Distribution and habitats of Unio caffer Krauss, 1848 (Bivalvia: Unionoida: Unionidae) in South Africa based on the records in the database of the National Freshwater Snail Collection - de Kock, K.; Wolmarans, C. (Suid-Afrikaans Tydskrif vir Natuurwetenskap en Tegnologie / South African Journal of Science and Technology, 2010)