Limited grazing pressure by native herbivores on the invasive seaweed Caulerpa taxifolia in a temperate Australian estuary

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Abstract

Caulerpa taxifolia is an invasive alga threatening biodiversity in invaded regions. Its proliferation in recipient communities will be due to several factors including limited grazing effects by native herbivores. However, little is known about grazing pressure exerted by native herbivores on C. taxifolia relative to native macrophytes or its attractiveness to them as habitat. The present study determined which herbivores co-occurred with invasive C. taxifolia in a temperate Australian estuary and documented their abundance, relative grazing effects, habitat preference and survivorship on C. taxifolia compared with native macrophytes. Four herbivores co-occurred with C. taxifolia and their densitieswere often lowor zero at the sites studied. Feeding experiments showed that compared
with C. taxifolia: the fish, Girella tricuspidata, preferred Ulva sp.; the sea-hare, Aplysia dactylomela, preferred Laurencia sp.; whereas the mesograzers, Cymadusa setosa and Platynereis dumerilii antipoda, both consumed Cystoseira trinodus and Sargassum sp. at higher rates. The two mesograzers also showed strong habitat preference for C. trinodus and Sargassum sp. Cymadusa setosa had poor survivorship on Caulerpa taxifolia whereas P. dumerilii antipoda had 100% survivorship on C. taxifolia after 41 days. We consider that the low diversity and abundance of native herbivores, their weak grazing pressure on C. taxifolia and its low attractiveness as habitat may facilitate further local spread in this estuary, and potentially in other invaded locations.