Some Discoveries Come Unexpectedly …

28.05.2020 | Dr. Ira Richling

With a smile and a heavy heart – the story of the discovery of a new species in Baden-Wuerttemberg

The Interstitial Pea Mussel Pisidium interstitiale Bößneck, Groh & Richling 2020 (Picture: J. Bahia / SMNS).

An unexpected find

In March 2017 in my last blog on Lost diversity in spring snails? (german only) I reported on research resulting in the elimination of a good number of species – in Baden-Wuerttemberg alone – because based on genetic results they turned out to be synonyms, i.e. to be identical to known species. The following story is about an opposite case. Unexpectedly and rather by accident we came across a new and in every respect virtually overlooked species: the Interstitial Pea Clam Pisidium interstitiale from the Odenwald in Baden-Wuerttemberg.

Here, “we” represents a whole team which includes the authors of the new species: my freelance colleagues Klaus Groh from Rhineland-Palatinate, Dr. Ulrich Bößneck († 2019) from Thuringia and myself, as well as Drs Catharina Clewing and Christian Albrecht, both in the Department of Animal Ecology and Systematics at the Justus-Liebig-University Giessen, who contributed the genetic investigations.

Wet biotopes along a small creek as typical habitat of the Interstitial Pea Mussel (Picture: K. Groh).

Whorl Snails and Pea Mussels

But starting at the beginning- What led to this discovery? Klaus Groh and myself conducted a professional malacological survey of the Steinach valley, Odenwald, searching for potential occurrences of protected land snails, specifically the Narrow-mouthed Whorl SnailVertigo angustior– a species listed in Appendix 2 of the Flora and Fauna Habitats Directive. Because this species dwells in wet meadows rich in sedges, we sampled these habitats, although the rather acidic soil conditions were not promising with regard to positive records. Confirming our expectations, most plots turned out to be characterized by an assemblage of ubiquitous species, including numerous small mussels of the genus Pisidium.

Colloquially, Pisidium mussels are called Pea Mussels. This is misleading because typical peas are true giants compared to the size of most species of the genus, which vary between 2 and 3 mm in length. The mollusc section of Naturportal Sued-West gives an overview of the species of pea mussels found in Germany. Due to very few distinguishing characteristics combined with high morphological variation, species are notoriously difficult to identify. Despite extensive experience with their identification, we could not convincingly attribute a strange form in our samples to any of the known possible species. Therefore my colleague sent these specimens to our friend Dr. Ulrich Bößneck, THE expert for the genus Pisidium. He promptly confirmed the atypical appearance of the specimens, and we all were fired up to get to the bottom of the problem.

The Interstitial Pea Mussel Pisidium interstitiale Bößneck, Groh & Richling 2020 (Picture: J. Bahia / SMNS).

The hunt for the unknown pea mussel

In September 2012, there was time and occasion for the three of us to re-visit the Steinach valley to “hunt” pea mussels at the sites known from 2011, in order to obtain more material and especially specimens properly fixed for genetic studies. Since pea mussels – like all mussels – are aquatic creatures, we used dip nets in small temporary puddles and swampy depressions in the wet meadows to catch the molluscs. Back home in the lab, specimens were sorted under the stereomicroscope and the results were seriously disappointing: Although we collected a fair number of pea mussels, they belonged almost exclusively to two known species previously found co-occurring with our potential new species.

This did not match our expectations of the proportion of species in our previous samples, and only allowed two explanations: Either we searched at unfavourable sites, or the differences were caused by differences in methodology. Previously, we did not use dipping nets since we were searching primarily for land snails – we took substrate samples with the removal of the upper soil layer (simply expressed: we used a spade to cut out sod); these samples were subsequently washed, screened and sorted for snails. Was it possible that the unusual pea mussel differed not just in appearance, but also showed a different life-style?

We thought this second working hypothesis was more convincing, and a year later the core team met again in the Odenwald for another more strategically planned “hunt”. We chose the same sites and dug spade-deep substrate soil samples out of the more-or-less water-saturated ground. These we separated into different layers for the analyses: surface with vegetation, topsoil with roots, and the mineral layer. It was more difficult to sample living animals using this technique because they had to be picked out of the wet screening residues instead of being easily separated in dry conditions under the stereomicroscope.

Reward for our efforts

However, our efforts were fully rewarded: Not only did we gain enough material for genetic and further morphological investigation, but acquired interesting data on the ecology of the species as well. As the sequencing of mitochondrial gene fragments by our colleagues from Giessen also confirmed that this strange pea mussel differed from other samples, we had three independent lines of evidence to postulate the existence of a new species: conchology (shell shape), genetics and life-style. Adding to our satisfaction, Ulrich Bößneck discovered this new species in a similar habitat in Thuringia and its similarity to the Odenwald material was genetically confirmed.

So the tedious critical assessment of all previously published names of pea mussels remained to be done. These existing names had been treated as synonyms of similar known species, but errors could have been made and one of these names could actually represent the same animal as “our” new species. Surprisingly, all 21 checked names did not qualify and the description of a new Pisidium-species was recommended – the first for Central Europe in nearly 100 years. Unfortunately and for various reasons largely out of our control, the process of publication took forever, leading to the sad situation that our co-author and dear friend Dr. Ulrich Bößneck did not live to see the final publication, because he passed away much too young after a severe and incurable illness.


The Interstitial Pea Mussel Pisidium interstitiale Bößneck, Groh & Richling 2020 (Picture: J. Bahia / SMNS).


Groh, K., Bössneck, U., Clewing, C., Albrecht, C. & Richling, I. (2020): A new pill clam from an unusual habitat: the interstitial Pisidium interstitialis n. sp. (Bivalvia: Sphaeriidae) from southwestern and Central Germany. – Journal of Molluscan Studies, 86 (2): 104-119 + 2 pp. Supplementary material. doi:10.1093/mollus/eyz036

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