Characterisation of the supply-settlement relationship for Semibalanus balanoides (L.) along a wave swept coast in Fife, East Scotland

Abstract

This thesis describes the results of a three year study which collected larvae of the acorn barnacles Semibalanus balanoides on a rocky coastline in Fife, East Scotland. The nauplii larvae of S. balanoides are released from their parent in springtime in the United Kingdom and develop in the plankton for approximately one to two months. During this period they are transported some distance from the parent population and eventually return to the intertidal shoreline as a cyprid larval stage. The ‘decision’ to settle is a crucial point in the life history of the organism as most sessile organisms cannot move once this has occurred. The supply of larvae to intertidal shorelines was historically neglected until the 1980s. Patterns of settlement were largely considered irrelevant to distribution patterns of adults relative to postsettlement processes such as predation and competition. Despite the resurgence in “supply-side ecology” in the past 20 years there has been little development towards the measurement of larval supply. Consequently there has been very little description of fine scale or large spatio-temporal studies involving larval supply. This study demonstrates the first study directly addressing larval supply independently from larval settlement at mesoscales (metres to kilometres; days to years). Improvements were made to the passive larval trap described by Todd (2003). A conical opening was combined with the spiral trap design and a number of inlet areas were trialled (0.25cm², 0.5cm², 1cm² and 2cm²). These were tested across typical wave regimes measured with a new autonomous pressure sensing wave transducer and the 1cm² inlet was selected as the most appropriate trap design due to a balance between an increased per unit inlet larval capture and sufficient absolute larval capture so as to identify daily variation in larval supply to a site. Larval settlement has been studied extensively and is often used as a direct measure of rates of larval supply. It is widely assumed therefore that rates of settlement are a direct reflection of rates of supply, as long as settlement substrates and adult conspecific responses remain uniform. This thesis provides a means of accurately characterising the supply-settlement relationship for S. balanoides independent of substrate based responses and demonstrates that this is not the case. The relationship was found to be asymptotic, even at sites where there was low larval supply. It was concluded that density-dependent larva-larva interactions were present during the settlement of larvae and were relevant at daily temporal scales, limiting the rates of larval settlement proportional to larval density. There was no obvious effect on this relationship due to wave action however differences were observed between sites and years. Saturation of preferential environments within the tiles was observed resulting in a settlement preference cascade, with larvae being forced to settle in increasingly less preferable areas of the tiles with increasing larval density. Larva-larva interactions are demonstrated as having a considerable effect on the rates of settlement of S. balanoides.