A new mussel: Scabies songkramensis

Mussels of the family Unionidae are commonly referred to as pearly mussels or unionids. There are about 700 species worldwide with the highest diversity in North America, where there are approximately 290 species. 

Unionids are found in freshwater sources such as lakes, streams, and rivers. Interestingly, they are not found in high mountain lakes, probably due to a lack of proper fish hosts for their parasitic larval stage called glochidium. A glochidium has hooks, enabling it to attach itself to fish (e.g to the gills) for some time before it detaches and falls to the substrate where it changes the typical form of a juvenile mussel. This passive dispersal process helps distributing these mussel species into areas they otherwise would not be able to reach.

The species name of our newcomer refers to the Songkram River in Thailand, where this new species was found.

For the experts: Morphological and chromosomal characteristics of a number of unionid freshwater mussels were studied from northeastern Thailand. Karyotypes of eight species from seven genera (Chamberlainia, Ensidens, Hyriopsis, Physunio, Pseudodon, Scabies and Trapezoideus) were examined. Six species possess 2n = 38 karyotypes, whereas Scabies crispata and an unidentified Scabies sp. lack three small chromosome pairs, giving a diploid number of 32. Moreover, the karyotypes of the unidentified Scabies differ from S. crispata as it exhibits a telocentric chromosome pair (6m + 7sm + 2st + 1t). Most of the conchological characters also differ between the two species – adult size, colour pattern, muscle scars, pseudocardinal and lateral teeth. The name Scabies songkramensis sp. n. is proposed for the unidentified species, and its description is included in this paper. Interestingly, seven species contain mostly bi-armed chromosomes, but only the mud-dweller in stagnant water, Ensidens ingallsianus, contains predominantly five telocentric pairs. In addition, the marker chromosome characteristics of an unbalanced long arm, twisted centromere, a wider angle 180° arrangement, a twisted arm and telomeric end union reported in this study are described for the first time for unionid mussels.