Responses of common SE Australian herbivores to three suspected invasive Caulerpa spp.

We sought to determine whether common intertidal and shallow subtidal zone grazers would consume extracts or fronds of three invasive Caulerpa spp., all of which are now resident in southern New South Wales, Australia. We examined the responses of herbivorous fishes, echinoderms and molluscs to C. filiformis. A subset of these organisms was tested with extracts of C. scalpelliformis and C. taxifolia. Polar (seawater) extracts of C. filiformis deterred a single herbivore, Aplysia sydneyensis, but confirmed that the biological activity reported from some Caulerpa spp. is not restricted to the lipophilic fractions. The large turbinid Turbo torquatus was deterred by an ethanol extract of C. filiformis, while the small congener T. undulatus demonstrated a significant preference for palatable agar discs containing ethanol extracts of C. filiformis. However, when T. undulatus were offered a choice of fronds from five algal species in the laboratory, they readily consumed Ulva spp. and Sargassum sp., showing the lowest preference for C. filiformis. Solvent extracts of C. scalpelliformis and C. taxifolia did not significantly deter any grazers. However, the overall trend was for reduced consumption of discs containing solvent extracts of these seaweeds. Indeed, for the large urchin Centrostephanus rodgersii and in the fish trials these effects were very near significant (P<0.06). We conclude that common herbivores associated with hard substrata are highly unlikely to intercede in the spread or control of these invasive algae.Communicated by M.S. Johnson, Crawley