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.