Soulpearls:

Both Shallows and Deepsea mer come with the concept of “soulmates”, too, but it's dangerous. They're reincarnated every time they die and are reborn into new bodies — the soul lives on. Soulmates are chosen, not assigned. They are born with this “pearl” (it's not really a pearl, it just looks like a fleshy/cartilage one with the consistency of an eyeball) inside their bodies that, if given to their given designated soulmate and it's eaten, means they'll have vague memories of each other in the next reincarnation cycle. They'll dream of each other, hear each other's voices singing to them, and have a kind of bond with them, where they know if they're alive or not, upset or not, etc. You can also have more than one soulmate. If everyone swaps pearls and everyone gets one, you'll all be tied together. You just have a strong bond with the person whose pearl you ate. The pearl is dangerous, though. If you're not careful, you'll bleed to death getting it. It's a risk. It's meant to prove how serious you are about the other person. It's commitment.

It's also worth noting that the pearl has magic involved with it. If you eat someone else's pearl, you become the same kind of mer as they are. So if you're a Deepsea mer and you eat a Shallow's pearl, you become a Shallow. This is important in that Deepsea and Shallows can only interact in the middling zones of the ocean. Deepsea disfigure and die (like the blobfish) because their bodies were made to deal with a certain amount of pressure of the ocean atop them. Shallows mer have less pressure seeing as they live near the surface in the shallow waters. A human can also be involved in this process. They can become mer if they eat a pearl and be involved in the reincarnation cycle, born again as a human or a mer depending on preference. For them, the dreams and memories of past lives haunt them their entire lives. Some go mad from it because they struggle to accept and move past it.