Short-term threats for the sustained survival of freshwater Mollusca in the Olifants River and selected tributaries.

Abstract

The conservation status of less than 2% of the more or less 7000 mollusk species known worldwide have been properly assessed. Consequently the general level of imperilment is poorly documented and almost certainly underestimated. Freshwater mollusks live permanently in water, have limited means of movement and are exposed to a variety of anthropogenic waste products due to the fact that waterbodies often act as sinks for a large array of harmful pollutants. The Olifants River is often described as one of the most polluted rivers in Southern Africa and is progressively subjected to extremely high pressure with regard to natural resources and associated rural transformation and pollution. Little is on record regarding the diversity of the Mollusca in the Olifants River; therefore, in the present study, four surveys of the molluscs were conducted in this river and selected tributaries during two consecutive years at three localities situated on the Highveld and four localities situated in the Lowveld respectively. The pH and electric conductivity of the water were determined during each survey at each one of the localities and values ranged from 6.93 to 9.50, and 110 µS to 1336 µS, for pH and conductivity respectively. A total of 25 mollusk species were collected during the four surveys which included the exotic invader species Lymnaea columella, Physa acuta, Aplexa marmorata and Tarebia granifera. The latter species yielded the highest number of specimens by far, mainly at a locality which could be described as largely transformed. The results of this investigation can serve as a point of departure for future surveys to evaluate the impact of anthropogenic disturbances on the mollusc diversity in the Olifants River and catchment.