Researchers from the University of Sydney and Newcastle University have discovered a unique birth mechanism that allows seahorses to control the process of removing fry from the brood sac. Research published in the journal Placenta.
In most animals, including reptiles and mammals, birth is regulated by hormones. First of all – oxytocin, which causes contractions of the smooth muscles of the uterus. These contractions are involuntary and spontaneous. A 1970 study found that exposing seahorses to oxytocin (isotocin) analogues triggered birth behavior in them. Pregnancy and birth in seahorses are of interest to men. Embryos develop in a special sac on their tail, similar to the mammalian uterus.
The authors of a new study conducted an experiment: parts of a seahorse’s brood sac were exposed to isotocin. The hormone caused contraction of intestinal tissue, which the scientists used as a control sample. But it had no effect on the skate “womb”. Analysis of the microscopic structure of the brood sac revealed very little smooth muscle, much less than in the womb of female mammals. This observation explained why the uterus did not contract in response to isotocin.
The scientists then compared the body anatomy of male and female bellied seahorses. Seahorses have three special bones located near the mouth of the brood sac. Skeletal muscles are attached to them – unlike smooth muscles, seahorses can contract them arbitrarily. In fish these muscles and bones control the anal fin, which is essential for movement, but in seahorses these muscles are not used much when swimming.
The muscles and bones of the anal fin were much larger in male straws than in females. Their directions indicate that they can control the opening of the bag. This process is triggered by isotocin. Inducing a courtship-like behavior in which males fill their brood sacs with water and then begin to rock, contracting their muscles to remove the young from the pouch. As a result, hundreds of babies are expelled in a short time.
The scientists’ discovery could mean that seahorses consciously control the expulsion of their young at the end of pregnancy; this is a unique way of managing childbirth.