First record and biology of Unio gibbus Spengler, 1793 in Tunisia

In the framework of a recent review of the systematics and global diversity of the freshwater mussel species in Tunisia, the species Unio gibbus Spengler, 1793 has been recorded for the first time. This overlooked Unio species inhabits several rivers in North of Tunisia, Northwest Morocco and Southwest Spain. Shell characters were examined in adult and juvenile specimens from several North Tunisian rivers. The glochidia is conic, to elongate triangular, intermediate between glochidia shapes in Unio and Anodonta. The hook is triangular, with large basis. The entire internal cavity of the external demibranchs (homogeneity) acted as a marsupium (ectobranchy). The favourable water temperature at the end of fall in Tunisia involves the apparition of a high number of gravid female in winter, which minimizes the fertilization of the eggs and the larvae release. Moreover, the drying of an important part of the mussel’s habitats in summer involves a mortality of a high number of specimens each year and threatens severely the populations. May and June constituted the most favourable period for reproduction.