Multilevel selection on offspring size occurs when offspring fitness depends on both absolute size (hard selection), and size relative to neighbours (soft selection). We examined multilevel selection on egg size at two biological scales: within clutches and among females, for an external fertilising tubeworm. We exposed clutches of eggs to two sperm environments (limiting and saturating) and measured their fertilisation success. We then modelled environmental (sperm) differences in hard and soft selection on individual eggs, as well as selection on clutch-level traits (means and variances). Hard and soft selection differed in strength and form depending on sperm availability – hard selection was consistently stabilising; soft selection was directional and favoured eggs relatively larger (sperm limitation) or smaller (sperm saturation) than the clutch-mean. At the clutch-level, selection on mean-egg size was largely stabilising; whilst selection on within-clutch variance was weak, but generally negative – although some correlational selection occurred between these two traits. Importantly, we found that the optimal clutch-mean egg size differed for mothers and offspring, suggesting some antagonism between the levels of selection. We thus identify several pathways that may maintain offspring-size variation: environmentally (sperm)-dependent soft selection; antagonistic multilevel selection; and correlational selection on clutch-means and variances. Multilevel approaches are powerful, but seldom used, tools for studies of offspring size and we encourage their future use.

Data used to model multilevel selection on egg size under sperm saturating (H) and sperm limiting (L) conditions for the broadcast spawning polycheate, Galeolaria caespitosa. We used a split-clutch design to conduct in vitro fertilisations, whereby each female's clutch was split across 2 replicate fertilisations (Rep) per sperm treatment. We then measured the size-specific fertilisation success of all eggs within these clutch subsamples as a binary response variable (0 = unsuccessful eggs; 1 = successfully fertilised eggs). Note that values of clutch-mean in the dataset represents the mean-egg size of each female's clutch measured from photographs taken prior to fertilisation. Egg size represents the inital size of each indiviual egg prior to fertilisation, and has been back-calculated from measurements taken after fertilisation using equations that describe these relationships.