the evolutionary radiation of the family syngnathidae was accompanied by a diversification of structures involved in male parental care whose anatomical variations may signal differences in reproductive strategies, with increasing egg protection possibly affecting female investment in offspring or larvae quality at the end of the embryonic development phase. an analysis of egg numbers showed significant differences between syngnathids with and without marsupium, suggesting that the brood pouch, besides increasing protection to the male and its eggs, also introduced the ability to carry an additional number of offspring per pregnancy, from one or more females. curiously, even though larvae sizes do not significantly differ between the considered brooding structures (controlling for male length), seahorse larvae were smaller than expected, given the large volume of the pear-shaped eggs. these observations suggest that the sealed seahorse pouch, although allowing the transport of a great number of offspring and capable of multiple consecutive pregnancies, may impose serious functional constraints, namely those related with oxygenation of the developing larvae, thus explaining the increased egg surface together with the presence of specific salinity control mechanisms inside the marsupium.