<P> The fertilized eggs are then embedded in the pouch wall and become surrounded by a spongy tissue . The male supplies the eggs with prolactin, the same hormone responsible for milk production in pregnant mammals . The pouch provides oxygen, as well as a controlled environment incubator . Though the egg yolk contribute nourishment to the developing embryo, the male sea horses contribute additional nutrients such as energy - rich lipids and also calcium to allow them to build their skeletal system, by secreting them into the brood pouch that are absorbed by the embryos . Further they also offer immunological protection, osmoregulation, gas exchange and waste transport </P> <P> The eggs then hatch in the pouch, where the salinity of the water is regulated; this prepares the newborns for life in the sea . Throughout gestation, which in most species requires two to four weeks, his mate visits him daily for "morning greetings". </P> <P> The number of young released by the male seahorse averages 100--1000 for most species, but may be as low as 5 for the smaller species, or as high as 2,500 . When the fry are ready to be born, the male expels them with muscular contractions . 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 nurture their young after birth . Infants are susceptible to predators or ocean currents which wash them away from feeding grounds or into temperatures too extreme for their delicate bodies . Less than 0.5% of infants survive to adulthood, explaining why litters are so large . These survival rates are actually fairly high compared to other fish, because of their protected gestation, making the process worth the great cost to the father . The eggs of most other fish are abandoned immediately after fertilization . </P> <P> Reproduction is energetically costly to the male . This brings into question why the sexual role reversal even takes place . In an environment where one partner incurs more energy costs than the other, Bateman's principle suggests that the lesser contributor takes the role of the aggressor . Male seahorses are more aggressive and sometimes "fight" for female attention . According to Amanda Vincent of Project Seahorse, only males tail - wrestle and snap their heads at each other . This discovery prompted further study of energy costs . To estimate the female's direct contribution, researchers chemically analyzed the energy stored in each egg . To measure the burden on the males, oxygen consumption was used . By the end of incubation, the male consumed almost 33% more oxygen than before mating . The study concluded that the female's energy expenditure while generating eggs is twice that of males during incubation, confirming the standard hypothesis . </P>

How many seahorses are born at one time