Population size, not density, serves as cue for sex ratio adjustments in polychaete worms

Local Mate Competition theory (LMC) predicts female-biased offspring sex ratios when mate competition occurs locally between siblings. The mating system of the marine polychaete worm Dinophilus gyrociliatus apparently fits LMC theory well. Females lay egg capsules with 3–6 large eggs (which produce females), and 1–2 small eggs (which produce males), resulting in female-biased sex ratios. However, mating occurs between larvae, inside egg capsules, and brothers fertilize sisters, possibly preventing any competition between unrelated males and obviating the need for sex ratio adjustments to local population size. Therefore, we tested whether mothers adjusted their offspring sex ratio to local population size, controlling for density. As predicted by LMC theory, sex ratios of single females were less male-biased than those of populations of multiple females, suggesting that males also compete with unrelated males. Sex ratio adjustments occurred irrespective of density, revealing sophisticated perception of the social environment in these worms.