Artículo de publicación ISIOxygen availability in the aggregation of offspring has been shown to be a critical fac- tor affecting the survival and development of embryos in aquatic systems. It is not yet known, how- ever, to what extent the capacity to provide O2 to embryo aggregations may act on the time of parental protection (here encapsulation), ultimately determining indirect and direct embryonic development. We assessed O2 conditions during encapsulation, the factors determining those condi- tions, and the consequences on embryo survival in 2 gastropod species with contrasting developmen- tal modes: Crepidula fornicata, an indirect developer, and Crepidula coquimbensis, a direct devel- oper showing adelphophagy. Results showed that intracapsular O2 conditions decreased to almost hypoxic conditions throughout development in C. fornicata, in contrast to the oxygenated conditions observed in C. coquimbensis during the entire encapsulation period. These contrasting patterns between species were explained by: (1) differences in metabolic rate of the embryos, (2) differences in total metabolizing material per capsule throughout development, and (3) differences in wall thick- ness and rates of decay throughout development, which may affect O2 diffusion. Moreover, when the low O2 conditions observed at the end of the encapsulation period were maintained after hatching by artificially extending encapsulation for 3 d, a dramatic negative effect on embryonic survival was observed in the indirect developer. In contrast, no effect on juvenile survival was observed in the direct developer. We suggest that hatching at intermediate stages of embryonic development in C. fornicata may be a response to increased O2 constraints during the encapsulated period.