Gamete Release and Spawning Behavior in Broadcast Spawning Marine Invertebrates

The release of eggs and sperm into the environment for external fertilization is common (Giese & Kanatani 1987) and thought to be the ancestral mating strategy (Wray 1995). Most phyla have at least some species that release sperm into the environment (free spawning or spermcasting) and a majority have representatives that also release eggs (broadcast spawning) for external fertilization (Levitan 1998a). A common feature of broadcast spawning taxa is a lack of sexually dimorphic adult characters (Strathmann 1990; Levitan 1998a). The rare instances of adult morphological differences among sexes occur in pair-spawning species and include body size differences or specialized appendages for clasping females (Levitan 1998a). The general absence of sexual dimorphism has been used as evidence that broadcast spawning invertebrates are not under sexual selection (Darwin 1871). In contrast to this view on the nature of sexual selection based on sexual dimorphism, recent studies documenting the rapid evolution of gamete recognition proteins in external fertilizing species have invoked sexual selection and sexual confl ict as the driving force of positive selection (Palumbi 1999; Swanson & Vacquier 2002; Haygood 2004). Only recently have data been collected on the patterns of reproductive variance and the intensity of sexual selection in male and female broadcast spawners (Levitan 2004, 2005a, b; Levitan & Ferrell 2006; Levitan 2008). The results suggest that sexual selection can be intense in these species and that the nature of sexual selection is dependent on the distribution and abundance of individuals. Given the nature of external fertilization, where the competition among individuals for mates is played out among gametes in the water column, it is not surprising that the primary sexual characteristics targeted by selection are associated with the spawning behaviors that mediate gamete competition and the traits of the gametes themselves. The infl uence of sperm availability and sexual selection on gamete traits has already received some attention (Levitan 1998b, 2006). Below we briefl y summarize the relationship between gamete traits and fertilization and direct readers to the relevant literature. The remainder of the chapter is focused on the less studied question of how sexual selection might infl uence the evolution of the duration, release rate, and timing of gamete release. Because these features determine the size, shape, and duration of an individual’s gamete distribution, it defi nes the extent to which one individual can interact with mates and mate competitors.