Intracapsular embryonic development is common in marine gastropods. In many species, embryos obtain nutrition during ontogeny by ingesting substances contained in the intracapsular fluid. South American volutids spawn egg capsules containing relatively few eggs, and development is entirely intracapsular, with young hatching as crawl-away juveniles. The spawning habits, egg capsules morphology and embryonic development of the South American volutid Adelomelon ancilla is described in this work. Egg capsules of A. ancilla (n = 42) contained 3.0 ± 1.4 (mean ± SD) embryos with a range of 1–6 eggs or embryos. The embryonic morphological changes and body spiralization occur simultaneously with embryo feeding. All embryos within a single egg capsule shared the same developmental stage. No evidence of cannibalism or intracapsular competition was found, suggesting that food is not a limiting resource as in other volutid species. Estimated intracapsular developmental time varied between five and six months (between 12–14°C). The long developmental time and low number of embryos inside the egg capsules, together with late reproductive maturity and longevity of adult individuals, reinforce the statement that the neogastropod Adelomelon ancilla is a fragile resource that must be conserved in Argentinean benthic communities.