Settlement of the invasive mussel Mytilus galloprovincialis in a warm temperate estuarine embayment in South Africa

Abstract

Mytilus galloprovincialis, a globally successful marine alien invasive species, occupies all artificial hard substrata in the embayment of the Knysna estuary, South Africa. The current study compared temporal and spatial settlement patterns of M. galloprovincialis within this environment. Settlement was monitored monthly from November 2015 to June 2017 at three locations within the embayment using plastic scouring pads. Each month the pads were deployed for seven days over a spring tide. Numbers of settlers differed significantly among sites and time. Peak settlement occurred in summer (December) and early autumn (February or March), but number of settlers varied between years possibly owing to differences in larval supply and/or environmental factors. During peak settlement, most M. galloprovincialis collected on the pads were primary settlers (<0.5 mm long). Settlement was greatest at the site closest to the entrance of the embayment (Thesen Island Wharf) and lowest at the site farthest away from the entrance of the embayment (Railway Bridge). Variation in settlement among sites and years could be a response to hydrographical conditions and/or environmental conditions and detailed studies on the planktonic phase are now required.

Keywords: invasive species, Knysna Estuary, Mollusca, Mytilidae, primary settlers, secondary settlers