It's true that male seahorses never play catch with their children or help them with their homework. But they do outdo human dads on one count: Male seahorses undergo pregnancy and give birth to their sons and daughters.
The trait is unique in these strange and fascinating fish that inhabit tropical and temperate coastal waters worldwide.
Seahorses, which range from less than an inch to a foot (one to 30 centimeters) in length, have evolved a series of unusual adaptations—a prehensile tail for clinging to underwater vegetation, a tubelike mouth for sucking in tiny crustaceans, and protective bony plates in their skin. There are 32 species of seahorse, all in the genus Hippocampus.
"They're such an unusual-looking fish, people sometimes don't realize they're real fish," said Alison Scarratt, curator of fishes at the National Aquarium in Baltimore.
The aquarium is currently featuring "Seahorses: Beyond Imagination," an exhibit about seahorses, pipefish, and sea dragons that will end next year. Scarratt said it has been the most successful exhibit in the history of the Baltimore Aquarium, which opened in 1981.
Although the bony plates covering its body make the seahorse unpalatable to most other animals, its survival is under threat from human predation, especially for use in traditional medicines.
No statistical data on seahorse populations is available because relatively little research on seahorses has been done until recently, but fishers have reported a decline in the number and size of seahorses they catch, according to a network of scientists from various institutions who conduct research under a program called Project Seahorse.
Breeding seahorses in captivity is a problem, in part because the babies are so tiny it's hard to keep them alive. The marine scientists in Baltimore are working to develop effective methods that will help ensure the creature's survival.
The male seahorse has a pouch on its stomach in which to carry babies—as many as 2,000 at a time. A pregnancy lasts from 10 to 25 days, depending on the species.
The reproductive process begins when a male and a female seahorse do daily pre-dawn dances, intertwining their tails and swimming together. Eventually they engage in a true courtship dance, which can last as long as eight hours. It ends with the female depositing her eggs in the male's pouch.
"Their mating ritual is quite beautiful," said Sarah Foster, a research biologist at McGill University in Montreal who is involved in Project Seahorse.
Scientists think the courtship behavior is designed to synchronize the movements of the two animals so that the male can receive the eggs when the female is ready to deposit them. The eggs are then fertilized in the dad's pouch.
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.