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,...