Unio elongatulus


The Naiad Lot of the Month 

In this place particular lots from the Unionida collection (freshwater mussels, naiads) of the Senckenberg Museum are featured. They come out of the ongoing catalogue listing and revision of that part of the malacological collection. 
Strange, remarkable, curious, sometimes even tragical: these are the circumstances that cling to these particular lots and their history. Each one represents part of the staggering richness of this collection and testifies to the enthusiasm and seriousness of many scientists, both formally trained or expert amateurs.

That a rich collection (in this case of freshwater bivalves) is also a challenge and a mission that may reduce one to despair, was clearly expressed by Charles Torrey Simpson. He tried to put order to the European Anodonta “species” of his time, but he ended up saying:
The number of specific names bestowed on the Anodontas of Europe must run up into the thousands. I have gone over the literature as carefully as possible, and large series of specimens from many localities, and I confess that I am absolutely unable to separate these forms specifically. The variations of form, size, color, solidity, and even texture are sufficient for a large number of species if they were not everywhere connected by intermediate examples. The new school of conchologists has named every conceivable variation, and in very many cases distorted individuals. Europe is a densely settled region, and it is possible that the waters of the streams and ponds in many places may be so affected by sewage and other offal as to produce many changes in the mollusks living in them. (Simpson, 1900: 621, footnote)

Simpson, C. T. (1900): Synopsis of the Naiades, or Pearly Fresh-Water Mussels. – Proceedings of the United States National Museum, 22: i-viii, 501-1044, Washington. (with Plate XVIII).



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And here, poor fool, I stand once more,
no wiser than I was before.
It is not known if he thought of South American freshwater mussels of the genus Diplodon. But they exemplify the many unanswered questions concerning the definition and recognition of species, their genetic relationship and phylogeny. This month’s naiad is just one example of the shell shapes (species? subspecies?) assigned to this genus that often resemble each other closely. Although modern molecular methods have shed light on many of these problems, the importance of the classical museum collections has not declined. The rediscovery of the shells that served to describe a Diplodon species 186 years ago – being unrecognized or simply forgotten about in the meantime – exemplifies once more the importance and mission of a scientifically curated collection. 
Miyahira, I. C., Mansur, M. C. D. & Santos, S. B. 2013. Revision of the type specimens of Diplodon ellipticus and Diplodon expansus (Bivalvia, Unionoida, Hyriidae). Spixiana 36 (2): 173-182.
Alike and still very different … The spines on the shell valve of the (marine) European thorny oyster (Spondylus gaederopus, left) and the “spines” (better: tubules) of a river oyster from Africa (Etheria elliptica, right) both convey the impression to be careful when messing around with these animals. Living in different environments they nevertheless develop similar shapes. But as spines can fulfil a variety of functions the two mussel species may just coincidentally look alike. Especially the biology of the river oysters is far from being understood so that the function of their “spines” is still a matter of speculation, particularly because completely smooth specimens exist as well.
Being in time with the start of the summer holiday season it may be useful to remind the reader that also sea urchins and cactuses like to “decorate” themselves in this way. It is certainly healthier to avoid too close a contact with their spines, too.
There is so much to discover in the fantastic world of freshwater mussels, and there are still a lot of new and unexpected findings coming to light. Sometimes one even might lose their mind while asking: How is this possible? Like in this Month of June edition where the specimen is said to be „partly gravid“ = slightly pregnant?? Well, the solution is quite simple (brooding gills only partly filled with eggs or larvae) and this is not as uncommon a phenomenon among freshwater mussels. This month’s species, Gonidea angulata (Lea, 1838), is remarkable also for other features. Although living in the west of North America, it also seems to be closely related to some Asian species.
More odd and surprising facts from the freshwater mussels‘ reproductive behaviour can be seen in the Unio Gallery of M. Christopher Barnhart at: http://unionid.missouristate.edu/.
With only a few exceptions (see previous month’s NLoM) most river mussel species look very like the same (admittedly at a first glance only). Such convergent shapes indicate similar living conditions that the animals have to deal with. As they are typically inhabiting rivers and permanent lakes the main factors to determine their distribution and appearance are flow conditions, composition of the river or lake bottom, chemical properties of the water and the amount and quality of food. The Naiad Lot of this Month gives an example of transatlantic similarities. While Potomida littoralis (up) can (often: could) be found in Western Europe and in the Mediterranean area, Elliptio crassidens is (was?) widespread in the Interior Basin and the eastern Gulf Coastal Region of North America. Despite their similarities both species are not closely related and the correct classification of Potomida littoralis still under debate. To answer this question (and many others) the collection of the Senckenberg Research Institute keeps material ready.
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While most species of river mussels do not catch the observer’s eye with remarkable colour or shape, there are some exceptions. One of them is Hyriopsis bialata whose commercial name „shark’s fin mussel” gives quite a good idea about its appearance. „Sailing boat mussel” (or „capsized sailing boat without mast”) would do likewise. But a really strange feature are the two “wings”, a large one above the back end and a small one at the front end of the shell.  Since the animals live on sandy bottom in running water (source: http://www.iucnredlist.org) they may serve to somehow stabilize the position of the mussel. However, the upper posterior part of the shell and the large wing obviously project out of the substratum, because they show a fine cover of silt. Any idea or observation to solve this problem will be appreciated.
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The various collections of the Senckenberg Research Institute also contain objects that were mounted for educational purposes and these were either on display at the museum or used for university teaching. The latter is the case for the Naiad Lot of March 2015. Almost 130 years ago at the Zoological Institute of the University of Heidelberg, a freshwater pearl mussel (Margaritifera margaritifera) was dissected in a way to show its heart (the dark coloured section of the tube running from top = front end to bottom = back end in the midline of the body). In addition, the cut shell valves show layers separated by dark lines (“winter rings”) that the animal deposits annually.
At the time when this specimen was prepared, the freshwater pearl mussel was a common sight in some of the streams in the Odenwald mountains near Heidelberg. This is now history. There, and in many other former pearl mussel rivers of the Central German Uplands, no traces of these animals are left.
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A short month, a late start, but a real big thing: the lectotype of Hyriopsis goliath Rolle, 1904, currently: Hyriopsis cumingii (Lea, 1852), an economically very important species from China and northern Vietnam. Its shell can be nearly as thick as a finger.
This time we shall start the new year with a real no 1. The specimen belonged to the last remaining population of thick-shelled river mussels (Unio crassus) in the entire catchment area of the Weser River. The population comprises a few thousand animals that live in the sub-catchment of the river Eder in Hessen. Between 1999 and 2005 host fishes were infected with glochidium larvae and released into a tributary. In 2011 young mussels were recorded there for the first time. In 2010 a programme started to establish another population in a different part of the Weser drainage area. To this end infected fishes are being released in a river in Lower Saxony. The vouchers that are now deposited in the collection of the Senckenberg Research Institute may become an important comparison material, because they document the shell characters of the animals that are being used to reintroduce the thick-shelled river mussel in the Weser system.
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Sometimes it is possible to assist (subsequently) at the birth of a new species. The labels attached to this particular lot, shows that this process needed its time. In the present case, it took 9 years to arrive at the conclusion that some peculiar shells from Thailand and Laos could not be placed within any known species, and that the differences in shell characters would justify its recognition as a new species, Physunio modelli Brandt 1974. Compared to the speed of species description in former times (e.g. the notorious „Nouvelle École“ of the late 19th c.) and to the potential future “turbo-taxonomy” thanks to molecular and digital techniques, this seems quite slow, but there is hope that our species will have an equally long life in science.  
The revision of the Senckenberg Collection of Unionida arrives at more exotic (which is of course a relative term) regions beyond Europe. But also there, to be precise: in Uzbekistan, we find a variation of an old theme: “Anodonta cygnea” as a collecting basin for different species that are hard to discriminate. In the (scarce!) material from the Zeravshan at Bukhara a species, Cristaria tenuis (Griffith & Pidgeon 1833), turned up that is widely distributed in Southeast Asia. As it obviously was not expected to live in Central Asia the shell was finally catalogued as a variety of the “excessively variable” „Anodonta cygnea“ (Haas 1969: 329, 348). Gerhard Falkner (1994: 144) already pointed to other taxa of pond mussels (Anodontinae) from the Zeravshan whose identity is still unclear. A part of the mystery now seems to be unraveled but the question remains how the species arrived there, if naturally or as a “cultural import”. Falkner, G. (1994): Systematik vorderorientalischer Najaden als Vorstudie zur Bearbeitung archäologischer Funde. — Forschungen und Berichte zur Vor- und Frühgeschichte in Baden-Württemberg, 53:135-162, Stuttgart.
Haas, F. (1969): Superfamilia Unionacea. In: Das Tierreich 88. De Gruyter, Berlin. i–x, 1–663.
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This month’s Naiad Lot of the Month is “by chance” a lot containing freshwater pearl mussels (Margaritifera margaritifera L. 1758) from England, UK. In 1927  Harry Howard Bloomer sent the specimens to Fritz Haas and asked him if the form that he had named siluriana corresponded to parvula of Haas, described in 1908 from shells of the Odenwald mountains in Germany. Haas denied it, and today siluriana as well as parvula are regarded as local modifications or ecophenotypes of M. margaritifera, a species from clean running waters that is (in many cases: was) widespread in the temperate zones on both sides of the Atlantic.
200 km east of the sampling area of these mussels the 7th Congress of the European Malacological Societies will take place on 7-11 September 2014 at St. Catharine’s College, Cambridge, UK (http://euromal2014.malacsoc.org.uk).
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To get you in the mood for the holiday season this month’s web page features the “Ship in the  Mussel”. It was designed with loving attention to detail by Hans Jürgen Schrader who certainly was inspired by the (prevailing) association of mussel and sea. 
The shell was found in a historic pond), the “Waschweiher” near the little town of Birstein in the Vogelsberg region (Hessen, Germany). The pond is home to a big population of the Swan mussel (Anodonta cygnea) that has been living there for at least the past 40 years. This is documented by voucher specimens in the Senckenberg Collection (for more details see Nagel 2012).
Nagel, K.-O. (2012): Die Najaden des Vogelsbergs. 1. Erforschung, Verbreitung und Bestandssituation. — Beiträge zur Naturkunde in Osthessen 48: 59-93, Fulda.
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European waters are inhabited by a relatively small number of freshwater mussel species. In this place that situation was already complained of (see NLoM November 2013). Two ways out seem realistic, but only the second one mentioned hereafter should have our full support. On the one hand there are (few) invasive species that conquer new river systems and whole continents, often as a consequence of human activities (fish stocking, linking of river systems by canals). The Chinese Pond Mussel (Sinanodonta woodiana) is the most well-known example and relevant for the European fauna (see NLoM May 2013). On the other hand recent studies on European freshwater mussels demonstrate higher species diversity than previously assumed. Some taxa with broad distribution range (e.g. Anodonta anatina, Unio pictorum, U. mancus and U. crassus) may actually contain additional biological species. A case in point is Unio tumidiformis (Reis & Araujo 2009) that was previously (Haas 1969) thought to belong to Unio crassus, a notoriously variable species. 
Haas, F.  (1969): Superfamilia Unionacea. – Das Tierreich, 88: i-x, 1-663.
Reis, J. & R. Araujo (2009): Redescription of Unio tumidiformis Castro, 1885 (Bivalvia, Unionidae), an endemism from the south-western Iberian Peninsula. – Journal of Natural History 43: 1929–1945.
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Sometimes one has to apply more radical methods to see and show to others how an object is composed of different parts. In our case Wilhelm Hohorst (who discovered among other things important steps in the life cycle of the sheep liver fluke) ground off the left valve of a Painter’s mussel (Unio pictorum) in two steps. In this way the dark outer organic layer (periostracum, left) nicely contrasts with two subsequent layers of calcium carbonate, i.e. the prismatic layer (center) and the nacre or mother-of pearl (right). The Hohorst collection is part of Senckenberg’s collection of Unionida.
This time the Naiad Lot of the Month, May 2014, does not come from the collection of the Senckenberg Research Institute but from the Museum für Naturkunde in Berlin. With over 7.500 lots the collection of freshwater mussels (Unionida) is second only to the Senckenberg collection. It contains important historical specimens and material essential for the regional faunistics of Central Europe. In the cabinets many discoveries await researchers, but also known problems, as the mindful reader of these pages will surely guess (solution provided upon request).
Museum für Naturkunde Berlin
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River mussels or naiads are notorious for making an ‘April fool’ of those who try to determine them to the correct species. This is due to the enormous variability of their shell shape and other external features such as the colour of the shells or the morphology of the hinge apparatus. Even species that otherwise are clearly distinguishable when growing to their “typical” form or by other means, e.g. molecular methods, are joking in this way. Here the Painter’s Mussel, Unio pictorum, and the Swollen River Mussel, Unio tumidus, are examples of this issue.
Figure A shows the Painter’s Mussel (above) and the Swollen River Mussel (below) that probably lived together in the lotic section of a large river (here the river Mosel). The intense wear of their shells is evidence for this. Under these conditions, river mussels tend to build compact shells with reinforced hinge teeth.
Figure B shows another “joke” for which the animals are only partly responsible. In the same lot, two different species (Unio tumidus top left, all the rest U. pictorum) from two different sampling stations were lumped together.
Unfortunately, these cases are not isolated ones and they show the need for revising a historic collection.
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The second half of February brought a welcome change to the work at the desk. And for several freshwater mussels the holiday trip to Nepal was the starting point for their career as members of the Senckenberg collection of Unionida. To search and find the animals required the same skills as in many other places. Michael Pfeiffer shows it in the shallow waters of lake Phewa. Also the mussels’ appearance in their habitat, in this case a small river in the Nepalese midlands, was quite familiar. But in many cases it is still unclear to which species the animals belong. This  is another exciting subject for collection-based research. 
The diversity of freshwater mussels in Nepal.
The collection of large freshwater mussel or naiads of the Senckenberg Research Institute and Natural History Museum is essentially shaped by the work of Fritz Haas who was its curator from 1910 to 1936. As a proof of his early focus on these animals the collection contains a Painter’s Mussel (Unio pictorum) that the 13 year old schoolboy had picked up (alive!) from the banks of the River Main near Frankfurt. In 1969, right at the end of an unwavering fascination for these animals Haas published his treatise “Superfamilia Unionacea“. This summary of nearly 700 pages is still the major reference work for everybody who studies naiads.
Sometimes notes or whole letters with detailed descriptions of sampling localities or explanations of sampling circumstances are lying in the boxes together with the shell material. In one case there is even the protocol of a (unintended) trial about growth and the formation of growth rings that Wilhelm Israel carried out over 100 years ago. On 1 December 1909 he put a young Painter’s Mussel (Unio pictorum) of 1 cm length into an aquarium. On 6 September 1910 he rediscovered the animal and counted 3 growth rings that the mussel had produced during the past 9 months (W. Israel: “There is no possibility of a mistake!”). One can check this with our Najad Lot of the Month January 2014. Since then Israel was in doubt about the validity of such rings as markers of the mussel’s age.
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The Painter’s Mussel (Unio pictorum) and the Swollen River Mussel (Unio tumidus) are still widespread throughout Europe and species recognition by means of shell characters is – often – easy. As is the case with the species pair Anodonta anatina/A. cygnea (cf. NLoM June 2013) a detailed look at the juvenile shell will help – sometimes – when in doubt. But when otherwise distinctive characters – here the umbonal sculpture in U. tumidus (lower shell) – are only weakly developed, discrimination between related species is very much about gestalt recognition and cannot be based on single, often variable characters alone. Molecular genetic methods clearly identify U. pictorum and U. tumidus. But also this approach has shown its limits in the case of U. pictorum and its relatives in western and southern Europe (U. mancus, U. elongatulus, see  Prié & Puillandre, 2013).  
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North America is the eldorado for freshwater mussels with nearly 300 known species. In contrast, the number of 16 species currently recognized in Europe appears quite modest. In North America, too, the way towards a widely accepted view of the real species diversity was long and not always straight. The Naiad Lot of the Month November 2013 (specimens of Lampsilis fasciola Rafinesque, 1820 from the Clinch River in Tennessee, USA) attests to this fact. In an addendum to the list of names that the donor of the shells provided with the material, he stated: “You may form our own opinion. I have mine.” This sentence once again shows the limits of species definition and recognition of unionid mussels that are solely based on shell characters.
To learn more about the varied forms of freshwater mussels and their regional distribution the “Mussel Project” web pages (http://mussel-project.uwsp.edu) offer a wealth of information.
And now for something completely different:
Nephronaias ravistella, the Veracruz Kidneymussel, is a Central American unionid. This group is poorly known and its taxonomy and systematics are mainly based on shell characters for which reason the determination of genera (ca. 20) and species (ca. 100) is conventional.  There are no precise ideas about the real diversity of freshwater mussels in this region. For many of the presumed species the only known information is the (approximate) sampling locality and the sampling often took places many decades ago.
The Unionida collection of the Senckenberg Research Institute holds possible syntypes of N. ravistella that might become very useful as comparison material in a future revision of the Central American unionids.
Half of the month has already passed, therefore this time an easy case that will trouble no one who paid attention to NLoM June 2013: While the lectotype of Anodonta ventricosa is A. anatina, the paralectotype is A. cygnea.
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Collecting shells perhaps always had an aesthetic appeal and some people even had their artistic impulses released, even towards the shells themselves, as shown with the Naiad Series of the Month August 2013. This is also to wish you a pleasant holiday season, at the seaside, on the lakeshore, in the mountains or wherever you are looking for recreation.  
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Some rather dull and unattractive looking objects have nevertheless an important documentary value and an interesting story to tell. This NLoM shows old (subfossil) shells of the thick shelled river mussel (Unio crassus) of the northern Upper Rhine. At the beginning of the 20th century Fritz Haas described this variety as Unio pseudocrassus. According to Haas they lived on the gravelly bottom of the main channel of the River Rhine. The present shells were collected at a time (1981) when Unio crassus had already died out there and remains of the original mussel fauna could only be collected from the sand banks of the main channel. During a visit at the Senckenberg museum the finder of the shells, Dr. Hasko Nesemann, had again a look at the material that he had collected 32 years ago. He added some details to the sampling methods of that time (a 100 km round trip to the sampling station with his bicycle – within one day).
Small shells sometimes create less problems than big ones. The two species Anodonta cygnea (Swan mussel, Big pond mussel, upper shell) and A. anatina (Duck mussel, Common pond mussel, lower shell) can be distinguished quite easily when young – as adults they were given over 400 synonyms. It is more difficult to label such small objects (less than 1 cm), but a skillful hand managed it as proven in the Naiad Lot of the Month June 2013.
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The scientific value of a collection is determined by the quality of the documentation for every object. With incomplete data one is often left to wonder about the origin of a specimen and how it made its way into the collection. The only hope then is to find somewhere other evidence that may shed light on it and ultimately give the object a place and a date – or at least a country and a century.  
The Naiad Lot of the Month May 2013 could be a little sensation, if place and time of sampling were known. Meanwhile it remains a curious fact that a Chinese Pond Mussel (Sinanodonta woodiana) is present in the collection with an old label that could indicate “Baden” (now a part of Baden-Württemberg/Germany) as the sampling place – long before the first record of the species for Germany in 1999.
There are frequently strange gaps in the distribution of a species. If  the determination of the species is difficult or the interpretation of forms that should belong to it changed over time, written information about its occurrence or absence will not help much to clarify the situation. It will remain unclear if lack of detection is real or if the species was overlooked or misidentified. In these cases only well documented vouchers can help. To hold them ready is a key function of any scientific collection.
The Naiad Lot of the Month April 2013 shows that the Swan mussel (Anodonta cygnea) was collected already in 1848 in its very isolated location in coastal freshwater lagoons in central Portugal. The next confirmed findings are in France, about 800 km to the east .  
One of the heroes of naiad taxonomy and systematics, Fritz Haas, surrendered to the European Anodonta and he called (nearly) all of them „cygnea“. In the historical part oft he Unionida collection about 2.000 lots are named in this way. But it is has been known for a while that Western, Central, Northern and Eastern Europe are inhabited by two different indigenous Anodonta species, namely A. cygnea (Swan mussel) and the by far more common A. anatina (Duck mussel). These species are not easy to distinguish. Therefore all “cygnea” in the collection have to be redeterminded in order to get a reliable picture of the species‘ true distribution and abundance of the species. Only then data about sampling localities, dates and other additional information are useful. The NLoM March 2013 features a shell collected by Fritz Haas that changed its name.