Fertilization ecology of egg coats

reproductive risk of incomplete fertilization (Mortensen, 1938). Given evidence of at least occasional sperm limitation in marine environments (Levitan and Petersen, 1995; Yund, 2000), recent work has examined whether this risk is reduced by particular features of adults or gametes that could improve chances of sperm–egg encounter (Denny et al., 1992; Levitan et al., 1992; Petersen et al., 1992; Buckland-Nicks, 1993; Levitan, 1993; Babcock et al., 1994; Eckelbarger, 1994; Thomas, 1994a; Thomas et al., 1999). One common and conspicuous feature of free-spawned eggs is an extracellular ‘accessory’ structure that surrounds the ovum and is shed early in development (Strathmann, 1987). Although several functions have been proposed for these structures (Szollosi, 1964; Lambert and Lambert, 1978; Chia and Atwood, 1982; Buckland-Nicks, 1993; Thomas et al., 1999), most research on their role in fertilization has focused on chemical constituents and their effects on sperm–egg interaction. Compounds contained in or released by accessory structures have been shown to influence sperm physiology, respiration, motility, longevity, chemotaxis, species specificity and acrosome activation (SeGall and Lennarz, 1981; Suzuki, 1989; De Santis and Pinto, 1991; Patricolo and Villa, 1992; Villa and Patricolo, 1993; Bolton and Havenhand, 1996). In addition to mediating chemical interactions, however, accessory structures can also alter the egg as a physical target for sperm. Jelly coats, follicle cells and egg hulls can double or triple the effective diameter of a free-spawned egg (Strathmann, 1987), substantially increasing cross-sectional (‘target’) area while also altering egg buoyancy. The potential importance of egg size for fertilization success was highlighted by Levitan (1993), who reasoned that under sperm limitation, larger eggs would be fertilized with greater probability because they present a larger target for sperm. As an alternative to the conventional view of accessory structures as primarily chemical in their action, here I test the hypothesis that accessory structures around free-spawned eggs can enhance fertilization success through a change in physical target size. This hypothesis has received speculation in previous studies (Rothschild and Swann, 1951; Vogel et al., 1982; Epel, 1991; Podolsky and Strathmann, 1996), and elsewhere I showed that overall target size is a good absolute predictor of the probability of fertilization (Podolsky, 2001). Previous studies have not, however, evaluated the importance of physical 1657 The Journal of Experimental Biology 205, 1657–1668 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB4070