Chapter 11 Larval Supply and Dispersal Dustin J. Marshall, Craig Styan, and Christopher D. McQuaid 11.1 Introduction Most marine organisms have planktonic larvae that spend between minutes and months in the water column before settlement. For over 50 years, marine ecologists have recognised that the number of larvae that are produced, disperse and recruit successfully is extremely variable (Thorson 1950). More recently, it has been realised that variation in larval supply can drive the dynamics of marine populations and communities (Underwood and Fairweather 1989). To understand how marine populations and communities vary in time and space, we must first understand how propagule supply and dispersal are influenced. This chapter examines the causes of variability in larval production, survival in the plankton, and scales of dispersal and their consequences for marine organisms living on hard substrata. 11.2 Variability in the Production of Larvae The enormous variability in production of larvae by any species can come from a variety of sources with two basic elements: (1) variation in fecundity (the production of gametes) and (2) variation in fertilisation success (the production of zygotes). 11.2.1 Variation in Fecundity Despite the realisation that variation in egg production helps determine larval supply, surprisingly few patterns of variation in fecundity have been identified for marine organisms. This may be because fecundity is often difficult to quantify (Ramirez-Llodra 2002) but the paucity of identifiable patterns may also arise because data are scattered across studies on individual species. Studies of variation in fecundity show that there can be dramatic differences among populations and individuals (Ramirez-Llodra 2002). For example, the number of eggs produced by individual M. Wahl (ed.), Marine Hard Bottom Communities, Ecological Studies 206, 165 DOI: 10.1007/978-3-540-92704-4_11, © Springer-Verlag Berlin Heidelberg 2009 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26