Seahorses

The seahorse is noted for strange reproduction:
The males, not the females, give birth.
What’s less widely known is the way that that works.
Here’s the answer, for what it is worth:

Their courtship involves a particular sit-up-like
Dance on the part of the male.
The female inserts all her eggs in a uterine
Pouch on the front of his tail.

He floods them with sperm, and the fertilized join
A placenta within the male pouch.
After 24 days’ incubation and nourishment,
Then is the time to say “ouch.”

That dance is repeated to crunch and expand
The male’s abdomen, letting in water
And pushing out thousands of babies in minutes.
(The number’s in case of a slaughter.)

There’s no oxytocin or smooth muscle tissue
At work in the process. Instead,
Where the skeletal muscles connect to the anal fin,
Three little bones help the spread.

The likeliest reason for using the system:
The male giving birth must allow
The female to make extra eggs for survival.
That answers the why and the how.