Meet the seahorse

Photo by Hans Hillewaert. Retrieved from Wikipedia on 06-29-2023. Shared under CC BY-SA 4.0.

Mothers usually get all the credit for raising their offspring, but there are some pretty amazing fathers out there, too!

In honor of Father’s Day, let’s talk about one of the best animal fathers around: the seahorse.

There are approximately 50 species of seahorses found in shallow, coastal waters worldwide. I personally associated seahorses with tropical waters, but you can find them as far north as the British Isles! Seahorses range in size from tiny (2 cm or 0.8 in) to large (35 cm or 14 in).

All seahorses are part of the genus Hippocampus, which comes from the Greek words for ‘horse’ (hippos) and ‘sea monster’ (kampos). Fun fact: the hippocampus, a memory region in your brain, got its name because it looks like a seahorse!

Funny-looking fish

Even though they may not look like it, seahorses are a type of fish. But they are definitely a unique type of fish!

Technically a fish. Photo by Nhobgood. Retrieved from Wikipedia on 06-29-2023. Shared under CC BY-CA 3.0.

Unlike other fish, seahorses are covered not with scales but with consecutive rings of bony plates. They also have long, tubular snouts and no teeth. To hunt, seahorses simply suck small organisms up through their narrow mouth. They can eat up to 3,000 crustaceans a day!

Seahorses aren’t that great at swimming: they only have a tiny fin on their back (for propulsion) and pectoral fins on the side of their head (for maneuvering). Their little back fin can move up to 50 times a second, but that doesn’t go far when it’s so small! In addition, they swim upright, which increases the amount of water resistance they encounter. Unfortunately, seahorses can become fatally exhausted from swimming in storms and rough waters.

But while they may not be great at swimming, seahorses can still travel large distances in the ocean. Their prehensile tail can grab onto floating debris, which then floats to a new location. This tail also lets seahorses cling to plants or corals, waiting patiently for a small organism to swim by. Some seahorses can even change their skin color to match their surroundings; this ability gives them a leg up for ambushing prey and avoiding becoming prey themselves.

Males give birth

It’s true: seahorses and a few species of pipefish and seadragons are the only animals in the world where the males give birth.

After courtship, the female deposits her eggs in the male’s brood pouch. It’s here that the eggs are fertilized and the young develop. During development, which takes between 10 days and 6 weeks depending on the species, the young are entirely dependent on their father for nourishment and protection. Over time, the father changes the fluid inside his brood pouch into salt water. The eggs then hatch inside the male, and he proceeds to expel them through a single opening in his pouch.

Male seahorse with a brood pouch. Photo by opencage. Retrieved from Wikipedia on 06-29-2023. Shared under CC BY-SA 2.5.

How do we know that it’s the male giving birth? Biologists define male and female based on who makes sperm and who makes eggs. And in seahorses, the same individual who produces sperm is the one who eventually gives birth.

But how?

Here’s how birth usually works: the hormone oxytocin is released in the female’s body, which causes the smooth muscles in the uterus to contract. This pushes the young out of the body.

But seahorses are different. A 2022 study found that the brood pouch of male seahorses doesn’t respond to oxytocin like a uterus typically does. In addition, the brood pouch has surprisingly little smooth muscle tissue.

But what the male seahorse does have is three small bones at the opening of the brood pouch. These bones attach to skeletal muscles and the fin on the back of the seahorse. When it’s time to give birth, the male uses these skeletal muscles to do some crunches; this pressing and relaxing of his abdomen eventually pushes his young out.

And why?

Scientists think that male birth is a response to the high mortality rate of young seahorses. Seahorses are born fully-formed and ready to take on the ocean by themselves; unfortunately, they are tiny and unprotected. Many of them die – less than one in a thousand make it to adulthood.

In most animals, the female would need time to make more eggs after giving birth. But by having the male carry the young, the female can get straight to work creating more eggs without having to waste time in the birthing process. This ultimately increases the number of eggs and young that a seahorse pair can create in a breeding season. In fact, a male seahorse is often pregnant again soon after giving birth.

Not many animals have perpetually pregnant males, but it seems to be working for seahorses!



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