Foraging decisions of a native whelk, Trochia cingulata Linnaeus, and the effects of invasive mussels on prey choice

Biological invasions, which are occurring at an increasing rate, are recognised as major drivers of environmental change. Impacts from non-native species are particularly pertinent to species interactions, such as those between predators and prey. In this regard, impacts of invasive predators on their native prey have been widely examined, while the impacts of invasive prey on native predators have been largely overlooked. Here we investigate the impact of invasive mussel species on foraging decisions of a native predatory whelk, Trochia cingulata, on the West Coast of South Africa. This coastline has been subject to a number of mussel invasions, resulting in changes to intertidal communities and hence the foraging landscapes of mussel predators. We compared present day survey data with that from 30 years ago and found significant changes in the mussel assemblage available to the whelk. The native mussel Choromytilus meridionalis was no longer present on the shore, and there were reduced abundances of the native Aulacomya atra. On the other hand there were increases in the invasive Mytilus galloprovincialis, with the detection of a second invader Semimytilus algosus. We then examined predation by whelks on the different mussel species in laboratory feeding trials. When presented with single prey species, whelks consumed greater numbers of the invasive M. galloprovincialis and S. algosus compared to A. atra, which was previously their preferred prey. Similarly when these mussels were provided in combination, greater numbers of the invasive species were consumed. Chemical cue trials indicated that whelks did not select prey based on chemical recognition, indicating that tactile stimulation was an important driver of prey choice. Although there was no overall difference in shell thickness at drilling sites among mussel species, drill holes were concentrated at the centre of the invasive mussel shells, while this was not observed in the native shells. No differences in energy content were found between mussel species, suggesting that whelks drilled in locations that maximised energy gain. Overall we found that native predatory whelks that previously preferentially consumed the native mussel had shifted their selection of prey towards the invasive species. Notably, familiarity with one invasive mussel appears to have facilitated the assimilation of a second morphologically similar invasive mussel into the diet of whelks.