Type specimens are the permanent reference materials that zoological taxonomy depends upon, serving as the foundation for identifying and classifying species. Their preservation, documentation, and accessibility are essential for maintaining taxonomic stability and enabling future systematic research. Yet museum collections often contain materials that remain overlooked, misplaced, or insufficiently curated for decades, sometimes because of financial limitations but also because of a lack of specialist taxonomic knowledge, an unknown repository, or the absence of type designations. In the nineteenth century, designating types was not even a standard practice, as the first authoritative zoological code was not published until 1905. The rediscovery of such material provides important opportunities to reassess historical descriptions using modern methods and to clarify unresolved taxonomic questions.

The story begins in 1889, when Wilhelm Georg Kükenthal, newly appointed Ritter Professor für Phylogenie at the University of Jena, and Alfred Walter embarked on an Arctic expedition to East Spitzbergen. Financially supported by the Geographical Society in Bremen, the expedition departed from Tromsö on 7 May, guided by the famous walrus hunter and captain Nils Johnsen. The journey suffered an unintended interruption when the ship ran aground and was crushed by pack ice on 11 June near the Kong Ludvigøyane. Fortunately, the small crew was picked up four days later by the Cecilie Malene from Tromsö, whose captain allowed them to continue their expedition under the same conditions as originally agreed.

The specimens brought back to Jena contained a variety of groups, including annelids, birds, mammals, anthozoans, echinoderms, mollusks, nemerteans, poriferans, bryozoans, tunicates, and some flatworms. The material included numerous specimens new to science. The polyclads, free-living marine flatworms, were given to Arnold Lang, a former student of Ernst Haeckel and a specialist on polyclads who had published a systematically significant monograph on the polyclads of the Gulf of Naples in 1884. Lang, in an unusually progressive manner for the period, passed the polyclads to his talented student Marianne Plehn, who published her examinations in her 1896 dissertation, describing several new species. As was commonly practiced in the nineteenth century, the exact details of where the specimens were stored after examination were omitted, and the locations of such type specimens were often lost.

Marianne Plehn was born on 30 October 1863 in Lubochen, then part of West Prussia. At the age of 27, following the death of her parents, she moved to Zurich to study zoology, botany, and geology. With financial support from her uncle, she finished her studies and passed the exam as a special instructor in natural sciences in 1893. From 1894 to 1896, she worked at the Zoological Institute and the Comparative Anatomical Institute of both the University of Zurich and ETH Zurich, which was highly unusual for a woman at this time, and she was the first to achieve this. She received her doctorate in zoology in 1896 after submitting her dissertation Neue Polycladen in 1895, becoming the first woman to be awarded a doctorate by ETH Zurich.

In 1898, Plehn became an assistant at the Bavarian Biological Experimental Station at the School for Veterinary Medicine in Munich. Together with Bruno Hofer, she is considered one of the founders of fish pathology. She dedicated her life to research and left public appearances to her director, even turning down a position in Vienna. As the first woman in Bavaria, she was awarded the title of royal professor by decree of King Ludwig III on 30 December 1913 for her extraordinary contribution to fish pathology. One year before her retirement, she was promoted to chief curator at the Starnberg research station in 1927, and two years later she received an honorary doctorate from Ludwig Maximilian University Munich, becoming the first woman in Germany to receive the Doctor medicinae honoris causa in veterinary medicine. She died on 18 January 1946 in Grafrath, close to Munich, at the age of 82.

The rediscovery itself came about through the systematic overhaul of previously overlooked collection material at the Phyletisches Museum Jena. During the COVID-19 pandemic, when the museum was closed between 2020 and 2021, the reorganization of some collection material led to the rediscovery of specimens that had been forgotten for decades. Among these, a jar was found containing an almost dried polyclad from Kükenthal’s collection that had been examined and described by Plehn. The small ground-stopper jar still had its historical label attached, stating Turbellarien Acelis arctica Plehn nov. gen; nov. spec. nördl. Eismeer. K. u. W., with a blue 1 written and marked by Haeckel on its right corner. Further proof that the specimen is from the type series examined by Plehn is a note by Haeckel in the catalog of the Zoological Institute, where he added an entry to the zoological collection in 1896 and made a remark confirming the specimen came from Kükenthal’s polyclad collection, examined by Marianne Plehn.

The rediscovered specimen was reexamined and compared with Plehn’s original description. High-resolution photographs of the type specimen were taken for the first time, using a Canon EOS 6D mounted on a macro rail with a Canon MP-E 65 mm macro objective, with single focused images stacked using Zerene Stacker. The remaining fluid in the jar was measured with a manual alcohol meter, and the specimen was carefully transferred into a 70 percent ethanol solution to meet best-practice standards for fluid preservation, starting with 90 percent ethanol and decreasing in 5 percent steps. The historical label was carefully detached from the jar using medical-grade cotton wool soaked in warm water, dried between glass plates, and reinforced on Japanese paper. The preserved specimen measures approximately 55 mm in length and reaches a maximum width of 36 mm, with a thickness of approximately 3 mm in the center. The body is elongated, grayish with white patterns, and largely opaque even when strongly illuminated. The mouth opening is situated centrally on the ventral side, the male and female gonopores are separate, and the female gonopore is surrounded by small concentric shallow furrows. No eyes were visible, consistent with Plehn’s original description.

Three specimens were originally available to Plehn, of which only the specimen from the collection of the Phyletisches Museum Jena is known to remain today. Swedish zoologist Karl Alfred Sixten Bock listed seven locations in his 1913 monograph where P. arctica has been found, who collected it, and where the specimens are stored. Only for the specimens collected by Kükenthal and Walter was no location of storage given. To locate possible further syntypes, inquiries were sent to different institutions where Plehn had worked or that were suspected of possibly having stored a specimen through connections to Haeckel or Lang, including institutions in Germany, Sweden, Denmark, Poland, Switzerland, and Norway. Responses confirmed that no additional type specimens could be located, clarifying the whereabouts of the syntypes and establishing the Jena specimen as the only known remaining syntype.

The rediscovery of this original syntype restores important reference material for polyclad taxonomy and once again underscores the scientific importance of preserving and revisiting historical museum collections. Polyclad systematics started with one of the first phylogenetic hypotheses suggested by Lang in 1884, and since then the classification has undergone various changes, but the phylogeny remains unresolved despite recent molecular approaches. None of the recent phylogenetic studies included a specimen of P. arctica, and the now available type material provides the opportunity to further resolve the phylogenetic relationships of polyclads. Beyond its taxonomic value, the specimen also serves as a tribute to the legacy of Marianne Plehn, an extraordinary woman who was a successful pioneer in an academic world dominated by men, where women were largely excluded from scientific careers. Today, a street in Munich bears her name, a PhD support program commemorates her, and four genera and one family in Polycladida, as well as one genus of Diplostomida, celebrate her contributions to science.

Subject of Research: Rediscovery and reexamination of the type specimen of the polyclad flatworm Plehnia arctica described by Marianne Plehn in 1896.

Article Title: Rediscovery of a type specimen of Plehnia arctica (Plehn, 1896) ( Polycladida , Plehniidae ) described by Marianne Plehn, a pioneering woman in German zoology

Article References: Bock, B. L. (2026). Rediscovery of a type specimen of Plehnia arctica (Plehn, 1896) (Polycladida, Plehniidae) described by Marianne Plehn, a pioneering woman in German zoology. Natural History Collections and Museomics, 3, Article e200580. https://doi.org/10.3897/nhcm.3.200580

Image Credits: AI Generated

DOI: 10.3897/nhcm.3.200580

Keywords: Plehnia arctica, Marianne Plehn, polyclad flatworms, type specimens, Phyletisches Museum Jena, natural history collections, Spitzbergen expedition, Ernst Haeckel, taxonomy, women in zoology, Rediscovery, type

The study, conducted by Bernhard L. Bock of the Institute for Zoology and Evolutionary Research at Friedrich Schiller University Jena, centers on Atolla wyvillei, a scarlet, bioluminescent deep-sea medusa first described by Haeckel in 1880 and named in honor of Sir Charles Wyville Thomson, the chief scientist of the Challenger expedition. Housed at the Phyletisches Museum Jena, the surviving type specimen has now been reexamined and documented with high-resolution imaging for the first time. More remarkably, the research reveals a previously unknown series of 29 microscope slide sections of a second syntype, discovered in the holdings of the Ernst-Haeckel-Haus in Jena, that directly connects Haeckel’s published plates to laboratory work performed on the original specimens.

The story begins with the Challenger expedition itself, the first scientific voyage to explore the deep sea on a global scale. Between 1872 and 1876, a crew of nearly 250 sailors, engineers, and officers, accompanied by a small scientific team, visited more than 500 stations across nearly 80,000 miles of ocean. They measured depths and temperatures, collected water samples, and dredged the seafloor, ultimately gathering more than 100,000 samples and specimens. The expedition permanently disproved Edward Forbes’s azoic hypothesis, which had held that no life could exist below 300 fathoms, and revealed an immense diversity of deep-sea organisms. It also discovered the Mariana Trench and the Challenger Deep, the deepest known point of the seabed, and even compiled the world’s oldest collection of micrometeorites using magnets to sample dredged sediments.

The expedition’s scientific output was staggering: a 50-volume report totaling nearly 30,000 pages, published between 1885 and 1895 by more than 75 contributing authors, describing around 4,700 new species. A set of these samples was sent to Jena, where Haeckel, already a leading authority in marine zoology, produced extensive illustrated monographs on radiolarians and deep-sea medusae. In total, Haeckel contributed 2,790 pages and 230 plates, nearly 10 percent of the entire publication. His plates, drawn largely with the camera lucida by himself and his lithographer Adolf Giltsch, remain among the most recognizable images of the entire expedition, and many were later adapted for his famous Kunstformen der Natur, a work that influenced artists from Kandinsky to Klimt and Munch.

Yet Haeckel’s methods attracted fierce criticism during his lifetime and afterward. Detractors argued that he fabricated non-existent taxa, lost primary type material, or produced illustrations useless for science. Much of the controversy has focused on his embryological drawings, but the charges bled into assessments of his entire body of work. The new study addresses these critiques head-on by physically reevaluating the type material of A. wyvillei. The surviving syntype, preserved in 70 percent ethanol, has shrunk to 40 millimeters in diameter, a reduction of 20 to 41 percent from the measurements Haeckel recorded for the Station 157 specimens, leaving it structurally resembling, as the author notes, a potato chip. Shrinkage artifacts are visible in the tentacles, lobes, umbrella, and gonads.

The decisive discovery came in 2025, when a small box containing microscope slide sections labeled in Haeckel’s handwriting as Atolla Wyvillei Discomedusae was found in the Ernst-Haeckel-Haus. The 29 slides, embedded in Canada balsam and bearing handwritten annotations describing each anatomical cut, were in poor condition, with cracked mounting medium, loose coverslips, and fragmented specimens. After careful restoration, the slides were examined and matched against Haeckel’s published descriptions. The orientations he noted for Plate 29 of the Challenger report, including radial, horizontal, and tangential sections, correspond precisely with the inscriptions on the slide cardboard, confirming that these slides represent at least one of the original individuals Haeckel examined and cementing their status as part of the syntype series.

This physical evidence demonstrates that several of Haeckel’s published observations could only have been made under a microscope, using histological sections to view the ovaries, distinct tissue layers such as epidermis, mesoglea, and muscle fibers, and the tiny sensory structures called rhopalia. Haeckel credited his preparator Reinhold Teuscher with making the sections, and his own remarks reveal the difficulty of the work: he succeeded only with considerable trouble in determining the rhopalia’s existence, and their anatomical nature could not be resolved because of their small size and the poor preservation of the umbrella margin. The number of rhopalar pedalia in the preserved type, 22, matches what the slides would be expected to show, though the historical cuts failed to intersect the sense clubs precisely.

The study also situates Haeckel’s illustrations within the broader visual culture of nineteenth-century marine biology. Comparing his plates with those of contemporaries such as Alfred Goldsborough Mayer, whose Medusae of the World relied heavily on simplified line drawings that highlighted only key diagnostic features, reveals a fundamental methodological difference. Haeckel’s plates overflow with detail and lettering, choreographed almost like a Baroque high altar, in a tradition of scientific realism aimed at enabling systematic comparison of sets of objects. His compositions move logically from whole-specimen overviews to cutaway views and magnified anatomical details, using stippling and hatching to convey three-dimensional structure. Drawing, the study argues, is fundamentally an exercise in abstraction, and Haeckel’s approach fully conformed to the scientific standards of his era, at a time when photography was expensive, logistically difficult for gelatinous specimens, and not yet widespread in natural science.

The author acknowledges the limits of the reevaluation. More than 150 years of storage have degraded the material, only one complete specimen of the former type-series survives, precluding new histological preparations, and the precise details of Haeckel’s preparation workflow remain unrecorded. The exact influence of Giltsch on the final plates also remains unquantified. Cautious extrapolation is therefore required when evaluating Haeckel’s broader output, and the study does not erase the legitimate debates surrounding his embryological figures. But for A. wyvillei, the verdict is clear: the species can be readily identified and distinguished from other members of the genus based on Haeckel’s work, and his illustrations were grounded in real morphological and histological examination rather than invention.

The broader implications reach into modern biodiversity science. The Challenger samples remain an indispensable historical collection, and historical baseline data from the expedition are now critical to understanding how modern ocean acidification is affecting marine ecosystems. The discovery underscores the hidden value of smaller institutional archives such as the Phyletisches Museum Jena, whose unexamined holdings may contain further unrecognized type specimens. An illustrated catalogue of the museum’s type collection is planned to give researchers broader access. As the study concludes, identifying and systematically reevaluating missing type material can resolve persistent systematic questions and recover invaluable cultural assets, ensuring that the legacy of the Challenger expedition, and of the controversial genius who illustrated it, continues to inform ocean science today.

Subject of Research: Historical reexamination of Ernst Haeckel's type specimens and illustrations of the deep-sea jellyfish Atolla wyvillei from the HMS Challenger expedition

Article Title: Portrayal of the deep sea: Haeckel’s Atolla wyvillei, the challenger expedition, and the boundaries of science and illustration

Article References: Bock, B. L. (2026). Portrayal of the deep sea: Haeckel’s Atolla wyvillei, the challenger expedition, and the boundaries of science and illustration. The Science of Nature, 113(5), Article 109. https://doi.org/10.1007/s00114-026-02154-8

Image Credits: AI Generated

DOI: 10.1007/s00114-026-02154-8

Keywords: Ernst Haeckel, Atolla wyvillei, HMS Challenger expedition, deep-sea jellyfish, scientific illustration, type specimens, histology, marine biology, history of science, taxonomia, museum collections, Coronatae