Glasby, T.M., and Gibson, P.T., 2007. Limited evidence for increased cold-tolerance of invasive versus native Caulerpa taxifolia. Marine Biology 152: 255–263.

Summary

Over the last 20 years, the seaweed Caulerpa taxifolia has invaded many temperate waterways throughout the world, spread mainly via the aquarium trade. Because C. taxifolia is native to many tropical and subtropical parts of the world, it has often been argued that the invasive strain of C. taxifolia must be genetically different to enable it to withstand cool temperate waters and to enhance its survival in aquaria. C. taxifolia is native to the coast and outer reefs of Queensland, extending as far south as Moreton Bay. Thus, the first sighting of C. taxifolia in Port Hacking, NSW in April 2000, represented a 750 km southern extension in the seaweed’s known distribution. For this reason, and because over the preceding 15 years C. taxifolia had been spreading throughout the Mediterranean, C. taxifolia was deemed to have been introduced into NSW (probably via the aquarium trade), rather than having spread naturally southward from Moreton Bay.

From an ecological and managerial perspective, it is the rate of spread and impacts of invasive species which are of prime importance. Thus, it is important to examine growth characteristics of native and invasive populations of C. taxifolia because these affect its ability to invade. If C. taxifolia in NSW is a particularly cold-tolerant invasive strain, one would expect that, in cool water, NSW C. taxifolia would grow better than native C. taxifolia from Queensland. Conversely, native C. taxifolia from Queensland should grow better in warm water than would invasive NSW C. taxifolia.

To test these predictions, native and invasive C. taxifolia were collected from numerous locations and, in the laboratory, their growth rates in warm (22 – 25oC) and cool (15 – 18oC) water were compared. In warm water, invasive and native C. taxifolia grew up to 10 times more than in cool water. In general, the growth of native C. taxifolia was as good as or better than the growth of invasive C. taxifolia in warm or cool water. Also, the appearance of C. taxifolia changed considerably in response to temperature, so much so that native and invasive samples that were physically distinct at the start of the experiment became indistinguishable after 69 days in warm water. Therefore, there is little evidence to suggest that NSW C. taxifolia could spread faster in cool water than could native C. taxifolia. Indeed, there was every indication that native C. taxifolia would be capable of surviving and becoming invasive if it were to be introduced into temperate estuaries. It seems very unlikely that NSW C. taxifolia is a genetically different cold-resistant strain. In fact, the results raise serious doubts about the existence of a distinct “aquarium strain” of C. taxifolia. Linked with recent genetic studies, the results presented here suggest that C. taxifolia from Queensland has been spread around the world and, because it is naturally resistant to cool water, has become invasive in many temperate locations.