Seahorses are teleost fish, even though most people are only vaguely aware of this fact, confounded by the horse-like head, a prehensile tail and an upright posture swim! Seahorses are viviparous fish – i.e. produce young which develop within the body of the parent – their reproduction is unique among vertebrates because these animals have refined paternal care to such an extent that the males become pregnant.
A female seahorse transfers eggs into the male’s brood pouch, which is a skin pocket situated on the male’s abdomen or tail (depending on the species), the eggs are subsequently fertilized and the embryonic development is initiated. After mating the opening is sealed and the embryos develop inside the pouch, isolated from the external marine environment, effectively preventing more than one female from depositing their eggs into a male’s pouch. After the mating has occurred, however, the circulating androgen concentration decreases rapidly and the male has to perform a completely different function, namely, to support pregnancy. After 2-3 weeks, depending on the species, the pregnant males can give birth to up to 1,000 tiny seahorses.