Alien invertebrates are invading South African rivers, competing with endemic species, and if their spread is not controlled, it could damage the habitat.
Prof. Chris Appleton, of the School of Biological & Conservation Sciences at the University of KwaZulu-Natal's Westville campus, said the first record of freshwater jellyfish (Craspedacusta sowerbyi) in the Midlands had been in Midmar dam in 1978. In 1989 it was discovered in Albert Falls dam and has spread to several Midlands rivers.
Explaining how these jellyfish were transported, Appleton said in their polyp stage they often got stuck in nets, or boat trailers.
"However, the real villains of the piece are the aquariums," said Appleton.
Scientists have found the polyp is able to survive the most unfavourable environments to flourish in a good year and believe they were part of a wave of post-war introductions of freshwater organisms to South Africa. "They were possibly introduced along with sport fish and aquatic plants destined for ornamental ponds or aquaria," he explained.
Other places where these jellyfish have made inroads, he said, were the Berg River in the Cape (where it was first found in 1958) and some rivers in the Johannesburg area.
According to Appleton, the major aliens are freshwater snails, which have thrived around the major ports from Cape Town to Durban and have also invaded some of the inland cities.
Two of the longest-established such species have expanded their range to include much of South Africa. Their ecological impact is that as microphagous feeders (grazers) they share their food resources with indigenous snails and some insects - which are being deprived of resources as the aliens proliferate.
Appleton will be speaking on this topic tomorrow at the J.S. Steel Auditorium Natural Science Museum at the City Hall, Smith Street entrance at 11am.