Table 1.

Sources of variation in fertilization rates when sperm have direct access to eggs (no-choice experiment) and the direction of sperm movement and subsequent fertilization rates in dichotomous choice trials (egg-choice experiment) in the mussel, Mytilus galloprovincialis. For each model, sums of squares (SS) and degrees of freedom (d.f.) were computed individually for each block and summed to estimate the mean squares (MS) for each analysis. Significance levels for male and female effects were assessed by dividing their respective mean squares by the interaction mean squares [21]. Significant effects are presented in bold text. The d.f. associated with the error variance for each block was calculated from the formula N_M N_F (n − 1), where N_M = 2, N_F = 2, and n = number of replicate crosses = 2. As with d.f. associated with main effects, error d.f. were summed across blocks for the final analysis [21].

source of variation	d.f.	SS	MS	F	p
(a) no-choice experiment: fertilization
male	15	0.091	0.006	1.076	0.445
female	15	3.548	0.237	41.94	<0.0001
interaction	15	0.085	0.006	1.945	0.036
error	60	0.174	0.003
(b) egg-choice experiment: sperm numbers
male	15	3 544 513	236 301	2.820	0.027
female	15	2 169 863	144 658	1.727	0.151
interaction	15	1 256 763	83 784	2.049	0.026
error	60	2 453 150	40 886
(c) egg-choice experiment: fertilization
male	13	1.560	0.120	4.065	0.008
female	13	1.062	0.082	2.767	0.039
interactiona	13	0.384	0.030	1.472	0.160
error	52	1.043	0.020

^aThe interaction term for fertilization rates in the egg-choice experiment was marginally significant (p = 0.05) when data from one block with high residual error variance (on average 5.2 times higher than all other replicates) was removed from the analysis.