Shell morphometrics and growth rate of the invasive bivalve mollusc <i>Mytilus galloprovincialis</i> in the Knysna estuarine embayment, South Africa

Abstract

Shell morphometrics of the invasive mussel Mytilus galloprovincialis were compared at five sites and growth rate at four sites (in four seasons) in the Knysna estuarine embayment. Mussels from two sites (The Heads, Leisure Isle) where wave action was present had shells significantly lower for any length when compared with other more sheltered sites. There was no significant difference in shell width of mussels for any given length among sites. Mussels from The Heads had thicker shells than other sites, and those from Leisure Isle thicker shells than three other embayment sites where shells did not differ in thickness. Growth rate of mussels at two embayment sites (Thesen’s Wharf and Thesen Islands Marina) was greatest in autumn and summer whereas at The Heads and Leisure Isle there was little seasonal difference in growth rate. Growth rate of mussels at Thesen’s Wharf and Thesen Islands Marina was mainly greater in all seasons when compared with mussels at The Heads and Leisure Isle. The more rapid growth rate of mussels at the sheltered embayment sites might in part explain why M. galloprovincialis now dominates the mid- to lower intertidal on hard substrata in this region of the Knysna estuary.

Keywords: Mediterranean mussel, Mytilidae, shell height, shell length, shell thickness