Abstract
Comparing five species of pipefish, egg size was significantly larger in species with brood pouches ( Syngnathus typhle , Syngnathus acus and Syngnathus rostellatus ) than in species without brood pouches ( Entelurus aequoreus and Nerophis ophidion ). Egg size correlated positively with female body size in species with brood pouches, but was similar across female sizes in the species lacking pouches. These results may reflect differences in offspring competition as a consequence of variable offspring relatedness within a brood, due to the mating systems adopted by the different species and the presence or absence of a brood pouch.