Did you know that male seahorses become pregnant? Read on to find out more on this unique feature and how the male seahorse can transfer nutrients to their babies during pregnancy.
Skalkos, Z., Van Dyke, J., & Whittington, C. M. Paternal nutrient provisioning during male pregnancy in the seahorse Hippocampus abdominalis (2020). Journal of Comparative Physiology B. 10.1007/s00360-020-01289-y
It’s a man’s world
When I first saw a seahorse on the ocean floor while scuba diving, my jaw dropped. These magical animals seem too tiny to be so intricate with their yellow body armour, horse-like body, and curled tail wrapping around whatever they can grab to stay stable. Their incredible individuality extends to their reproduction as well. Seahorses and other members of the Syngnathid fish family, which includes pipefishes and sea dragons, are the only vertebrates with pregnant males.
Seahorses are also viviparous, as opposed to oviparous. In oviparous animals, like chickens or most fish, the female parent lays eggs that hatch externally. In viviparous animals like seahorses, the female deposits thousands of eggs inside the male seahorse parent. He then carries the eggs for up to 45 days in a fully enclosed ‘brood pouch’ that incubates and grows the embryos internally. Eventually, the eggs hatch (while still inside the pouch!) and the male gives birth to live babies by ejecting them into the ocean. The male brood pouch is an incredible feat of nature, however, how this pouch actually transfers nutrients from father to baby is poorly understood.
How do the babies eat?
In an effort to better understand the brood pouch, scientists compared the weights of newly fertilized eggs to the weights of newborns. By comparing these weights, scientists can show how much is transferred to the baby from the father.
Male seahorses do not have an umbilical cord like humans, but the embryos are closely embedded in the internal tissue of the father’s pouch. Scientists believe this close proximity acts like a placenta and thus is how nutrients can transfer easily between father and young. Because male seahorses pass nutrients, like rich fats and lipids, to their young, the newborns develop to be much heavier than the eggs.
Like human, like seahorse
This is the first ever study to show that seahorses, like humans, transfer nutrients to their babies while pregnant. This is called matrotrophy when it is from mother to embyro, but in the case of seahorses, it is patrotrophy, from father to embryo.
By transporting nutrients to their offspring, seahorses provide them with energy, oxygen, and immune protection. It is incredible to think of how one seahorse can provide so many nutrients; one male can release 5 to 2500 babies! Interestingly, fathers abandon babies after birth, and newborns have a 0.5% of surviving to adulthood.
As key seahorse habitats like coral reefs and seagrass beds decline with trawling and other destructive fishing practices, the seahorse population is likely decreasing as well. As many as 37 million seahorses are estimated to be killed through bycatch annually. Studies like this provide key information into understanding the evolution behind seahorse male pregnancy and viviparous animals, and helps contribute to future conservation strategies. There are also ways you can help take action to protect these unique animals at home, like choosing sustainable seafood options (avoiding shrimp), reducing waste or organizing beach cleanups, and refusing to buy seahorses as souvenirs or pets.
I have always been happiest in nature – exploring forests, traveling to the ocean, or working with wildlife. After obtaining my MSc in Marine Environmental Management at the University of York, I have worked in conservation and marine science around the world. I have a special affinity for corals, cuttlefish, and cetaceans.
The sea, once it casts its spell, holds one in its net of wonder forever. – Jacques Cousteau