytosing bacteria and encapsulating foreign bodies, while eleocytes participate in immune regulation and reflect the metabolic state of the coelomic cavity. A skewed ratio between these populations could impair an earthworm’s ability to fight off infections, respond to other pollutants, or maintain normal physiological function, even in the absence of visible illness.

The ecological implications extend well beyond a single species in a laboratory feeding trial. Eisenia fetida serves as a surrogate for the broader community of soil-dwelling organisms that face chronic exposure to plastic contamination. Microplastic particles are now documented in agricultural soils across the globe, introduced through sewage sludge, plastic mulch films, irrigation water and the atmospheric deposition of fragmenting debris. Earthworms ingest soil particles indiscriminately as they feed, which means that plastic fragments in the 25 to 75 micrometer range fall squarely within the size class these animals routinely consume. If chronic exposure erodes immune competence in wild populations, soils could become more vulnerable to pathogen outbreaks, and the decomposition processes that underpin nutrient cycling could slow.

The study also carries a broader warning for how microplastic toxicity research is conducted. A growing body of literature has argued that testing pristine, spherical, additive-free particles systematically understates the risks posed by environmental plastics, which arrive pre-loaded with stabilizers, plasticizers, pigments and flame retardants. By deliberately including Irgafos 168 in their experimental design and demonstrating enhanced toxicity, the Bayreuth team has provided concrete experimental support for that argument. The findings suggest that regulatory assessments of microplastic risk, which often focus on the polymer alone, may need to account for the full chemical package that real-world particles carry. For now, the image that emerges from this research is a sobering one: the earthworms that quietly sustain the world’s soils are swallowing our plastic waste, and the chemical hitchhikers riding on that waste appear to reach deep into their cellular defenses, killing the very immune cells that would normally keep them safe.

Subject of Research: Effects of microplastic particle ingestion and plastic additives on earthworm immune cells

Article Title: Microplastic ingestion induces changes in coelomocyte composition of Eisenia fetida

Article References: Fritsche, J. K., Döring, M. V. R., Mauel, A., Senker, J., Feldhaar, H., Freitag, R., & Jérôme, V. (2026). Microplastic ingestion induces changes in coelomocyte composition of Eisenia fetida. Microplastics and Nanoplastics. https://doi.org/10.1186/s43591-026-00224-2

Image Credits: AI Generated

DOI: 10.1186/s43591-026-00224-2

Keywords: microplastics, polystyrene, Irgafos 168, Eisenia fetida, coelomocytes, immune cells, flow cytometry, ecotoxicology, soil health, plastic additives, earthworms, environmental toxicity