Seahorses aren’t odd simply for their horselike shape. They (and their syngnathid fish kin, seadragons and pipefish) are the only vertebrates in which the males take on the burden of pregnancy. You might think that since having pregnant guys is really weird, some keen scientists would have figured everything out about the phenomenon. But a lot of questions remain, in part because it doesn’t look the same in all species. In some, the seahorse brood pouch — the male alternative to the uterus — serves as a simple site for fertilized eggs to attach while they develop. In others, the eggs, after mom deposits them in the brood pouch, are fully enclosed and protected from the outside world.
The big-belly seahorse (Hippocampus abdominalis) is at the far end of that spectrum, with the most complex type of seahorse pregnancy. To get a better picture of what is going on during this period of seahorse life, Camilla Whittington of the University of Sydney and colleagues analyzed gene expression (the patterns of genes turned on and off) in the big-belly seahorse during conception, egg development and birth. They found several commonalities linking the seahorse, reptiles that give live birth and us. The study appeared September 1 in Molecular Biology and Evolution.
A male seahorse becomes pregnant when a female transfers her eggs into his pouch, where they are fertilized. (It’s a clever system for males that ensures that they are taking care of their own kids.) The pouch has to undergo some restructuring, such as growing bigger, to accommodate and protect the eggs, and there are changes in genes that regulate cell growth, proliferation and death, Whittington and her colleagues found.