Although it is common for male fish to play the dominant parenting role, male pregnancy is a complex process unique to the fish family Syngnathidae, which includes pipefish, seahorses and sea dragons.
When seahorses mate, the female inserts her unfertilized eggs into an external brood pouch that grows on the body of the male. The outer shell of the eggs then breaks down, and tissue from the male grows up around the eggs in the pouch. Afterwards the male seahorse releases its sperm into the pouch for fertilisation.
During this whole process, the male keeps blood flowing around the embryos, controls the salt concentrations in the pouch, and provides oxygen and nutrition to the developing offspring through a placenta-like structure until he gives birth.
Male pregnancy has interesting implications for sex roles in mating.
In most species, males compete for access to females, so the evolution of secondary sex traits is usually seen in males with brightly coloured ornamentation on male fishes.
In some species of pipefish, the sex roles are reversed because males become pregnant and there is limited brood pouch space. So the colourfully ornamented females compete for access to available males.
Seahorses, however, are monogamous within a breeding season, each seahorse only mates with one other seahorse. Competition does not occur very often for this species.
One of the interesting things about the brood pouch is that it appears to have evolved independently multiple times. There are two major lineages of seahorses and pipefish: trunk-brooding and tail-brooding. It is believed that this structure independently evolved in each of these groups.