Some life-history parameters of the non-native amphipod Platorchestia platensis (Talitridae) in a warm temperate South African estuary

Abstract

Density, population structure, growth, mortality and aspects of reproduction of the non-native amphipod Platorchestia platensis were studied for 20 consecutive months (October 2008 to May 2010) in the supralittoral wrack of the Knysna Estuary, South Africa. Amphipod density varied over the sampling period with the lowest numbers recorded in summer (January and February). Ovigerous females with embryos, and juveniles were found in most months with peaks in recruitment found in both April and October, suggesting that in Knysna this species is a biannual breeder. Average female size was significantly greater in winter, with larger females tending to brood more embryos. The largest adults (13.5 mm total length) were always male although the monthly sex ratio was nearly always female biased. Growth rate estimated from a cohort analysis was about 1 mm per month and monthly survival about 69%. The ability to reproduce all year round may be one reason why this introduced species has become established within this warm temperate estuary.

Keywords:

• Crustacea
• reproduction
• density
• sex ratios
• wrack
• Knysna Estuary

ACKNOWLEDGEMENTS

We would like to thank Professor Charles Griffiths (University of Cape Town) for the identification of P. platensis, and South African National Parks for granting permission to carry out this study. Nicholas Hodgson, Catherine Hodgson, and John Dickens are thanked for their assistance in sorting some of the wrack samples, and Bronwyn McLean (Rhodes Graphics Services) for the production of Figure 1. The study was made possible by a research grant from the National Research Foundation. We thank the three anonymous reviewers whose constructive comments greatly improved the paper.