Foraging decisions of a native whelk, Nucella cingulata and the effects of invasive mussel species on prey choice
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Invasive species are considered one of the top threats to global biodiversity altering native habitat structure and functioning in a number of natural systems. The open coast along the west part of South Africa has been invaded by two mussel Mytilus galloprovincialis and Semimytilus algosus. These species have altered the native species composition causing declines the native population of ribbed mussel Aulacomya atra and several other species. The alteration of mussel species composition on the shore has altered prey availability to native predators such as the dogwhelk Nucella cingulata. The overarching aims of this study were to determine the foraging behaviour of the native whelk and how a shift in species composition towards invasive mussel species has altered the whelks prey preference. Foraging behaviour was analysed to determine if prey selection by whelks was based on chemical cues (y-maze testing), tactile responses, (simultaneous prey presentation) or energy content gains (bomb calorimetry). Whelks did not show preference towards any particular mussel species when prey choice was made using chemical cues. This method of prey selection reflects the turbidity of wave exposed rocky-shore habitats where such cues would be unreliable. Prey selection during simultaneous presentations indicated a strong preference towards the two invasive mussel species with the invasive species being consumed more often than the native mussels. There was, however, no preference between both mussel species when presented simultaneously. Energy content did not vary between the three mussel species, therefore other variables must play a role during prey selection. The introduction of invasive mussel species has unmistakably altered native communities along South Africa’s coastline not only in structure, but also in functioning with regards to species interactions. It is therefore important to keep in mind these interactions during decisions regarding management and removals of these mussel species for sound conservation of native biodiversity and the complex interactions that are maintained.