Seahorses are a genus (Hippocampus) of fish belonging to the family Syngnathidae, which also includes pipefish and leafy sea dragons. 

There are over 32 species of seahorse, mainly found in tropical and subtropical coastal and reef waters all over Pacific, Atlantic and Indian oceans. Colonies have also been found in European waters such as the Thames Estuary. From North America down to South America there are approximately four species, ranging from very small in size (dwarf seahorses are only about an inch long) to those much larger, found off the Pacific Coast of Central America (the foot-long Hippocampus ingens). Hippocampus erectus are bigger and fatter seahorses found anywhere from Nova Scotia down to around Uruguay.

These fish form territories, with males staying in about one square meter of their habitat while females range about one hundred times that area. They bob around in sea grass meadows, mangrove stands, and coral reefs where they are camouflaged by murky brown and grey patterns that blend into the sea grass backgrounds. During social moments or in unusual surroundings, seahorses turn bright colors. According to co-founder of Project Seahorse, Amanda J. Vincent, mates can blush a shade of creamy yellow when meeting each other in the morning. She even encountered one male who took the shade of the orange tape she used to mark the grid in the study area.

Physical description.

Seahorses and pipefishes are notable for being the only species in which males become "pregnant".

Courtship.

When two parties discover a mutual interest at the beginning of breeding season, they court for several days, even while others try to interfere. During this time they have been known to change color, swim side by side holding tails or grip the same strand of sea grass with their tails and wheel around in unison in what is known as their “pre-dawn dance”. They eventually engage in their “true courtship dance” lasting about 8 hours, during which the male pumps water through the egg pouch on his trunk which expands and cleaves open to display an appealing emptiness. When the female’s eggs reach maturity, she and her mate let go of any anchors and snout-to-snout, drift upward out of the seagrass, often spiraling as they rise. "The female inserts her ovipositor into the male’s brood pouch, where she deposits her eggs, which the male fertilizes. The fertilized eggs then embed in the pouch wall and become enveloped with tissues."

New research indicates the male releases sperm into the surrounding sea water during fertilization, and not directly into the pouch as was previously thought. Most seahorse species' pregnancies lasts approximately two to three weeks.

As the female squirts anywhere from dozens to thousands of eggs from a chamber in her trunk into his pouch, her body slims while his swells. Both seahorses then sink back to the bottom and she swims off. Scientists believe the courtship behavior serves 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 father’s pouch which is coursed with prolactin, the same hormone responsible for milk production in pregnant women. He doesn’t supply milk, but his pouch provides oxygen as well as a controlled environment incubator. The eggs then hatch in the pouch where the salinity of the water is regulated. This prepares the babies for life in the sea. Throughout the male’s pregnancy, his mate visits him daily for “morning greetings”. The female seahorse swims over for about 6 minutes of interaction reminiscent of courtship. “They change color, wheel around sea grass fronds, and finally promenade, holding each other’s tails. Then, the female swims away until the next morning, and the male goes back to vacuuming up food through his snout.”

In some recent studies, it has been noted that male seahorses frequently seem to take unusual interest in each other, particularly in the months leading up to the breeding season. They have been observed engaging in rituals in which they pair up and appear to orally stimulate the egg pouches of their partners. It has been speculated that this might serve to clean the pouches in preparation for breeding, as well as to provide sexual pleasure. Homosexual behavior has been observed in some 1500 other species throughout the animal kingdom, so to researchers these findings come as no great surprise.

Birth.

The male seahorse can give birth to as many as 2,000 babies at a time and pregnancies last anywhere from 40 to 50 days, depending on the species. When the babies are ready to be born, the male undergoes muscular contractions to expel the “fry” from his pouch. He typically gives birth at night and is ready for the next batch of eggs by morning when his mate returns. Like almost all other fish species, seahorses do not care for their young once they are born. Infants are susceptible to death from predators or being swept into ocean currents, where they drift away from rich feeding grounds or into temperatures too extreme for their delicate bodies. Fewer than five infants of every 1,000 born survive to adulthood, helping to explain why litters are so large. The survival rates of these infants are actually fairly high compared to fish standards, because they are initially sheltered in their father’s pouch during the earliest stages of development, while the eggs of most other fish are abandoned immediately after fertilization. This makes the process worth the great cost to the father of incubating his offspring.