How seahorse fathers use their unique body and behavior to give birth – Interesting Engineering

What we found

First, we exposed pieces of seahorse pouch to isotocin. While isotocin caused our control tissues (intestine) to contract, surprisingly this hormone produced no contractions in the brood pouch.

The result led us to wonder about the anatomy of the pouch. When we examined the pouch under a microscope, we found it contains only scattered small bundles of smooth muscle, far less than the uterus of female mammals. This explained why the pouch did not contract in our experiments.

Using 3D imaging techniques combined with microscopy, we then compared the body structure of male and female pot-bellied seahorses.

In males, we found three bones positioned near the pouch opening, associated with large skeletal muscles. These types of bones and muscles control the anal fin in other fish species. In seahorses, the anal fin is minuscule and has little or no function in swimming.

So, the large muscles associated with the tiny seahorse fin are surprising. The anal fin muscles and bones are much larger in male seahorses than in female seahorses, and their orientation suggests they could control the opening of the pouch.

Seahorse courtship is an elaborate process. Males open and fill their pouch with water by bending forward and contracting their bodies to force water into the pouch, before dancing with the female.

Similarly, during labor, male seahorses bend their body towards the tail, pressing and then relaxing. This pressing behavior is accompanied by brief gaping of the pouch opening, with a series of whole-body jerks. This movement combined with pouch opening allows seawater to flush through the pouch.

Jerking and pressing continue, the pouch opening gets gradually bigger, and groups of seahorse babies are ejected with each movement. Many hundreds of babies are ejected in a short time.

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How seahorse fathers use their unique body and behavior to give birth - Interesting Engineering