Perhaps the most unique and unusual feature of seahorse biology is the fact that it is the male and not the female who becomes pregnant. When mature, males develop a pouch on the belly, known as the brood pouch. In this species the pouch is formed after 6 months of life, but males don’t breed until they are about one year of age (3). Breeding takes place in spring and summer; the female inserts her ovipositor into the male’s pouch and lays her eggs. The male then fertilises them and they become embedded into the wall of the pouch. The pouch is very similar to the womb found in female mammals; a placental fluid removes waste products and supplies the eggs with oxygen and nutrients. As pregnancy progresses, this fluid gradually becomes similar to the surrounding seawater, so that when the young seahorses are ‘born’ the change in salinity will not be such a shock (9). After around 30 days of pregnancy the male goes into labour, typically at night when there is a full moon (5) (3). After hours of thrusting, the miniature seahorses, which look exactly like the adults, are released from the pouch (3). The most numerous brood reported numbered over 1100 offspring (7). The offspring are fully independent after birth and must fend for themselves (3) (9). They are pelagic in the first stage of life, or hold onto floating debris at the surface with their tail (5). They settle on the bottom after they reach a length of 30 mm (3).
The big-belly seahorse is more active at dusk and night than in the day (3). They feed on crustaceans, such as amphipods and shrimp, which are sucked into their tube-like snouts and ingested whole (10).