Ocean Love Story Series丨Seahorse: Dad gets pregnant and gives birth

Author: Scientific scraps

Reviewer: Zhang Wei, Associate Professor of Marine Biology, Shandong University

In human society, there is one thing that women must do and men can never replace, and that is giving birth to children.

Even in the animal world, giving birth seems to be the privilege of the mother. Even for animals like kangaroos where the father has a pouch, it is still the mother who gives birth to the baby kangaroos.

However, the protagonist we are going to talk about today is not like that. This is an animal that actually gives birth to its children. That is the seahorse.

Poseidon chose it as his mount, but he might have made the wrong choice...

The seahorse is indeed a very unique animal. However, with its bent head and straight chest, it really looks a bit like a chess horse. No wonder in Greek mythology, the seahorse is regarded as the mount of the sea god.

However, have you ever wondered what kind of animal the strange-looking seahorse is?

Although you would never associate it with a fish based on its appearance, and even if you say it is a dragon, some people would believe it, but the seahorse is indeed a fish.

But the seahorse is indeed a very special fish.

Seahorses often have a raised spine on top of their heads that looks like a crown. Each seahorse has its own unique crown, and scientists can use these crowns to distinguish individual seahorses, just like fingerprints in humans [1].

Perhaps it is because the crown on his head makes him look majestic, so the sea god in Greek mythology chose the seahorse as his mount? However, it is certain that the sea god would regret choosing the seahorse as his mount, because no matter where he wants to go, he may be late.

Why? Because seahorses are the slowest swimming fish in the world.

The way seahorses swim is different from other fishes. They don't have a tail fin, so they can only push their bodies slowly with the dorsal fin on their backs. And when seahorses swim, they often stand very straight, which makes people anxious for them.

According to the Guinness World Records, they are the slowest swimming fish in the world. Among them, the pygmy seahorse (Hippocampus zosterae) is surprisingly slow to swim because of its small size, and can only swim a dozen meters per hour[2].

Such weak swimming ability makes seahorses have to face a problem, they may be washed away by the current. But fortunately, their tails have evolved to be very flexible, and they can grab the seaweed, seaweed or other fixed objects beside them like hands to prevent themselves from being carried away by the current.

So if you see Poseidon's horse stuck in seaweed while swimming in the sea, don't be too surprised. Okay, after making fun of the seahorse's swimming ability, let's get back to the seahorse's love.

Seahorse's courtship dance

Although seahorses have poor swimming abilities, they are very determined when it comes to seeking love.

Their courtship ritual is generally divided into four stages[3]. However, for ease of understanding, it can be simply divided into two stages: before the mating day and on the mating day.

First, let's take a look at the courtship rituals that precede mating day.

This ritual usually begins one to two days before the official mating. During these two days, every morning at dawn, the male and female seahorses gather together, shaking their bodies as they get closer.

Eventually, the tails of both seahorses will grab onto the same fixture and begin to spin in circles, though the male and female seahorses still have to go their separate ways before mating.

On the day of mating, the male and female seahorses will perform a series of courtship dances, culminating in the two seahorses letting go of their tails, leaving their anchorage, and swimming a few centimeters upward. The female seahorse will insert her ovipositor into the male's brood pouch, handing over her precious eggs to the male [3].

With the eggs, the male seahorse can fertilize them. After that, the male seahorse will wait for the baby seahorses to be born with a big belly. Most seahorses can give birth to 100 to 1,000 baby seahorses at a time. It can be said that the male seahorse really completes the pregnancy and birth process.

So why did seahorses evolve such a unique reproduction mechanism?

Scientists are still studying this issue and there is no conclusion yet.

One hypothesis is that seahorses do this to shorten the interval between births and produce more offspring.[4] If there are enough male and female seahorses, male pregnancy can save more time than female pregnancy and produce 17% more offspring.

Since the father seahorse no longer takes care of the baby seahorses after giving birth and leaves them to fend for themselves, the more offspring it produces, the better.

Are seahorses monogamous?

Most wild seahorses are monogamous, at least during the courtship and breeding phases, and they stick together[5].

After all, seahorses are animals with relatively weak swimming abilities. They mainly rely on hiding in seaweed and staying motionless to escape predators. It is too dangerous for them to go out and look for a new mate. It is safer for them to stick to one fixed mate and mate more often to produce as many offspring as possible.

However, if the female seahorse in a pair can no longer smell her husband’s scent, she may go off the rails and look for another male seahorse[6]. After all, if someone “disappears” in the ocean, they are likely to be eaten.

Protecting seahorses is imperative

It can be said that seahorses have racked their brains to survive. They have even evolved a unique reproductive mechanism, such as male seahorses becoming pregnant. But even so, this strange creature in the ocean has run into big trouble due to human fishing and environmental changes.

In traditional Chinese medicine, seahorse is considered a medicine that can treat impotence and asthma. It is also associated with "yang-strengthening". Ancient people recorded that "seahorses are in pairs of male and female, and their sexual warmth has the meaning of sympathy" and "they are often used in sexual intercourse for yang deficiency, and can replace the effect of gecko". This "magical effect" that has not been verified by evidence-based medicine has brought fatal disasters to seahorses.

According to estimates by the American Fisheries Society, approximately 37 million seahorses are caught each year[7]. Worse still, relatively little research has been done on seahorses, and a comprehensive database has not been established for many species of seahorses, so people have no way to assess their endangered status.

But the good news is that seahorses have now been listed in the Convention on International Trade in Endangered Species of Wild Fauna and Flora (Appendix 2), restricting the fishing and trading of seahorses. In our country, all species of seahorses are classified as national second-level key protected wild animals and cannot be caught and traded at will.

If you also like this magical animal that gives birth to babies, and like the world's slowest swimming "sea god's mount", as ordinary people, we should also boycott all trade activities related to seahorses.

References:

[1] Freret-Meurer, Natalie (2013). "Seahorse Fingerprints: A New Individual Identification Technique". Environmental Biology of Fishes. 96 (12): 1399–1405. doi:10.1007/s10641-013-0118-6. S2CID 13917616.

[2] https://www.guinnessworldrecords.cn/world-records/70705-slowest-fish

[3] Masonjones, Heather D.; Lewis, Sara M. (1996). "Courtship Behavior in the Dwarf Seahorse, Hippocampus zosterae". Copeia. 1996 (3): 634–640. doi:10.2307/1447527. JSTOR 1447527.

[4] Vincent, Amanda CJ (1994). "Operational Sex Ratios in Seahorses". Behavior. 128 (1/2): 153–167. doi:10.1163/156853994X00091. JSTOR 4535169.

[5] https://ocean.si.edu/ocean-life/invertebrates/seahorse-lovebirds

https://www.newscientist.com/article/2280826-female-seahorses-cheat-on-their-mate-when-they-can-no-longer-smell-him/

Lawson, JM; Foster, SJ; Vincent, ACJ (January 2017). "Low bycatch rates add up to big numbers for a genus of small fishes". Fisheries Magazine. American Fisheries Society. 42 (1): 19–33. doi:10.1080/03632415.2017.1259944.

This article is produced by Science Popularization China-Starry Sky Project (Creation and Cultivation). Please indicate the source when reprinting.