The eggs hatch in the pouch. The father cares for the young as they grow, regulating the water salinity in the pouch to prepare them for life in the sea. "It's quite costly energetically," Foster said.
When the tiny seahorses are ready to be born, the male undergoes muscular contractions to expel the young, known as "fry," from the pouch.
Cutting the Ties
While seahorse dads go the extra mile to give birth, the parents do not provide their tiny offspring with any care or protection after they are born.
Infant seahorses are susceptible to death from predators and being swept into ocean currents, where they drift away from feeding grounds rich in microscopic organisms.
Fewer than five infant seahorses in every 1,000 survive to adulthood, which helps explain why the litters are so large, said James Anderson, manager of the seahorse program at the National Aquarium in Baltimore.
By fish standards, however, seahorses have a fairly high survival rate because they are sheltered in the father's pouch during the earliest stages of development. The eggs of other fish are abandoned immediately after fertilization.
Although seahorses are the only fish that experience true male pregnancy, the males of two close relatives, sea dragons and pipefish, carry eggs attached to an area beneath their tails.
Anderson said other members of the family Sygnathidae, to which these three fish belong, have a number of different types of enclosed areas on their bodies in which to hold eggs; these spaces range from a flat spot to a trough, suggesting how a pouch may have evolved.
Scientists are not sure what evolutionary advantage male pregnancy gives seahorses. One theory is that it enables a shorter cycle of reproduction by distributing the costs of the process between the two partners.
While the male is bearing the young, the female can prepare more eggs to implant soon after the male has given birth to the last litter. Anderson said some seahorses can give birth in the morning and be pregnant again by evening.
Aquariums have only recently developed the technology to raise seahorses in captivity, according to Scarratt. "The husbandry for seahorses is extremely difficult," she said.
The main challenge has not been getting the seahorses to breed, but to help the fry survive, she explained. The infant seahorses are so small they cannot eat most of the tiny plankton that are fed to the adults. Special food has to be grown so the fry do not starve.
The Seahorse Breeding Lab at the National Aquarium in Baltimore has raised eight species of seahorses and three species of pipefish. Populations of these fishes have been sent to institutions in the United States and Portugal.
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