Kurt Ockelmann's unpublished studies on the Thyasiridae (Mollusca: Bivalvia) of the North Atlantic, held in the Natural History Museum of Denmark

239 European Journal of Taxonomy 1007: 239–278 https://doi.org/10.5852/ejt.2025.1007.3011 europeanjournaloftaxonomy.eu ISSN 2118-9773 2025 · Oliver P.G. This work is licensed under a Creative Commons Attribution License (CC BY 4.0) Received: 15 November 2024 • Accepted: 12 June 2025 • Published: 18 August 2025 Topic editor: Magalie Castelin • Section editor: Thierry Backeljau • Desk editor: Kristiaan Hoedemakers Research article urn:lsid:zoobank.org:pub:011AE27D-E48D-415D-8809-BD81904B8D28 Kurt Ockelmann’s unpublished studies on the Thyasiridae (Mollusca: Bivalvia) of the North Atlantic, held in the Natural History Museum of Denmark P. Graham OLIVER Honorary Research Fellow, Amgueddfa Cymru, National Museum of Wales, Cardiff, CF10 3NP, United Kingdom. Email: [email protected] Abstract. The archive of illustrations and graphics relating to the research of Kurt Ockelmann (1924– 2021) on the bivalve family Thyasiridae is reviewed. Although he published only one short paper on the family, the archive reveals that he undertook a substantial series of studies and shows his continuing influence on the taxonomy and life history of this family in the North Atlantic. Ockelmann was the first to recognise amphi-Atlantic species, their life habits and larval biology. He mapped the distribution of 19 species, drew their prodissoconchs and made superb drawings of the adult shell and their hinges. He made accurate anatomical diagrams of representative species of the genera. Given the excellence of illustrations many are reproduced here, as they are among the best available and a valuable resource when identifying species. Keywords. Ockelmann, history, Thyasiridae, retrospective, archive. Oliver P.G. 2025. Kurt Ockelmann’s unpublished studies on the Thyasiridae (Mollusca: Bivalvia) of the North Atlantic, held in the Natural History Museum of Denmark. European Journal of Taxonomy 1007: 239–278. https://doi.org/10.5852/ejt.2025.1007.3011 Introduction Kurt Wolfgang Ockelmann (1924–2021) was one of Denmark’s most respected and well-loved marine biologists of the twentieth century. The photograph in Fig. 1 was taken at the celebration conference in Helsingør in 2014 to mark his 90th birthday. Ockelmann’s early life from German soldier to Danish marine biologist is told in Kristiansen (2021) and a full retrospective of his scientific life will be found in Jensen & Dinesen (in prep). His interests strayed across a wide range of taxa and found him following whatever took his curiosity throughout his forty years at the Marine Biological Laboratory, Helsingør. His primary interest was in the Mollusca, probably sparked by his mentor, the great Danish biologist, Gunnar Thorson (1906–1971) (Ankel 1972). Through Thorson, Ockelmann became familiar with the collections in the Zoological Museum, Copenhagen (now incorporated into the Natural History Museum of Denmark) and in particular the large collections from Greenland made by Thorson during European Journal of Taxonomy 1007: 239–278 (2025) 240 the Godthab expeditions of the early 1930s (Higgins 2010). Ockelmann began work on the bivalves of Greenland publishing an account on the genus Yoldia Møller, 1842 in 1954 (Ockelmann 1954) and a large monograph on all the bivalves in 1958 (Ockelmann 1958). It was in this latter study that he encountered the Thyasiridae Dall, 1900 (1895) and the taxonomic difficulties posed by these, rather nondescript, small white bivalves. Following up on this study, in 1959, he spent time in the United States National Museum studying the type material in the J.G. Jeffreys and A.E. Verrill & K.J. Bush collections where he recognised type material and, in some instances, selected lectotypes and neotypes. From this study, he wrote a short paper on the status of three species of Thyasira Lamarck, 1818 described by Verrill & Bush (1898) (Ockelmann 1961). Despite his long interest in the Thyasiridae, this was the last and only paper he published on them. However, Ockelmann became known as the expert on Atlantic Thyasiridae, probably through his generosity and willingness to help anyone who came to him for advice, a trait I personally benefitted from in my study on the Thyasiridae of the UK North Sea oilfields (Oliver & Killeen 2002). In 1961, McIntyre acknowledged Ockelmann’s help in identifying Thyasira equalis (Verrill & Bush, 1898) from the North Sea and many such acknowledgments can be found, e.g., in Bowden & Heppell (1968), Lande (1975), Lubinsky (1976), Southward (1986), Dando & Southward (1986) and Payne & Allen (1991). A reference in Bowden & Heppell (1968: 263) is especially informative: “The arrangement of British Thyasiridae given here will be modified considerably by Ockelmann, 1968 (Ophelia, in press)”, as it suggests a paper had been completed and submitted, but as we know, it was never published. The following acknowledgement is typical: “Thanks are also due to Dr. K. Ockelmann at Helsingør for the many ideas and the help he gave in identification during a short stay with him” (Blacknell 1973). C.M. Payne told me, when working on the deep-sea thyasirids (Payne & Allen 1991), she visited Ockelmann and recalled seeing some of the contents of his study. It is these papers that were archived by the Natural History Museum of Denmark (NHMD), under Archive number T 926 and which form the core of this paper. Fig. 1. Portrait of Kurt Ockelmann at the conference to celebrate his 90th birthday at Helsingør Marine Biological Laboratory, 2014. OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 241 The hope was that I would be able to publish some, if not all, of Ockelmann’s monograph in one or more taxonomic papers, but this will not be possible simply due to the advances made in techniques and papers published over the intervening years. Rather, this will be a commentary on Ockelmann’s work, but also an opportunity to publish many of the fine graphics and illustrations that he had prepared. We may not know why this monograph was not published, but from the note in Bowden & Heppell (1968) and annotations on the illustrations, it appears that it was Ockelmann’s intention to publish in the journal “Ophelia”, which was merged into “Marine Biology Research” in 2005. The Thyasiridae Dall, 1900 (1895) Students of the thyasirids are, and have always been, few. From a conchological focus, most are nondescript, small, white bivalves with a preference for the colder and deeper parts of the oceans. In historical literature, they appear mainly in expedition reports under descriptions of new species; typical of these are the works by Jeffreys (1876, 1881) and Verrill & Bush (1898) or as faunistic works from northern latitudes, such as Sars (1878). Ockelmann’s attention to this family was drawn by the very large numbers he encountered in East Greenland (Ockelmann 1958). Ockelmann remained the only serious student until the material collected from the deep Atlantic, by American and French cruises, was studied by John Allen and Cathy Payne (Payne & Allen 1991). Allen (2008) listed 27 named species and 52 unnamed thyasirid species from the deep Atlantic (500 to 5000 m), an indication of their diversity and importance in deep-sea benthic communities. Despite detailed anatomical observations made by Ockelmann and Payne & Allen, they did not recognise that many thyasirids are chemosymbiotic. This observation was first made by Eve Southward (1986), and kindled a new interest in thyasirids, with the work of Dufour (2005) seminal here. While most chemosymbiotic bivalve families are strictly so, Dufour (2005) showed that there is a range of dependency on chemosymbiois in the thyasirids, and this is reflected in their gill anatomy. In the last decades, most publications were taxonomic, with only a single paper attempting to look at phylogeny through DNA sequencing (Taylor et al. 2007). This study revealed that the current generic systematics is probably inadequate, a view supported by Huber (2015). Further research is hampered by the difficulty in acquiring suitably preserved material for DNA extraction, especially of the smaller taxa from deep water. An intriguing observation among the smaller taxa are the large geographic and bathymetric ranges cited. Some are considered cosmopolitan, others pan-Atlantic with ranges from shelf to abyssal depths. It was these that Ockelmann studied most, and along with Payne & Allen, being responsible for confirming their distributions. However, the benthic fauna of the continental margins is strongly zoned (Carney 2005) and in the North Atlantic, it is unusual to find the same species inhabiting the shelf on both American and European margins. A detailed study using DNA sequencing is needed to resolve the bathymetric and geographic ranges exhibited by the Thyasiridae. A final conundrum thrown up by thyasirids is the relationship between arctic and boreal populations of species such as T. gouldii (Philippi, 1845a) and T. dunbari Lubinsky, 1976. Much of the accepted position on North Atlantic thyasirids came from Ockelmann’s unpublished monograph via personal communications, and this paper is an opportunity to examine his work in detail. After some nomenclatural explanations and a description of the contents of NHMD archive number T926, the results are presented in five sections. The first provides details on four unpublished species that Ockelmann had identified, followed by a section on four Amphi-Atlantic thyasirids studied by him in detail. A section on larval shells is followed by an account of his studies on living thyasirids, in particular their anatomy, life habits and the ciliary currents they produce. Finally, a number of miscellaneous shell drawings are presented. European Journal of Taxonomy 1007: 239–278 (2025) 242 Material and methods Reference will be made throughout this paper to manuscript species names applied by Ockelmann, but which were never published. There is no intention here of introducing these names into nomenclature and their inclusion here should not be interpreted as a nomenclatural act. Ockelmann’s use of generic names, as seen in the archive, was rather inconsistent and he sometimes referred all species to “Thyasira sensu lato” while also using other genera such as Mendicula Iredale, 1924, Axinulus Verrill & Bush, 1898 and Leptaxinus Verrill & Bush, 1898 as genera or subgenera. When referring to Ockelmann’s annotations, I have not altered them to the current accepted nomenclature in MolluscaBase (2025) or added generic names to his labels. The archive consists of graphic material only; there are no texts, but most of the illustrations carry pencil identifications of species but are not allocated to a particular genus. Frequently, the pencil notations are very faint, so for the purposes of publication these have been enhanced to make them legible. Many of the items are on card, attached to larger pieces of card, such that on copying there is often a shaded border. Also, the colour of the card is no longer pure white, and some discolouration has taken place; in such instances, no attempt has been made to retouch the scans so some shadows and shading may be present. The archive is listed in the Natural History Museum of Denmark under accession T 926. An overview of the contents of this archive is provided here and presented in detail in the Results section. Pen and ink diagrams of anatomy Seven species were illustrated, labelled as: croulinensis, dunbari, gouldi, minutus, orbiculata, “perplexa”, pygmaea. Stipple illustrations of the shells, including those by K. Olsen and P. Winther Some of the shell illustrations can be directly linked to specimens in the collection and these are annotated with “Tegnet [= drawn by] K. Olsen” or “Tegnet P. Winther” and dated 1971 by Ockelmann. Those done by P. Winther must have been executed earlier as he died in 1966. The following species are represented as the entire shell and /or their hinges. The majority of these illustrations are probably by Ockelmann himself as he was known to be a good scientific illustrator (Jensen & Dinesen in prep.). Stipple drawings of twenty-two species labelled as follows are present: brevis, croulinensis, dunbari = “frigida”, equalis, eumyaria, ferruginea, flexuosa, gouldi, grandis, granulosa, incrassata, “ingolfi”, obsoleta, orbiculata, “perplexa”, perplicatus, pygmaea, sarsi, “scandica”, subovata, succisa, tortuosa. Distribution maps The distribution maps are all based on a map of the Atlantic Ocean or restricted to the North Atlantic and Arctic Oceans from 40° to 85° N. Occurrences relate to data associated with collections and are represented by single symbols. Open and closed symbols are present but not explained; I assume that the open symbols represent dead collected shells or literature records. Some maps pertain to a single species whereas others combine data for up to three species. The following species have distribution maps but only those in bold font are reproduced here: brevis, croulinensis, dunbari = “frigida”, equalis, eumyaria, ferruginea, flexuosa, gouldi, incrassata, “ingolfi”, obsoleta, orbiculata, pygmaea, sarsi, “scandica”, subovata, succisa, tortuosa. Some maps include more than one species and in some cases, these have been simplified by me. Line drawings of the prodissoconchs Two plates of line drawings illustrate the larval shells (prodissoconchs) of the following species and labelled by Ockelmann as: Plate 1: Axinopsida orbiculata, T. gouldi, flexuosa, sarsi, croulinensis, succisa, OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 243 granulosa, “frigida” = dunbari, equalis, subovata; Plate 2: tortuosa, “scandica”, minutis, incrassata, “ingolfi”, pygmaea, ferruginea, ?eumyaria, brevis. Data presented in graphical form Bar-charts indicating the bathymetric ranges of 10 species, gouldi, obsoleta, sarsi, “frigida”, flexuosa, orbiculata, equalis, eumyaria, pygmaea, ferruginea. Line charts of the relation between height and length of the prodissoconch of 18 species not annotated. Scatter diagram of the shell length against depth of 18 species. Life observations Pen and ink drawing of T. flexuosa (Montagu, 1803) in life position. Diagram of the ciliary currents of the mantle. Diagrammatic transverse section showing mantle currents. Methods The supplementary illustrations of photographed shells and scanning electron micrographs (SEMs) were made by the author. The photographs were made using a Leica Z6 macroscope producing digital images enhanced by Helicon Focus stacking software. The SEMs were made on a Jeol JCM-7000 Benchtop in the National Museum of Wales. Specimens from the Ockelmann collection of his unpublished species were not gold coated and were viewed at the low vacuum setting. Institutional acronyms MCZ = Museum of Comparative Zoology, Harvard NHMD = Natural History Museum, Denmark, Copenhagen NHMUK = Natural History Museum, London USNM = United States National Museum, Washington, USA Results Unpublished manuscript names by Ockelmann Among the archive and in his collection are four undescribed species annotated with the following manuscript names: T. “frigida”; T. “perplexa”; T. (Leptaxinus) “scandica”; T. (Mendicula) “ingolfi”. Thyasira “frigida” Ockelmann MS Figs 2–4 In Ockelmann’s monograph (1958) on the bivalves of East Greenland, he illustrated a shell from Jan Mayen as Thyasira equalis (Verrill & Bush, 1898) (Ockelmann 1958: fig. 7). He noted that this species was widely distributed in East Greenland and was in general a little larger than T. equalis from NE America. In McIntyre (1961), it became evident that Ockelmann changed his mind and that the T. equalis illustrated by him was probably undescribed. In his collections, Ockelmann labelled this form as “Thyasira “frigida” n. sp.” and selected a prospective holotype (NHMD-1175760) from “Ingolf” stn 126; 67°19′ N, 15°52′ W, at a depth of 535 m, N of Iceland. The Danish Ingolf Expedition of 1895/96 was reviewed by Wolff (2008). Ockelmann’s map of distribution for T. “frigida” gives a widespread range across the cold regions of the North Atlantic into the Arctic Ocean at 80° N. Then, around 1975/76 he corresponded with Irene Lubinsky who subsequently described T. dunbari Lubinsky, 1976 from Arctic European Journal of Taxonomy 1007: 239–278 (2025) 244 Canada. Ockelmann was gifted a specimen of T. dunbari (NHMD-1175909) and from then on dropped his name T. “frigida” in favour of T. dunbari. I find this surprising as the shapes of the shell are quite different, T. “frigida” being polygonal rather than the pyriform shell of T. dunbari. Furthermore, the prodissoconch sizes, which Ockelmann gave emphasis to throughout his studies, are quite different with a mean of 165 µm in T. “frigida”, but 193 µm in T. dunbari. Perhaps he considered T. dunbari to represent a larger, gerontic form of his T. “frigida”, but there are no shells resembling T. dunbari in any of the collections in the Natural History Museum of Denmark from East Greenland, northern Norway or Svalbard. Here, I present his illustrations (Fig. 2) and map (Fig. 4) from the archive along with photographs of the type specimen of T. “frigida” from Ockelmann’s collection (Fig. 3B) and a paratype of T. dunbari (Fig. 3A). From this simple comparison, I would not consider T. dunbari and T. “frigida” to be conspecific. However, samples of T. “frigida” do vary and are close to T. rotunda (Jeffreys, 1881), a taxon not considered by Ockelmann in his monograph. A full consideration of T. dunbari, T. “frigida” and T. rotunda will be presented in a forthcoming paper on the Thyasiridae of the UK Atlantic margin (Oliver in prep). Fig. 2. Stipple drawings from the Ockelmann archive of Thyasira “frigida” Ockelmann MS/ dunbari Lubinsky, 1976. A. Exterior of shell. B. Hinge of right valve. C. Diagram of the anatomy. D. Prodissoconch. Abbreviations: DD = digestive diverticula; SP = separate portion of anterior adductor. OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 245 Fig. 3. A. Paratype of Thyasira dunbari Lubinsky, 1976 (NHMD-1175909). B. Holotype of T. “frigida” Ockelmann MS (NHMD-1175760) selected by Ockelmann. Images by P.G. Oliver. Fig. 4. Ockelmann’s map of the distribution of his T. “frigida” MS. European Journal of Taxonomy 1007: 239–278 (2025) 246 Thyasira “perplexa” Ockelmann MS Figs 5–6 In the archive, this species is represented by stipple drawings of two shells, hinge and anatomy (Fig. 5). Drawn shells (2 valves) + 6 valves + 2 shells from Ria de Arosa, N Spain stn 1566 (all NHMD1175772), are present. There is no correspondence relating to these shells and further information on their provenance could not be found. The Ria de Arosa is situated on the north-west coast of Spain, approximately 40 km N of Vigo. Its deepest point is 60 m and T. flexuosa has been recorded from here (Cadee 1968). Fig. 5. Stipple drawings from the Ockelmann archive of Thyasira “perplexa” Ockelmann MS. A–B. Exterior of shell. C. Hinge of both valves. D. Diagram of the anatomy. Abbreviations: aa = anterior adductor muscle; apr = anterior pedal retractor muscle; ct = ctendium; ex a = exhalant aperture; f = foot; lbp = lateral body pouch; lp = labial palp; me = mantle edge; pa = posterior adductor muscle; ppr = posterior pedal retractor muscle; r = rectum. OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 247 At first, this did appear to be an undescribed species, but scanning electron microscopy of the prodissoconch revealed two morphologies, one with what I call a “menorah” sculpture of ridges (Fig. 6G) and the other almost smooth (Fig. 6F). The larger shell (Fig. 6A) with the “menorah” prodissoconch sculpture is not too dissimilar from T. equalis, but the serrated edge to the submarginal sulcus is unusual and may indicate that a distinct species is present in northern Spain. All the smaller shells, with a smooth prodissoconch, are similar to Axinulus alleni (Carrozza, 1981) (Fig. 6D–E) and the anatomy matches Fig. 6. A–C. Shells drawn by Ockelmann of his Thyasira “perplexa” MS (NHMD-1175772). D–E. Specimens of Axinulus alleni (Carrozza, 1981), Cañón de Motril, Mediterranean Sea off Andalusia Spain, 244–284 m depth, University of Malaga, AN258. F–G. Scanning electron micrographs of the prodissoconchs of shells labelled T. “perplexa” MS by Ockelmann. European Journal of Taxonomy 1007: 239–278 (2025) 254 never formally published these designations. Neither did he ever publish descriptions or illustrations comparing American and European examples. Comparisons with type material are essential to verify synonymy and identifications of amphi-Atlantic species. Many type selections have been validly made, but some have now crept into the literature following their incorporation through the curation of the collections. Ockelmann selected type material from the Jeffreys collection and some have been validly published by Warén (1980), as follows under their original genus: • Clausina croulinensis Jeffreys, 1847; neotype USNM 62048. • Axinus incrassatus Jeffreys, 1876; lectotype USNM 61969. • Axinus planatus Jeffreys, 1882: not found. • Axinus polygona Jeffreys, 1864; not found. • Axinus rotunda Jeffreys, 1881; not found, subsequently noted in collection under USNM 61942/a but see under T. granulosa below. • Axinus subovatus Jeffreys, 1881; syntype USNM 61895, subsequently noted as lectotype under USNM 61895. • Axinus succisa Jeffreys, 1876; lectotype USNM 61973. • Axinus tortuosus Jeffreys, 1881; lectotype USNM 61904 and paralectotype USNM 683898, but see note by Ellen Strong in USNM catalogue on line: EZID: http://n2t.net/ark:/65665/3ce7ffe52-e280-48e9-a33c-2423b208bff0. Although not cited in the USNM on-line catalogue, Ockelmann was responsible for selecting lectotypes from the Verrill & Bush collection, as indicated on annotations in the collection, but unlike Warén, he never published them: • Cryptodon obsoletus Verrill & Bush, 1898; lectotype USNM 159886, but this was never published and should be cited as a figured syntype. • Cryptodon (Axinulus) pygmaea Verrill & Bush, 1898; lectotype USNM 78368, but this was never published and should be cited as a figured syntype. • Cryptodon equalis Verrill & Bush, 1898; holotype USNM 74302, but this was not selected by Verrill & Bush and should be a syntype or figured syntype. A fourth amphi-Atlantic species to be confirmed by Ockelmann was Thyasira gouldii, originally described as Lucina gouldii Philippi, 1845 (Philippi 1845a). No type specimens have been isolated and there is no record of them in the MCZ in Harvard (Coan & Kabat 2017). Ockelmann did not visit the MCZ and we assume his concept of this species was based on material in the USNM. Thyasira equalis (Verrill & Bush, 1898) Fig. 13 Following his investigations in the USNM, Ockelmann began to apply some of Verrill & Bush’s names to European specimens. The first of these was to identify a species from the Fladen Ground in the North Sea as T. equalis following communications with Alistair McIntyre (McIntyre 1961). However, T. equalis was not included in the British fauna by Tebble (1966) but was listed by Bowden & Heppell (1968). It appears in the Norwegian literature in 1975 (Lande 1975), but the first figures of European specimens were published by Payne & Allen (1991), and illustrations of specimens from the North Sea are in Oliver & Killeen (2002). Comparisons of the shells from NE America, the North Sea and northern Norway show that they share the same characters and in particular the micro-sculpture of the prodissoconch. This has the characteristic “menorah” pattern (Fig. 22A) which Ockelmann did not see. Ockelmann’s map (Fig. 13) shows a discontinuous distribution with a western Atlantic range from New England northwards into the Labrador Sea but absent from the entire east of Greenland. In the east, it ranges from the North Sea into the Kattegat and all along the Norwegian coast into the Barents Sea. OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 255 Thyasira obsoleta (Verrill & Bush, 1898) Fig. 14 For a great many years this species (Fig. 14) was confused with Axinulus croulinensis (Jeffreys, 1847) and was widely recorded as such from the eastern Atlantic from the Bay of Biscay to northern Norway (see Payne & Allen 1991 for details). It was not until Ockelmann formally recognised this species in Fig. 13. Thyasira equalis (Verrill & Bush, 1898). A. Ockelmann’s map of the distribution, arrow indicates locality of figured shell, Skagerrak. B–C. Stipple drawings of the shell and hinge. Fig. 14. Thyasira obsoleta (Verrill & Bush, 1898). A. Ockelmann’s map of the distribution. B. Stipple drawing of the shell. European Journal of Taxonomy 1007: 239–278 (2025) 256 the British fauna (Ockelmann pers. comm. in Bowden & Heppell 1968: 263) that the name became established in Europe. Lande (1975) recorded this species from Norway and credits Ockelmann with its identification. Thyasira gouldii (Philippi, 1845) Fig. 15 This species was originally described from off north-eastern USA and was known from high latitudes in the eastern Atlantic as early as 1878 (Sars 1878) (Fig. 15). Records from farther south in Norway appear in Lande (1975) and from Scotland (Blacknell 1973), both noting Ockelmann’s confirmation of identifications. Mendicula pygmaea (Verrill & Bush, 1898) Fig. 16 Mendicula pygmaea was described from off the coast of New England and appears in the European literature in Lande (1975), and Høisæter (1986) recorded it from the coast of Norway between 60° and 63° N. Ockelmann’s unpublished map (Fig. 16) shows a wide range but sparse distribution across the N Atlantic Ocean. It was listed from the Skagerrak by Wikander (1989) and later was cited from the North Sea oilfields by Oliver & Killeen (2002). Unfortunately, both Ockelmann and Oliver & Killeen had overlooked the fact that the material from the North Sea was not M. pygmaea but was a neglected species of Adontorhina published subsequently by Barry & McCormack (2007) as A. similis. It is most unlike Ockelmann to have overlooked A. similis, but without recourse to the SEM, he probably could not discern the denticulate dorsal margin typical of Adontorhina. Oliver & Killeen (2002) did figure the denticulate margin, but failed to notice the significance taking the M. pygmaea identification for granted. Fig. 15. Thyasira gouldii (Philippi, 1845). A. Ockelmann’s map of the distribution. B–C. Stipple drawings of the shell and hinge. OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 257 Larval shells (prodissoconchs) Ockelmann was mentored by Gunnar Thorson who postulated Thorson’s rule concerning the relationship of egg size and larval development to latitude. Egg size and larval development can be inferred in bivalves from their prodissoconch size and this was an area of interest for Ockelmann throughout his research. In his thyasirid archive, we find two very detailed plates of the drawings and sizes of prodissoconchs of 20 species of Thyasiridae (Figs 17–18). In addition, there are scatter-charts of the length to height ratios of prodissoconchs (Fig. 19), shell size plotted against depth (Fig. 20) and bar charts of bathymetric ranges (Fig. 21). All of these can be related to prodissoconch size and ultimately to Thorson’s rule. Ockelmann would also have been familiar with Rass’s rule which relates to the variation in larval development within a species affected by latitude or temperature (Laptikhovsky 2006). Unfortunately, we only have the drawings and charts in the archive without any written commentary, so we cannot discern how Ockelmann was going to interpret his results. The effort he made in compiling this data was impressive for the charts in Fig. 21 were compiled from 21 252 individual observations on 10 species. This data was probably compiled before 1976, as it includes T. “frigida” rather than T. dunbari. By contrast, the legend on his prodissoconch plate cites “T. “frigida” n. sp. = T. dunbari” so at least the legend was added after 1976. A selection of these images was sent to the author and published in Oliver & Killeen (2002). At the time we (Oliver & Killeen) were puzzled by the faint radial lines indicated on Ockelmann’s drawing of the prodissoconch of T. equalis (Fig. 17: no. 9) for we were unable to see any. In a forthcoming paper on NE Atlantic thyasirids, using scanning electron microscopy, I compare the prodissoconchs of a number of species and will demonstrate that T. equalis has a very distinct arrangement of raised ridges on the apex of the larval shell that I will describe as the “menorah” pattern, as they resemble to arms on the Jewish candlestick of that name. Furthermore, far from the smooth appearance inferred by Ockelmann’s drawings, the prodissoconch displays a variety of distinctive microsculptures that can be used to differentiate species (Fig. 22). While Ockelmann’s drawings adequately indicate the sizes of the prodissoconchs, the details are missing, and this would have made it difficult to publish after the establishment of scanning electron microscopy. Fig. 16. Mendicula pygmaea (Verrill & Bush, 1898). A. Ockelmann’s map of the distribution, arrow indicates locality of figured shell, Ingolf stn 218. B–C. Stipple drawings of the shell and hinge. European Journal of Taxonomy 1007: 239–278 (2025) 258 Fig. 17. Fig. 17. Stipple drawings of 10 species of Thyasiridae from the Ockelmann archive, numbering is original and identifications as per annotations. 1. Axinopsida orbiculata (G.O. Sars, 1878). 2. Thyasira gouldi (Philippi, 1845). 3. T. flexuosa (Montagu, 1803). 4. T. sarsi (Philippi, 1845). 5. Axinulus croulinensis? (Jeffreys, 1847). 6. T. succisa (Jeffreys, 1876). 7. Parathyasira granulosa (Monterosato, 1874). 8. T. “frigida n. sp.” = T. dunbari Lubinsky, 1976. 9. T. equalis (Verrill & Bush, 1898). 10. T. subovata (Jeffreys, 1881). OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 259 Fig. 18. Stipple drawings of 10 species of Thyasiridae from the Ockelmann archive, numbering is original and identifications as per annotations. 1. Thyasira tortuosa (Jeffreys, 1881). 2. T. “scandica” Ockelmann MS. 3. Leptaxinus. minutus Verrill & Bush, 1898. 4. Leptaxinus. incrassatus (Jeffreys, 1876). 5. T. “ingolfi” Ockelmann MS. 6. Mendicula pygmaea (Verrill & Bush, 1898). 7. Mendicula ferruginosa (Forbes, 1844). 8. ? 9. Genaxinus eumyarius (M. Sars, 1870). 10. Axinulus brevis (Verrill & Bush, 1898). European Journal of Taxonomy 1007: 239–278 (2025) 260 Fig. 19. Scatter diagrams from the Ockelmann archive of the height and largest diameters of the prodissoconchs of eighteen species of Thyasiridae. The identities of the species are not given. OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 261 Fig. 20. Chart from the Ockelmann archive of the sizes of species, as shell length plotted against bathymetric distribution of nineteen species of Thyasiridae. Numbering is original and identifications as per annotations. 1. Axonopsida orbiculate (G.O. Sars, 1878). 2. Thyasira flexuosa (Montagu, 1803). 3. T. gouldi (Philippi, 1845). 4. T. sarsi (Philippi, 1845). 5. T. succisa (Jeffreys, 1876). 6. T. obsoleta (Verrill & Bush, 1898). 7. T. “frigida” Ockelmann MS. 8. T. equalis (Verrill & Bush, 1898). 9. T. granulosa (Monterosato, 1874). 10. T. subovata (Jeffreys, 1881). 11. T. tortuosa (Jeffreys, 1881). 12. T. croulinensis (Jeffreys, 1847). 13. T. eumyaria (M. Sars, 1870). 14. T. brevis (Verrill & Bush, 1898). 15. T. pygmaea (Verrill & Bush, 1898). 16. T. ferruginea. 17. T. “ingolfi” Ockelmann MS. 18. T. “scandica” Ockelmann MS. 19. T. incrassata (Jeffreys, 1876). European Journal of Taxonomy 1007: 239–278 (2025) 262 Fig. 21. Bar charts from the Ockelmann archive of the bathymetric distribution of 10 species of Thyasiridae. The depth is given on the x-axis and the percentage of individual recorded on the y-axis. The number on the bars are the actual numbers used. OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 263 The living animal Ockelmann was what one terms as a “whole animal zoologist” and making observations on living animals in the laboratory was, I believe, a passion of his and is evidenced in other papers of his (e.g., Ockelmann 1964a, 1965; Ockelmann & Muus 1978). He made observations on the burrowing behaviour of at least 3 species of Thyasiridae, T. flexuosa, T. equalis and what he called M. pygmaea. The latter two were modified from rough sketches and reproduced in Oliver & Killeen (2002). The fully worked up reconstruction for T. flexuosa is reproduced below (Fig. 23) but not in its original size that was portrait A3. This diagram shows the bivalve living at depth in the sediment with an anterior inhalant tube and the exhalant flow posteriorly into the surrounding sediment. Once buried in this position, the foot probes further into the sediment and makes a network of such tubes (Dando & Southward 1986). I do not know if Ockelmann had an explanation for such behaviour, but he probably thought that pedal feeding was involved where particles of detrital food were brought into the mantle by the foot. It was not until Southward (1986) and Dando & Southward (1986) discovered that the foot was probing into the anoxic zone to supply its commensal chemoautotrophic bacteria with sulphides that the behaviour Fig. 22. Scanning electron micrographs of four species of Thyasiridae. A. Thyasira equalis (Verrill & Bush, 1898) showing “menorah” pattern. B. T. obsoleta (Verrill & Bush, 1898). C. T. “frigida” Ockelmann MS. D. Mendicula ferruginea (Forbes, 1844). All by P.G. Oliver. European Journal of Taxonomy 1007: 239–278 (2025) 270 Fig. 27. Stipple drawings by Kurt Ockelmann of three species of Thyasiridae. A. Shell labelled “granulosa” is Parathyasira granulosa (Monterosato, 1874). B. Detail of the microsculpture. C. Hinge of P. granulosa. D–E. Shell and hinge of “succisa” is Thyasira succisa (Jeffreys, 1876). F–G. Shell and hinge of “orbiculata” is Axinopsida orbiculata (G.O. Sars, 1878). OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 271 Fig. 28. Stipple drawings by Kurt Ockelmann of four species of Thyasiridae. A–B. Shell and hinge of “brevis” is Axinulus brevis (Verrill & Bush, 1898). C–D. Shell and hinge of “croulinensis, Faeroes” is Axinulus croulinensis (Jeffreys, 1847). E. Shell of “eumyaria Skagerrak” is Genaxinus eumyarius (M. Sars, 1870). F. “Cleaned shell of Thyasira ferruginea” is Mendicula ferruginea (Forbes, 1844). European Journal of Taxonomy 1007: 239–278 (2025) 272 Fig. 29. Stipple drawings by Kurt Ockelmann of two species of Thyasiridae. A. Shell labelled “flexuosa Faroes” is Thyasira flexuosa (Montagu, 1803). B. Internal, external and dorsal views of the shell of T. flexuosa. C. Stipple drawing of the left and right hinges of T. flexuosa, the annotation indicates that the shell was from the Faroes at 62°11.2′ N, 6°58.5′ W and was from “Dana” stn 3322. D-E. T. sarsi (Philippi, 1845) shell and hinge, no locality indicated. OLIVER P.G., Ockelmann’s unpublished Thyasiridae studies 273 Discussion It is undoubtedly true that Ockelmann made many new observations on the Thyasiridae based on many detailed observations on both living and preserved specimens. It is also true that he had a great influence on others interested in the Thyasiridae. But what of his legacy and what can we derive from his surviving monograph? I fear that, by not publishing or actively collaborating, his research will be overlooked. Of the undescribed species, three, T. “perplexa”, T. “scandica” and T. “ingolfi”, are represented by too few specimens to sustain description today. The fourth, his T. “frigida”, should have been published as he had considerably more material than Lubinsky (1976) and from a far greater geographical and bathymetric range. He was the first to recognise amphi-Atlantic species inhabiting the continental margins of both northeast American and European waters but without published papers, he has no visible credit for this. With DNA sequencing, it will be possible to investigate the phylogenetic relationships of these amphi-Atlantic populations and test the validity of their distributions. He was among the first to observe the unusual burrowing behaviour of thyasirids and if he had collaborated with Southward and Dando, he would have been at the forefront of the investigations into the ecology of chemosymbiotic species. If he had continued his observations on the settling of thyasirid larvae he may have helped to discover when, and from where, they acquired their symbiotic bacteria. He was the first to document the larval shells, prodissoconchs, of all known N Atlantic species but his work has become largely irrelevant through the advent of scanning electron microscopy. I am sure he could have had access to scanning electron microscopy but he did not use it, rather choosing to stay with familiar techniques. Fig. 30. A. Chanellaxinus perplicata (Salas, 1996), stipple drawing by Kurt Ockelmann lacking any legend. B. Stipple drawing by Kurt Ockelmann of the hinge of Axinus grandis (Verrill & Smith, 1885), labelled holotype. European Journal of Taxonomy 1007: 239–278 (2025) 274 He was meticulous as evidenced in the thousands of shells he processed to compile bathymetric ranges for N Atlantic species and map their distribution. Yet, he published but one short paper specifically on thyasirids after his major work on the bivalves of East Greenland and because of this, his name does not feature as much as it should in the literature on the thyasirids. Despite this, he had a wide and lasting influence through his generosity of sharing his knowledge with others and being satisfied with mentions in acknowledgments of many papers. His beautiful stipple illustrations are far superior to any others published and his anatomical diagrams are striking for their clarity. Unfortunately, time passes, and new techniques and thinking have made it impossible to go back and publish his work as prepared by him. Others have now published on many aspects of his study and in some instances, new techniques have revised his work radically. In this retrospective, I hope by reproducing many of Ockelmann’s drawings, one can see the excellence of his work and powers of observation. I cannot explain why Ockelmann never published his monograph or any part of it. I did not know him well enough to know if it was down to a streak of perfectionism or perhaps the making of observations was enough for him and the laborious process of preparing papers was just too much. Tom Schiøtte recalls: “I believe this is what Jørgen Knudsen, Godtfred Høpner Petersen and others who asked Kurt about it were convinced was the reason. He said something like “These things lie very well in my drawers”. I once asked him if he wouldn’t like to be remembered for having solved the riddle of Thyasira, and he actually scolded me for thinking that such concerns should be on the mind of a true scientist”. Perhaps Kurt would not approve of publishing this retrospective, but I hope that it will be cited and give due acknowledgement to his enormous contributions to the study of these small, non-descript, but fascinating bivalves. Acknowledgements The study was supported by the grant from the European Commission’s programme ‘Transnational Access to Major Research Infrastructures’ to SYNTHESYS+ (grant no. DK-TAF-2719), which enabled my visit to the Natural History Museum of Denmark. My thanks are especially due to Tom Schiøtte and Martin V. Sørensen of the NHMD who facilitated access to the archive and to the collections. Thanks to Kathe Jensen (University of Copenhagen), Tomas Cedhagen (Aarhus University), Paul Dando (Marine Biological Association, Plymouth) and Cathy Cauldwell née Payne (University of Dundee) for their recollections of meeting Kurt and discussing thyasirids. I am very grateful to Yolanda Villacampa, Amanda Robinson, John Pfeiffer and Ellen Strong of the Department of Invertebrate Zoology, Smithsonian Institution, United States National Museum for access to the collections, loans and supply of SEM images of type material and for a pleasant and fruitful visit to Washington in 2023. 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Malacologia 51 (2): 271–290. https://doi.org/10.4002/040.051.0204 Printed versions of all papers are deposited in the libraries of four of the institutes that are members of the EJT consortium: Muséum national dʼHistoire naturelle, Paris, France; Meise Botanic Garden, Belgium; Royal Museum for Central Africa, Tervuren, Belgium; Royal Belgian Institute of Natural Sciences, Brussels, Belgium. The other members of the consortium are: Natural History Museum of Denmark, Copenhagen, Denmark; Naturalis Biodiversity Center, Leiden, the Netherlands; Museo Nacional de Ciencias Naturales-CSIC, Madrid, Spain; Leibniz Institute for the Analysis of Biodiversity Change, Bonn – Hamburg, Germany; National Museum of the Czech Republic, Prague, Czech Republic; The Steinhardt Museum of Natural History, Tel Aviv, Israël.