Among many remarkable attributes of seahorses, their male pregnancy has to be the most spectacular.
A female transfers eggs to a male’s enclosed brood pouch. The male then fertilizes the eggs, giving him certainty of paternity, a confidence that is rare among animals.
The embryos settle into the tissue of the brood pouch, where they are provided oxygen through a capillary network and nutrition through a placental fluid (with adult diet apparently affecting embryonic development).
Male pregnancy lasts about 10 days to six weeks depending on the species and water temperature, during which time the pouch environment is adjusted to become more similar to the ocean.
At the end of the pregnancy, the male goes into labour, pumping and thrusting for hours, to propel his young out of the pouch. He may release anywhere from a very few to several thousand embryos, depending on the species, and young of most species measure 8-10 mm.
The young are entirely independent from that point – most drifting around as plankton – and the male is ready to mate again.
In many species, but certainly not all, one male and one female form a long-term pair bond. This was first discovered (by Project Seahorse) in Hippocampus whitei from eastern Australia.
The female will approach the male early in the morning and they then move together in a greeting dance. Their bodies brighten visibly then they promenade with linked tails and pirouette around a shared holdfast for an average of six minutes. The animals then part for the rest of the day. This greeting dance continues throughout a male pregnancy and may be cueing the female; on the day that the male gives birth, the female has prepared new eggs and is ready to mate.
The pair bond is so robust that the female will continue to visit the male even if an injury precludes mating or pregnancy. It also seems to be beneficial; paired seahorses mate again quickly and long enduring pairs tend to have more young.
Mating in seahorses is a prolonged ballet, an expansion of the morning dance, again with brighter colours. Only now the pair rises through the water column, trying to align the female’s ovipositor (penis equivalent) with the male’s pouch opening. After many failed attempts, they eventually manage the egg transfer, then part. The male descends and sways gently to settle the eggs while the female swims away.
In general, pairs tend to be somewhat matching in size, with larger females (with more, larger eggs) mating with larger males (with larger brood pouches)1.
Read the story of how we were fortunate enough to witness this male seahorse giving birth in the waters off New South Wales in Australia.
Kvarnemo, C., Andersson, S. E., Elisson, J., Moore, G. I., & Jones, A. G. (2021). Home range use in the West Australian seahorse Hippocampus subelongatus is influenced by sex and partner’s home range but not by body size or paired status. Journal of Ethology, 39(2), 235-248.