ROBINSON et al. (2005) MARINE ALIEN SPECIES OF SOUTH AFRICA
This paper reviews the status of the Marine alien species from the South African coast, and hi-lights the ecological impacts of the alien invasions. The paper also includes for the first time what are known as “Cryptogenic” species, or species seldom seen or observed which are invasive.
This important paper indicates that marine organisms have been both intentionally and accidentally moved all over the world’s oceans since the first navigation of the seas and more specifically the increase in shipping over the last century. This includes the increased use of ballast water by ships, and the dumping thereof in different locations other than the original collection point.
Robinson et al. (2005) directed a survey of the present status of three invasive species that were last assessed 10 years ago. The European shore crab, Carcinus maenas, the Mediterranean mussel, Mytilus galloprovincialis, and the Australian whelk, Bedeva paivae were surveyed.
European shore crabs were first discovered in Table Bay Harbour in 1983, and Robinson et al. (2005) suggest that a previous study in 1990 proposed that the crabs “arrived via fouling of International oil exploration vessels, which had docked in the harbour since 1969.” Robinson et al. (2005) sampled both at intertidal and subtidal levels, and additional searches were implemented beyond the known range of the crab, but in similar appropriate habitat. Where populations were encountered in harbours, the size of the population was estimated using the mark-recapture method, which continued until the recapture rate of marked individuals was larger than 10%. No intertidal range extension for the European shore crab was documented, but the paper indicates the first discovery of this crab in the Hout Bay Harbour, which is further than what is previously suggested in my introductory article. The suggestion for the lack of crabs in intertidal areas, is that they may have difficulty in adapting to inhabitation of wave-exposed areas.
The Mark-recapture experiments indicated large subtidal populations in Table Bay Harbour (133 568 individuals) and Hout Bay Harbour (9180 individuals). It is suggested that the movement of small lobster-fishing vessels between these two harbours facilitated the invasion from the Table Bay Harbour to Hout Bay Harbour. What is interesting is that although subtidal sampling has been done for Saldanha Bay, Robinson et al. (2005) state that no specimens of Carcinus maenas were recorded. The authors indicate their curiosity for the apparent disappearance of Carcinus maenas from this area 12 years after its initial discovery and suggest that its potential for invasion could easily have disastrous impacts on the local biodiversity in Saldanha, however no suggestions are entertained regarding possibilities for their disappearance.
Was it biological? It is doubtful that it is physical since the habitat is very similar to that of Table Bay and Hout Bay, and Robinson et al. (2005) indicates a massive increase in their primary food source along the West coast, the invasive Mediterranean mussel, Mytilus galloprovincialis.
The authors conclude that Carcinus maenas is restricted to harbours and sheltered areas, though in general, the overall prevalence of alien marine species is lower than that of other parts of the world. One possible reason for these statistics is the lack of work that has been done in a predominantly unexplored region of the South Africa coastline where investigations for potential populations of alien species have not been carried out yet. The authors warn that another factor could be the relatively poor development of marine taxonomy, which is an additional limiting factor with only four full-time professional marine invertebrate taxonomists working in South Africa. It is suggested that the number and prevalence of marine invasive species will increase as more investigations are undertaken.
Senior aquarist, Quarantine
Two Oceans Aquarium
Cape Town, South Africa