Effect of oxygen availability in determining clutch size in Acanthina monodon

Most taxa of aquatic invertebrates exhibit (1) strong variation in number and size of offspring, and (2) an association between adult size and mode of development, and also between mode of development and size of the clutch. These patterns suggest that a similar set of constraints affects both critical life-history traits and physiology of most taxa. Evidence suggests that oxygen is limiting during early development in aggregated egg masses of marine invertebrates. We studied whether oxygen availability during early development could be a determinant of clutch size in the gastropod Acanthina monodon. We estimated intra- and extracapsular oxygen availability throughout embryo development using optic fibers and incubated capsules at different oxygen partial pressures (hypoxia, normoxia and hyperoxia) throughout development. We also compared the ratio between nurse eggs and embryos in capsules collected in situ as well as in experimental capsules. We found that intracapsular oxygen availability decreased as embryos developed (80% air saturation in early stages to 40% air saturation in late stages). The mean number of embryos increased as oxygen availability increased from hypoxia to hyperoxia, while the total number of eggs remained constant. The mean number of embryos that developed under normoxia was similar to that recorded in field-collected capsules. We suggest that oxygen availability determines clutch size in this species, probably as a result of oxygen competition among siblings. This may also occur in other taxa of marine invertebrates, since oxygen is limiting in egg masses in general.