Polyspermy, Egg Size, and the Fertilization Kinetics of Free‐Spawning Marine Invertebrates

at least two types of block may be involved: a fast electriRecent recognition of sperm limitation effects, and quancal block occurring within seconds of sperm contact and tification of resultant variation in fertilization success, has a slower cortical reaction (Schuel 1984). However, in the had important consequences for the understanding of the Vogel et al. model, polyspermy blocks are implicitly ecology and life history of marine organisms that repro‘‘perfect’’ in that, once an initial sperm fertilizes an egg, duce by free spawning (reviewed in Levitan 1995; Levitan there is (instantaneously) no possibility whatsoever that and Petersen 1995). Central to this developing theory is other sperm can also fertilize the egg. In reality, there will the idea that the likelihood of fertilization is a function be a finite time period between the initial fertilization of ambient sperm concentration. Vogel et al. (1982) proand polyspermy block activation while cellular changes duced a mathematical model of this relationship, based necessary to prevent further fertilizations take place. Duron a model of bimolecular kinetics and a random colliing this time, additional fertilizations (hence polyspermy) sion of gametes. This model is the basis of several analymight occur. At least one recent study (Brawley 1992) ses of individual and population fertilization success of found that up to 7% of eggs were polyspermic in a natufree-spawning invertebrates (e.g., Denny 1988; Denny ral population of free-spawning (fucoid) algae. A more and Shibata 1989; Denny et al. 1992; Babcock et al. 1994; realistic model of fertilization kinetics would take into Levitan and Young 1995; Morris 1994), reproductive account the finite time delay in the activation of polystrategies in hermaphroditic fish (Petersen 1991), and gaspermy blocks and be able to predict the likelihood of mete provisioning in invertebrates (Levitan 1993, 1996a, polyspermy. 1996b; Podolsky and Strathmann 1996). The Vogel et al. A more complete model of fertilization kinetics might model, however, examines only the consequences of also address one concern of Podolsky and Strathmann sperm limitation and does not consider the possibility (1996) regarding Levitan’s (1993, 1996b) incorporation that contact with more than one sperm may lead to polyof sperm limitation effects to account for intermediate spermy. egg sizes of at least free-spawning marine invertebrates. The Vogel et al. model assumes that all eggs that are Using the Vogel et al. model, Levitan (1993) suggested contacted and fertilized by at least one sperm develop that, because in sperm-limited conditions larger eggs into normal embryos when, in fact, we would ordinarily provide larger targets for sperm (and increase the rate of expect that fertilization by two or more sperm will result sperm-egg collisions), they should be selected for, counin polyspermy and either mortality or abnormal larval terbalancing other forces that favor small eggs. Podolsky * Present address: South Australian Research and Development Institute, South and Strathmann (1996) reasoned that, if eggs were sperm Australian Aquatic Sciences Centre, P.O. Box 120, West Beach, South Australia limited, why would they not have evolved to become bet5022, Australia; E-mail: Styan.craig@wpo.pi. sa.gov. au. ter targets in a manner that does not require an increase Am. Nat. 1998. Vol. 152, pp. 290–297.  1998 by The University of Chicago.