Early biotic interactions among introduced and native benthic species unveil cryptic predation and shifts in larval behaviour

Abstract

Recurrent introductions of non-indigenous species generate novel interactions that vary with local conditions and the composition of the receiving community. Most studies examine relationships of newcomers with native species, but interactions among introduced species could also affect community shifts. As early ontogenetic stages are particularly vulnerable to biotic interactions, we explored direct and indirect interactions across early life-history stages in space-dominating marine invertebrates. We used introduced ascidians and both native and introduced mussels. To increase generality, we ran our experiments in 2 distant locations, one in the northern and one in the southern hemisphere (Mediterranean and South Africa). We found no sperm interference between the ascidians, nor were there interspecific effects on settlement or metamorphosis success. However, larvae of the ascidian species reacted to each other by shifting from aggregated to random settlement. Juvenile mussels consumed large numbers of ascidian larvae, though larvae that avoided mussel predation showed higher settlement success. Mussel species in the southern hemisphere locality (native Perna perna and introduced Mytilus galloprovincialis) consumed more ascidian larvae than mussels in the northern locality (native M. galloprovincialis), with a tendency for ascidian larvae to avoid settling close to mussels in the northern locality. We conclude that larval consumption by mussels affects the establishment of ascidians, but that the magnitude of this effect is context dependant. These results emphasize the importance of the composition of the receiving community in determining its susceptibility to invasion. Whether the species comprising this community are native or introduced is, however, less important than what manner of species they are.