Marine invertebrates that lay egg masses provision their offspring in a variety of ways, and in some groups, egg size is not the sole determinant of offspring size at independence. Supplemental, maternally derived resources, such as extra-embryonic nutrients, may increase offspring size without increasing egg size-options not available to broadcast spawners. Extra-embryonic nutrients, such as nurse eggs, can increase offspring size and performance, but little is known about other forms of larval nutrition. Larvae of the sea slug Elysia stylifera have access to intra- and extra-capsular resources in the egg mass prior to an obligate planktonic feeding period. We investigated the effects of maternal body size on egg size, capsule size, extra-capsular yolk (ECY), and fecundity, and how variance in these traits affected larval size at hatching. Larger mothers produced larger and more eggs, but larval size was unrelated to maternal size, egg size, or fecundity. Larval size instead depended upon the size of the egg capsule and amount of ECY available to each developing embryo: when capsule size was small, increasing ECY per larva increased larval size, and conversely, when ECY per larva was small, increasing capsule size increased larval size. Under theoretical considerations, ECY appears to supplement egg provisioning. Intra-capsular fluid and ECY add to the already diverse set of known mechanisms of offspring provisioning in marine invertebrates, and may provide insight into the factors affecting life-history evolution in marine invertebrates.