Settlement of shore crab Carcinus maenas on a mesotidal open habitat as a function of transport mechanisms

Abstract

We evaluated the patterns of larval settlement of Carcinus maenas on natural substra- tum and its relationships with forcing factors related to larval transport mechanisms. Settlement was estimated as colonisation of traps containing defaunated substratum. Traps were deployed at low tide for 24 to 26 h, in the intertidal of the island of Helgoland (North Sea, German Bight), every day dur- ing the settlement seasons of 2003 to 2005. The forcing factors were wind direction, predicted resid- ual surface (0 to 8 m) currents, and days since spring tide. The settlement season lasted from the end of June to the end of August; colonisation rates varied among and within years, without a clear pat- tern. In 2003 and 2004, long periods of low colonisation rates occurred in association with strong and persistent (5 to 10 d) SW winds and eastward currents. In 2003, colonisation peaked at spring tide only when winds were not from SW and currents were not flowing eastwards. In 2005, colonisation rates peaked mainly at, or 1 to 3 d after, spring tide; periods of low colonisation were short, except at the end of the settlement season. According to an existing 2-step model and to local movement of megalopae, colonisation patterns in the intertidal may reflect transport processes: (1) settlement rates on Helgoland are primarily affected by wind-driven currents: SW winds transport larvae out of Helgoland waters, leading to low settlement rates, while other wind regimes transport them towards Helgoland. In the absence of strong SW winds, peaks of settlement at or after spring tide suggest (2) selective tidal stream transport or some form of tidally mediated flow. The absence of SW winds leads to a higher influence of local movement of megalopae, as the number of larvae reaching the benthic habitat is high. In consequence, colonisation of natural substratum may reflect larval trans- port processes and local movement of larvae in C. maenas and other invertebrates with mobile ben- thic stages.